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Reconstruction of Millennial Forest Dynamics from Tree Remains in a Subarctic Tree Line Peatland
Ecology
Abstract
Tree ring and growth form sequences of 319 black spruce (Picea mariana [Mill.] BSP.) stems buried in a treeless peatland at the arctic tree line of northern Quebec were used to reconstruct the development of a woodland in response to climate change and fires during the last 2500 years. A high frequency of diagnostic tree rings (light rings and narrow rings) allowed the cross-dating of 142 individuals and the construction of a master chronology spanning AD 690-1591. Three floating chronologies covering 964 (≃178 BC-AD 785), 349 (≃587-239 BC) and 210 (≃1274-1065 BC) years were also developed. The buried stems were classified as arborescent (75%), small fragments of unknown origin (13%), stumps (11%), and portions of stunted stems (1%). The high frequency of arborescent individuals indicates that the buried spruces were the remains of a former forest. The forest bordered the peatland between 600 BC and AD 1568, and successfully regenerated after two fires around 350 BC and 10 BC. Winter-damaged trees dominated during periods of suppressed growth at AD 760-860 and AD 1025-1400, whereas undamaged trees were more frequent during periods of rapid growth around AD 700-750, 860-1000, 1400-1450, and 1500-1570. Fast growth between AD 860-1000, along with the concurrent establishment of symmetrical trees, suggests well-defined boundaries for the Medieval Warm Period in northeastern Canada. The forest abruptly shifted to an open krummholz (stunted spruce) after the last fire in AD 1568, indicating that a climatic threshold inhibiting postfire regeneration was crossed between the second and the last fires. The maintenance of this old growth forest over 1500 yr, in the absence of external disturbances except climate change, was probably due to the buffering effect of aggregated trees on wind-drifted conditions at the snowpack line. With the exclusion of the forest influence on microclimate and local growth conditions, the AD 1568 fire caused the forest vegetation to shift to krummholz. The amplitude of these ecosystem changes at tree line does not mirror that of climate change. Hence, it is concluded that climate and vegetation reconstructions from proxy indicators cannot portray full ecological impact, because vegetation change at tree line is nonlinear relative to climate change.
... Millennial growth ring series are useful to reconstruct long-term climatic variations (Helama et al. 2005;Büntgen et al. 2011) and past ecosystem responses to climate change (Arseneault and Payette 1997a;Payette and Delwaide 2004). Long chronologies also allow dating of trees having died following natural disturbances such as wildfires, landslides, rockfalls (Brown et al. 1999;Corona et al. 2013), or wood structures in buildings and archeological sites (Delwaide and Filion 1999;Querrec et al. 2009;Martin-Benito et al. 2014;Rochner et al. 2017). ...
... Lavoie and Payette (1997) constructed seven floating chronologies using black spruce trunks preserved in permafrost peatlands ( Figure 2D). Arseneault and Payette (1997a) provided a chronology covering the period 690-1591 CE, as well as three older floating chronologies ( Figure 2E). A reconstruction of lake level fluctuations made it possible to develop nine chronologies from samples preserved in two ponds and at the bottom of two lakes ( Figure 2F; Payette and Delwaide 2004). ...
... Another chronology was prepared in 2010 using samples preserved in a palsa bog ( Figure 2G; Cyr and Payette 2010). Finally, Asselin and Payette (2006) constructed 10 chronologies from wetland sites ( Figure 2H) and successfully aligned them to the floating chronologies of Arseneault and Payette (1997a). ...
We present the longest tree-ring chronology to date in northeastern North America (2233 years; 227 BCE – 2005 CE), resulting from several research projects conducted at the subarctic treeline in northern Quebec. This raw chronology of tree-ring width includes 464 black spruce (Picea mariana (Mill.) B.S.P.) shrubs (krummholz) growing in wetlands and preserved within peatlands. An indexed series of 152 erect black spruce trees that have lived in wetlands is also presented, covering the period 216 BCE to 1619 CE. We compared these chronologies to a tree-ring series including 116 black spruce trees and krummholz having grown on well-drained lichen woodlands over the period 1304–2000 CE. These chronologies highlight the major climatic periods of the last two millennia. Floating chronologies dating from 2500 to 3500 years ago were also developed from trees preserved in frozen peat. Growth rings from this period are much wider than those of the last 2233 years, suggesting warm climatic conditions and permafrost-free peatlands during the transition from mid- to late Holocene. The three subarctic chronologies presented here underscore the relevance and usefulness of tree growth rings and growth forms as ecological tools to assess the influence of climate on subarctic ecosystems.
... Most light-ring research has focused on black spruce, a dominant tree species throughout the treeline in North America. Light rings are described for their annual occurrence frequency in black spruce (Filion et al., 1986;Delwaide et al., 1991;Arseneault and Payette, 1997), their regional distribution (Delwaide et al., 1991), and site condition differences. As a powerful crossdating tool, light rings have helped to establish long black spruce chronologies (Arseneault and Payette, 1997;Lavoie and Payette, 1997). ...
... Light rings are described for their annual occurrence frequency in black spruce (Filion et al., 1986;Delwaide et al., 1991;Arseneault and Payette, 1997), their regional distribution (Delwaide et al., 1991), and site condition differences. As a powerful crossdating tool, light rings have helped to establish long black spruce chronologies (Arseneault and Payette, 1997;Lavoie and Payette, 1997). Unlike eastern larch, black spruce at subarctic treeline produces light rings only when induced by low temperature, a fact which can be used to identify light rings induced by insect defoliation in eastern larch (Liang et al., 1997). ...
... The samples were collected in the Boniface River area (57°44'05"N, 76°07'30'W) (Fig. 1), in northern Quebec, 30 km east of Hudson Bay (see Arseneault and Payette (1997) for study area description). Spruce trees in three different states were collected for this study: living trees (17 radii of 13 samples), exposed dead trees (upright or those lying on the ground: 17 radii of 15 samples), and subfossil trees buried in peat (69 radii of 45 samples). ...
Light rings in black spruce (Picea mariana [Mill.] BSP.) at the arctic treeline are characterized by pale-colored latewood made of a single or very few latewood-cell layers with thin-walled cells. Their widespread occurrence and their high frequency greatly facilitate the cross-dating procedure in dendrochronological studies. In this study, black spruce tree-ring density and wood structure were analyzed for light ring characteristics along with the mechanism of their formation according to ambient temperature. Light rings were quantitatively categorized into three classes based on the maximum tree-ring density using a normalized standard distribution. A light-ring chronology was established according to this classification. The results indicate that the grade of light ring was positively related to the frequency of light rings obtained from visual light-ring chronologies. The following anatomic variables were examined: number of cell layers of latewood, number of cells of the whole ring, percentage of latewood in the total ring width, and mean latewood cell-wall thickness. Among these anatomic variables, the mean latewood cell-wall thickness represents the best quantitative descriptor of a typical light ring as recognized by optical examination. The main causal factors of light rings are insufficient length of the growing season or cool summers.
... Tree species presented are Alder (A), Ash (F), Larch (La), Oak (Q), Pine (P), and Spruce (Pi), and what context the material originates from, bogs/peatlands (b), mainly bogs/peatlands (m) and partly bogs/peatlands (p). Arseneault and Payette (1997) CA/57°44′ N, 76°10′ W Pi (b) 500 BCE-present 4. Baillie (1973) IE/54°50′ N, 6°50′ W a Q (b) 5000-1380 CE 5. Baillie (1977) IE/54°50′ N, 6°50′ W a Q (b) 5000-1001 CE 6. Baillie et al. (1983) IE/54°50′ N, 6°50′ W a Q (b) 5000-1000 BCE 7. Baillie and Brown (1988) IE/54°50′ N, 6°50′ W a Q (b) 5300-200 BCE 8. Barniak et al. (2014) PL/54°15′ N, 19°44′ E P (b) 940-460 BCE b 9. Bauerochse et al. (2015) DE/52°50′ N, 9°50′ E a P, Q (b) 2600-1500 BCE 10. Boswijk and Whitehouse (2002) GB/53°32′ N, 0°57′ W P (b) 2921-2445 BCE 11. ...
... Over the last 3000 years, however, relatively few tree establishment phases have been recorded in German and Irish peatlands, whereas trees appear to be close to absent at all known Swedish sites (Fig. 10). Despite this fact, tree colonization has been recorded at peatlands in the Netherlands (Sass- Klaassen and Hanraets, 2006), Poland (Barniak et al., 2014), Lithuania (Pukienė, 1997;Edvardsson et al., 2016a), and in one of the few regions documented in North America (Québec; Arseneault and Payette, 1997). With the exception of the Canadian records, these phases are relatively short, which may also explain why only the Dutch chronologies have been cross-dated successfully over long distances (Figs. ...
... BP; Mellström et al., 2015). Gaps also exist in Lithuanian (Edvardsson et al., 2016a) and Canadian (Arseneault and Payette, 1997) tree records during the same period, which may strengthen the hypothesis that the 2.8 ka event did indeed significantly influence large parts of the northern Hemisphere. ...
... MTCs have allowed long climatic reconstructions to be created from tree ring widths, maximum wood density, and stable isotopes for several biomes and continents (Grudd, 2008;Helama et al., 2008;Shao et al., 2010;Bü ntgen et al., 2011;Gagen et al., 2011). These long chronologies have also provided a dating tool for reconstructing past ecosystem responses to climate change and other disturbances (Arseneault and Payette, 1997;Lloyd and Graumlich, 1997;Payette and Delwaide, 2004;Gunnarson, 2008) and have allowed the radiocarbon timescale to be calibrated to calendar years (Reimer et al., 2004). Owing to their length, MTCs are especially useful for deciphering low-frequency climatic and ecological signals (Esper et al., 2002;Helama et al., 2005;Ogurtsov et al., 2005). ...
... Except for eastern white cedar (Thuya occidentalis L.), which may locally reach an age of over 500 years at its northern limit in the southern boreal forest (Archambault and Bergeron, 1992;Kelly and Larson, 2007), individuals of all other species only rarely live for more than 300 years. As tree rings of North American boreal tree species are responsive to climate conditions at several spatial and temporal scales (Payette et al., 1989;Arseneault and Payette, 1997;Huang et al., 2010) MTCs from the North American boreal forest are much needed. ...
... In contrast, LTRs have only been exploited marginally in boreal North America. Tree remains in ponds and small lakes associated with permafrost dynamics in peatlands have allowed the development of a 2183-year chronology at the eastern Canadian arctic tree line (Arseneault and Payette, 1997;Payette and Delwaide, 2004;Asselin and Payette, 2006). White pine LTRs have also allowed the construction of MTCs in the northern temperate forest near the southern limit of the boreal forest (Guyette and Cole, 1999). ...
A denser global network of millennial tree ring chronologies is needed to verify whether the warming of the last century is anomalously rapid or of large amplitude in the context of the last millennium. The North American boreal forest in particular has been poorly represented in long climatic reconstructions based on tree rings, because of short tree longevity, as well as frequent and severe wildfires. In this study, we exhaustively sampled tree remains in the littoral zone of two lakes and partially sampled seven additional lakes in the northern boreal forest of eastern Canada to verify if highly replicated millennial tree ring chronologies could be developed. In total, 989 black spruce [Picea mariana (Mill.) BSP.] tree remains were sampled, of which 529 could be crossdated, providing a master chronology spanning the AD 676–1991 time period. Radiocarbon dating of a floating chronology indicates that maximum residence time of tree remains outside of sediments is at least 1500 years. Past fires halted or reduced wood inputs at least once at all but one site and thus influenced the replication of local chronologies. However, fire effects can be minimized using appropriate site selection criteria (old-growth forest on the leeward shore, sharp deepening of the littoral zone, burial of tree remains by fine sediments), such that it would be possible to develop a network of several highly and evenly replicated multi-century local chronologies. These tree ring chronologies would have desirable properties for extracting low-frequency tree growth trends and for reconstructing forest disturbance histories. Copyright © 2013 John Wiley & Sons, Ltd.
... L'impact climatique à court terme de cette éruption a entraîné la seule séquence connue de deux cernes pâles consécutifs (Filion et al., 1986;Arseneault et Payette, 1998; voir Delwaide et Filion dans cet ouvrage) chez l'épinette noire (Picea mariana (Mill.) B.S.P.), dans le nord du Québec, cernes qui ont constitué un véritable tandem marqueur pour la majorité des études dendroécologiques effectuées dans cette région (Payette et al., 1989;Arseneault et Payette, 1997a;1997b;Asselin et Payette, 2006; voir Arseneault dans cet ouvrage). La formation de plusieurs glissements rotationnels en 1818 (3 ans après l'éruption du Tambora), survenus en pleine saison de croissance (bois de réaction à la limite du bois initial et du bois final), à l'embouchure de la Grande Rivière de la Baleine (côte est de la baie d'Hudson), fut interprétée par Bégin et Filion (1988) comme une conséquence hydroclimatique de ces quelques années d'anomalie climatique (saturation en eau des sols, hausse du niveau de la rivière et érosion des berges). ...
... Allard, Caron et Bégin (1996) ont montré l'influence de la forêt sur l'activité thermokarstique d'un plateau pergélisolé situé en bordure de la baie d'Hudson, laquelle s'est surtout exercée par le biais de la quantité de neige accumulée sur les versants et dans les dépressions. Arseneault et Payette (1997a;1997b) ont montré à l'inverse, grâce à l'analyse dendroécologique détaillée de tiges d'épinette noire fossilisées dans la tourbe, l'impact de la déforestation dans une tourbière où le pergélisol s'est développé après la disparition des conifères. De toute évidence, la présence/absence de végétation haute (arbres et arbustes) est un facteur déterminant de la dynamique du pergélisol dans le Subarctique et le Bas-Arctique, de sorte que la dendrogéomorphologie périglaciaire offre un grand potentiel pour l'étude des fluctuations climatiques dans les environnements froids. ...
... This relationship is related to temperature requirements, over a period of several summers, for successful seed production, germination and seedling establishment (Zasada et al., 1992;Scott et al., 1997). Appropriate temperatures are rarely attained at treeline environments, which causes the absence of sexual regeneration (Arseneault and Payette, 1997). In addition, extremely cold episodes have caused rapid treeline recession in the Swedish Scandes (Kullman, 1996). ...
... Long periods of treeline stasis punctuated by brief periods of change seem to be a common feature of treeline dynamics (Kullman, 1979(Kullman, , 1990Slatyer and Noble, 1992;Shiyatov, 1993;Lloyd and Graumlich, 1997). The described scenario would agree with a conceptual model based on non-linear treeline response to climatic thresholds (Slatyer and Noble, 1992;Arseneault and Payette, 1997). The treeline would remain static or ascend gradually during long periods, but it could descend suddenly in response to intense disturbance, including extreme climatic events (Kullman, 1990). ...
Spatial identifcation and description of ecological boundaries is fundamental to better understanding of treeline dynamics. Ecological boundaries across two contrasting subalpine Pinus uncinata forest-alpine grassland ecotones were delineated within the Central Pyrenees (Ordesa and Tessó sites). Boundaries were delineated using an edge detection algorithm for two-dimensional data (lattice-wombling). Tree density, size-structure, growth-form, and estimated age were used to reveal spatial location of boundaries for several size and growth-form tree classes. Overlap statistics were applied to quantify spatial relationships among boundaries determined for different sets of variables. The most significant and consistent boundaries were those for structural variables at the Ordesa site. At this site, the sequential disposition of bigger and unistemmed trees descending across the ecotone produced boundaries for size-structure and growth-form variables. These boundaries were located along an ordered spatial pattern (altitudinal diagonal). At the Tessó site, there were few consistent boundaries, most of which were developed along the slope. Overlap statistics showed that boundaries at the Ordesa site were more spatially related than were those at the Tessó site. This result held when any set of variables was considered. The studied ecotones describe sharp (Ordesa site) and gradual (Tessó site) structural changes in tree populations, related to situations similar to the ecotone and ecocline concepts, respectively. The possible environmental driving factors producing these patterns are the strong winds and reduced snow cover at higher altitudes at the Ordesa site, and snow avalanches at the Tessó site. Boundary detection through time in permanent plots might be a better tool for monitoring climate-change impact in the forest--alpine grassland ecotone than the subjective location of treelines.
... Such studies are limited in time and cannot reveal the long-term effects of alternative seed sources on forest landscape recovery, which may last for centuries (Seidl et al. 2014). Paleoecological studies may capture the long, historic changes in forest succession (Arseneault and Payette 1997;Niklasson et al. 2010;Dawson et al. 2016). However, separating the effects of different seed sources remains difficult, because succession can compound the effects of remnant seed source, making the effects (amount and pattern) diminished over time. ...
Context
The rate and trajectory of forest landscape recovery after a volcanic eruption rely largely on available seed sources and patterns of seed dispersal. However, quantifying the importance of various seed sources has not been attempted.
Objectives
We demonstrate a forest landscape modeling framework for spatiotemporal reconstruction of post-volcanic-eruption forest landscape. We quantify the effects of remnant seed sources on post-eruption forest landscape recovery at tree species level in Changbai Mountain, China.
Methods
We used a factorial experiment of modeled scenarios with/without remnant seed sources and/or matrix seed sources. We applied the modeling framework to reconstruct the post-eruption forest landscape from 1710 to 2010.
Results
Remnant seed sources’ contribution to basal area and density was minimum before year 1810 and 1770, and increased to maximum at 1940 (51%) and 1840 (79%), respectively. Remnant seed sources contributed > 40% to stand age from 1740 to 1870. Remnant seed sources’ contribution to species area was minimum at 1760 for all species, while it increased to > 40% at 2010 for some late-successional species.
Conclusions
Remnant seed sources have relatively small effects in the early stage of post-volcanic-eruption forest landscape recovery. This is different from what has been reported of other disturbances such as wind and fire. Remnant seed sources facilitate late-successional species recovery, advance forest succession, and increase stand age. These findings coincide with reports of other types of disturbances. Spatiotemporal historic landscape reconstruction provides a platform to simulate seed dispersal and quantify the roles of remnant seed sources on post-disturbance landscape recovery.
... Viau and Gajewski (2009) also recorded this warming period in northern Quebec. Rolland et al. (2009) inferred a warming period between 1160 and 1360 AD on Southampton Island (Nunavut, Canada) based on chironomid assemblages.In the central Northwest Territories(Canada), chironomid-inferred temperature reconstructions have shown a warming which occurred between 1 k -0.7 cal a BP.Arseneault and Payette (1997) also observed a warming trend based on tree rings at the treeline in northwestern Quebec. When comparing our record to other proxy records (Fig. 1.4A), the period around ca. 1.5 -1 k cal a BP is clearly identified as warmer than today with a concomitant timing(PAGES 2k Consortium, 2013). ...
Un futur climat planétaire plus chaud avec une sècheresse plus marquée durant toute l’année est aujourd’hui prévus, en particulier pour les hautes latitudes de l’hémisphère nord. Ces changements environnementaux seraient marqués par des fréquences et des intensités plus importantes des perturbations naturelles, qui pourraient menacer l’intégrité de certains écosystèmes forestiers boréaux. L’une des conséquences prévisibles serait une augmentation significative de l’occurrence des feux de forêt qui est déjà et deviendrait davantage encore l’élément perturbateur majeur de ces écosystèmes. Le devenir des écosystèmes boréaux suscite donc de nombreuses interrogations et des incertitudes, ce qui amène à souligner la nécessité d’une bonne caractérisation des changements climatiques spatio-temporels et de leurs conséquences sur ces écosystèmes.Afin de maintenir ces paysages forestiers dans les limites de leur variabilité naturelle, le Ministère des Forêts, de la Faune et des Parcs du Québec (MFFP) se propose, dans un contexte d’aménagement écosystémique durable, d’établir un Registre des états de référence. Dans cette perspective, l’objectif de recherche de cette thèse était de produire des reconstitutions des paléotempératures holocènes les plus robustes possibles au sein de la pessière à mousses de l’ouest du Québec. La recherche a porté sur une approche multi-indicateurs incluant les Chironomidae et le pollen contenus dans les sédiments des deux lacs retenus.Les résultats mettent en évidence la pertinence des capsules céphaliques de Chironomidae pour reconstituer à haute résolution temporelle, les paléotempératures holocènes du mois d’août provenant du Lac Aurélie à l’aide de deux Fonctions de Transfert (FT). La FT Est canadienne s’est avérée plus en accord avec le signal régional que la FT canadienne. De surcroit, les assemblages de Chironomidae du Lac Lili ont donné une reconstitution surestimée des paléotempératures en raison de la faible profondeur de ce lac. Enfin, la reconstitution quantitative combinée (Chironomidae et pollen) des paléotempératures du mois d’août a souligné trois périodes principales: (i) 8500-4500 ans AA, identifiée comme le Maximum Thermique de l’Holocène (HTM) durant laquelle les températures d’août sont plus chaudes que les températures actuelles; (ii) 4500-1000 ans AA, marquée par le début de la période Néoglaciaire froide, où les températures oscillent autour des valeurs actuelles et (iii) les derniers 1000 ans AA, caractérisés par une diminution générale des températures. Plusieurs événements climatiques courts ont également été identifiés pour chaque période : (i) l’évènement froid de 8200 ans AA; (ii) l’évènement chaud de 4200 ans AA; (iii) la période chaude dite Romaine (RWP, 1900-1700 ans AA) ; (iv) la période froide de l’Âge Sombre (DACP, 1700-1500 ans AA) ; (v) l’Anomalie Climatique Médiévale (MCA, 1100 ans AA) et (vi) le Petit Âge Glaciaire (LIA, 500-250 ans AA). Une série d’événements froids est également observée vers 5900, 4300, 2800 et 400 ans AA, se référant probablement aux évènements « Bond ». La durée et l’amplitude observées des événements climatiques rejoignent les enregistrements régionaux, nord-américains et plus largement ceux de l’Hémisphère Nord. Par ailleurs, la synthèse soulignant les relations entre climat-végétation-feu confirme l’influence d’une instabilité climatique sur les changements des régimes des feux, principalement causée par les variations saisonnières des précipitations et de l'irrégularité des épisodes de sécheresse. Toutefois, notre reconstitution combinée des paléotempératures indique que les grands feux coïncident avec l’évènement ponctuel de 4200 ans AA et ceux survenus à partir de 2000 ans (RWP, DACP, MCA et LIA). La température estivale conjuguée à d’autres variables climatiques (sécheresse estivale, ensoleillement estival, et précipitations annuelles) joue donc un rôle important dans la variation du régime des feux à l’ouest du Québec.
... In the central Northwest Territories (Canada), chironomid-inferred temperature reconstructions have shown a warming which occurred between 1 and 0.7k cal a BP (Upiter et al., 2014). Arseneault and Payette (1997) also observed a warming trend based on tree rings at the treeline in north-western Quebec. When comparing our record to other proxy records (Fig. 4B), the period around ca. 1.5-1k cal a BP is clearly identified as warmer than today with a concomitant timing (PAGES 2k Consortium, 2013). ...
Chironomid head capsules preserved in lake sediments were used to reconstruct 8200 years of summer temperatures in the boreal forest of north‐eastern Canada. Two training sets were used derived from Canadian and Eastern Canadian transfer functions. Both models reconstructed similar climate patterns, but the Canadian model provided temperatures generally 2–3 °C lower than the Eastern Canadian model. Three main thermal changes inferred by chironomids were: (i) the Holocene Thermal Maximum, which occurred between 8 and 5k cal a BP, with temperatures generally higher than today's, maximum temperatures between 8 and 6.5k cal a BP, and an average of + 0.9 °C; (ii) the Medieval Climate Anomaly around 1.1–1.2k cal a BP with an amplitude of + 0.7 °C; and (iii) a colder period reconstructed between the 14th and 19th centuries, corresponding to the Little Ice Age, with summer temperatures on average −0.5 °C lower than the climate normal. For each of these three climatic events, the timing and the amplitude of changes were similar with other published regional, North American and Northern Hemisphere records.
... Fires are a common phenomenon in boreal forests and fire frequency is predicted to increase under climate warming (Kasischke and Turetsky 2006;Kharuk et al. 2008;Balzter et al. 2010); (but see Bergeron and Archambault 1993). Generally, fires have the ability to suppress treeline advance (Payette and Gagnon 1985;Shankman and Daly 1988), although the strength of this effect might depend on the taxa forming the treeline (Landhäusser and Wein 1993) and the actual rate of climate warming (Arseneault and Payette 1997). For example, post-fire recruitment after high-intensity fires depends on taxa-specific traits such as the ability to survive fires as trees and/or seeds or the potential for long-distance seed dispersal (Galipeau et al. 1997;Brown and Vellend 2014). ...
Tree stands in the boreal treeline ecotone are, in addition to climate change, impacted by disturbances such as fire, water-related disturbances and logging. We aim to understand how these disturbances affect growth, age structure, and spatial patterns of larch stands in the north-eastern Siberian treeline ecotone (lower Kolyma River region), an insufficiently researched region. Stand structure of Larix cajanderi Mayr was studied at seven sites impacted by disturbances. Maximum tree age ranged from 44 to 300 years. Young to medium-aged stands had, independent of disturbance type, the highest stand densities with over 4000 larch trees per ha. These sites also had the highest growth rates for tree height and stem diameter. Overall lowest stand densities were found in a polygonal field at the northern end of the study area, with larches growing in distinct “tree islands”. At all sites, saplings are significantly clustered. Differences in fire severity led to contrasting stand structures with respect to tree, recruit, and overall stand densities. While a low severity fire resulted in low-density stands with high proportions of small and young larches, high severity fires resulted in high-density stands with high proportions of big trees. At water-disturbed sites, stand structure varied between waterlogged and drained sites and latitude. These mixed effects of climate and disturbance make it difficult to predict future stand characteristics and the treeline position. © 2017, Finnish Society of Forest Science. All rights reserved.
... Similar results (peatland macrofossil and pollen records) were obtained by Roy et al. (2012Roy et al. ( , 2015 on Dog Island and further north at Okak Bay. Arseneault and Payette (1997) identified a period of rapid Picea sp. tree growth between AD 860 and 1000 in their study of tree rings. ...
We reconstructed the late Holocene vegetation of the Nain region (northern Labrador, northeastern Canada) in order to assess the influence of climate and historic land use on past shifts in forest composition. Chronostratigraphy was used in combination with macrofossil and pollen data from monoliths sampled from four peatlands. Paleoecological reconstructions produced a vegetation history spanning 4900 years for the Nain region that is largely concordant with other studies in Labrador. An initial open forest tundra phase was followed by an increase in tree cover at around 2800 cal yr BP. Paludification began ∼200 cal yr BP. A decline in Picea and its subsequent disappearance from most of the sites occurred ∼170 cal yr BP (AD 1780) in a period of relatively mild conditions during the Little Ice Age. This event was followed by the establishment of Larix laricina in the region. Local anthropogenic factors are likely responsible for these later developments, as they were not observed in other regional studies. The period around AD 1780 corresponds to the establishment of the Moravian missionaries on the Labrador coast, which increased the need for fuel and lumber. We conclude that changes in land use are reflected in the patterns of vegetation and hydrological change at the study sites.
... Further east, Hughes et al. (2006) also reported a wet shift between 900 and 750 cal. BP. Arseneault and Payette (1997) reported greater tree growth associated to warmer conditions (1000-800 cal. BP) in northern Quebec, while Lemay-Tougas (2014) observed slightly cooler and wetter conditions from a record of summer SST (1200-800 cal. ...
Dust deposition in two ombrotrophic peatlands (Baie and Ile du Havre peatland (IDH) bogs) of the Estuary and Gulf of the St. Lawrence in eastern Canada was reconstructed using elemental geochemistry. The rare earth elements (REEs) and other lithogenic element concentrations were measured by ICP-oES and Q-ICP-MS along two peat cores spanning the last 4000 years. Principal component analyses on the geochemical profiles show that REEs display the same behavior as Al, Ti, Sc, and Zr, all conservative elements, which suggests that REEs are immobile in the studied peat bogs and can be used as tracers of dust deposition. Plant macrofossils were also used to infer past environmental and humidity changes. The dust fluxes were reconstructed using the sum of REEs (ΣREE). The range of dust deposition varies from 0.2 to 3.8 g m−2 yr−1 in the Baie bog, while the IDH bog shows lower fluxes ranging between 0.1 and 1.2 g m−2 yr−1. The highest dust fluxes in the Baie bog were recorded from 1750–1000 cal. BP to 600–100 cal. BP and occur at the same time as periods of high variability in the macrofossil record (i.e. successive layers dominated by Sphagnum or Ericaceae). The timing of these events in the dust and macrofossil records also corresponds to documented cold periods. These two periods have been identified as episodes of climatic instability, which could have been caused by changes in the wind regime.
... Predictions based on the entire set of candidate models gave a minimum difference of 15 years and a maximum difference of 40 years between age at 1 m and age at root collar (Fig. 3, mean ± SE: 26.2 years ± 7.0). The greater difference between age at 1 m and total biological age of the trees in older stands could be explained by the continuous development of adventitious roots throughout their life (DesRochers and Gagnon, 1997;Krause and Morin, 2005), the progressive sinking of trees under their own weight, or growing conditions that were less favorable to tree growth in the past (Arseneault and Payette, 1997), as our data show that it took seedlings longer to reach 1 m height in the oldest stands (Fig. 4). This difference between growth rates of old vs young trees was also observed by Fantin and Morin (2002) and Gagné (2000). ...
... Fires were frequent in the region during the Holocene period (Arseneault & Payette 1997;Arseneault & Sirois 2004), but the colder and wetter conditions of the last 1000 years reduced their frequency (Filion 1984). The Rivi ere-aux-Feuilles migratory caribou (Rangifer tarandus L.) herd (RFCH) declined sharply after peaking at 931 000 AE 427 000 individuals in July 2001 (Couturier et al. 2004; Aerial migratory caribou count, Qu ebec Government, 2011, data not published). ...
Expansion of shrub species is widely reported in northern regions, although its extent varies across the landscape. In subarctic Québec (Canada), where dwarf birch (Betula glandulosa Michx) is the main species responsible for shrub expansion, little is known about the causes and consequences of this phenomenon. This study aims to identify the drivers of dwarf birch densification heterogeneity at the landscape level and to evaluate its influence on other shrub species. We used model selection with Akaike's information criterion to rank ecologically relevant models including topographic, historical and edaphic variables. The influence of dwarf birch densification was evaluated through regression analysis. We found that the best model explaining the heterogeneity in dwarf birch densification in western Nunavik includes factors related to both historical conditions (initial shrub cover, time elapsed since last wildfire) and topography (type of environment). Among these factors, only the initial shrub cover had a significant positive influence on the shrub densification. Increase in dwarf birch cover was found to negatively influence the cover of other shrub species. However, no relation was found between dwarf birch densification and other shrub species richness, suggesting that the densification did not yet lead to the exclusion of less competitive species.
... Les mégafossiles ligneux constituent des archives paléo-écologiques, géomorphologiques et géologiques de première importance (Arseneault & Filion, 2001 ;Cubizolle, 2009). Les gisements des tourbières boréales ou d'altitude (Arseneault & Payette, 1997), des petits bassin-versants torrentiels (Miramont et al., 2004) ou morainiques (Le Roy, 2012) ont été largement exploités dans un optique paléo-écologique et/ou géomorpholo- gique. Dans les archives alluviales des grandes vallées, la présence de ces bois subfossiles a également été signalée de longue date (Coupé, 1809). ...
The buried subfossil trees of Bourret are the most important deposit discovered in the Garonne valley. Radiocarbon datings of 8 of the 17 tree trunks range from 8,300 to 4,200 cal. years BP. They attest the partially reworked character of the deposits. Some of these trunks have experienced one or more phase of storage ranging from more than 4,000 to 1,000 years before they were definitively buried into the floodplain. The history of the oldest subfossil trees is the most complex. After they reach the river channel, they were stored into temporal deposits. Then erosion occurred and they were removed and transported with bedload to their definitive place. The younger subfossil tree trunks were directly bring from the riparian forest to the river channel and then stored. The radiocarbon dates distributed into three clusters around 8,200, 5,200 and 4,200 cal. years BP synchronous with Rapid Climate Changes (RCC) suggest a possible climatic control (i.e.) on the deposit formation process, which lead us to propose a theorical model of subfossil tree deposit formation.
... Fires were frequent in the region during the Holocene period (Arseneault & Payette 1997;Arseneault & Sirois 2004), but the colder and wetter conditions of the last 1000 years reduced their frequency (Filion 1984). The Rivi ere-aux-Feuilles migratory caribou (Rangifer tarandus L.) herd (RFCH) declined sharply after peaking at 931 000 AE 427 000 individuals in July 2001 (Couturier et al. 2004; Aerial migratory caribou count, Qu ebec Government, 2011, data not published). ...
NDVI analysis and repeated aerial photographs have revealed significant shrub expansion in many subarctic regions. While the recent increase in temperature is usually considered to be the main driver of this phenomenon at regional scales, very little is known about the local heterogeneity of shrub responses across the landscape.
In this study, we aim to identify the climatic factors controlling the growth of the largely distributed shrub species B etula glandulosa in three types of environments (terrace, hilltop and snowbed). We also aim to evaluate the relationship between B . glandulosa growth and the NDVI data for the Boniface River region, in north‐western Québec, where the study took place.
In the field, we harvested 180 B . glandulosa individuals (20 per site, 3 sites per type of environment). We constructed specific growth‐ring width chronologies and mean axial growth rate chronologies for each site and used them for dendroclimatic analysis (response functions). We also used linear regressions to evaluate the relative influence of dwarf birch growth on the NDVI trend.
We found a sharp increase in B . glandulosa radial growth in the 1990s followed by a sharp decreasing trend from 2002 on terraces and hilltops, while growth remained high in snowbeds. B etula glandulosa growth was positively correlated with summer temperatures on terraces and hilltops, whereas winter precipitation promoted growth on snowbeds. The NDVI trend was largely correlated to B . glandulosa growth on terraces and hilltops for the period between 1986 and 2002 (71–80% explained variance).
Synthesis . Our results suggest that topography plays a major role in B . glandulosa growth and therefore in shrub community dynamics. Because terraces and hilltops represent 70% of the land surface, the sharp B . glandulosa growth increase at these sites promoted an important overall expansion of the shrub community in the region. However, the decline in B . glandulosa growth observed after 2002 suggests that the expansion could be slowed down in the near future, therefore limiting shrub growth contribution to the regional NDVI signal.
... Other studies have found that the lichen-spruce woodland is a self-perpetuating open forest at treelinethat is, they are relicts of past populations (Payette, 1993;Payette, Filion, Gauthier, & Boutin, 1985;Payette & Morneau, 1993). This phenomena has been found on the eastern side of Hudson Bay (Arseneault & Payette, 1997;Payette & Gagnon, 1979), which lends credence to the notion that some western Hudson Bay treeline populations could be out of sync with current climatesimilar to their eastern counterparts. There is also the confounding influence of post-glacial isostatic rebound of the area, in which the coastline moves seaward at a rate of approximately 40 m century −1 (Dredge, 1992). ...
Northern forested ecosystems are predicted to change dramatically in response to climate change during the next century. The purpose of this paper was to use logistic regression analysis to model the effects of climate and topography on the spatial distribution of the northern forest around Churchill, Manitoba, Canada. Climate maps were modelled using kriging interpolation of actual climate data collected from 34 long-term monitoring sites distributed throughout the study area and topographic information was derived from commercially available digital elevation models. Five of the 18 independent variables contributed appreciably (P < 0.15) to the final logistic regression model: summer soil temperature, snow density, snow water equivalent, slope and distance from the Hudson Bay coast. Current forest distribution was predicted with 66% accuracy using the final model, and Kappa statistics indicated significant agreement between modelled and actual forest extents. Significant explanatory variables demonstrate important synergistic effects of Hudson Bay, wind, and snow in determining forest distribution. Modelled forest extents were further south than actual forest limits, which suggest treeline is not likely in equilibrium with the present climate.
... Tree-ring datasets are widely used to reconstruct histories of disturbance events and forest dynamics [1][2][3], infer large-scale patterns of climate variation (dendrochronology) [4][5][6][7][8], assess trends in tree growth and forest management options [9][10][11], and regulate wood production and wood quality by controlling site, silviculture, and genetics. Tree-ring data based on microdensity profiles are collected in stems of a set of individual trees, which contains a number of successive annual rings [12] related to the age of the tree, since a new ring is added each year. ...
Tree-ring datasets are used in a variety of circumstances, including archeology, climatology, forest ecology, and wood technology. These data are based on microdensity profiles and consist of a set of tree-ring descriptors, such as ring width or early/latewood density, measured for a set of individual trees. Because successive rings correspond to successive years, the resulting dataset is a ring variables × trees × time datacube. Multivariate statistical analyses, such as principal component analysis, have been widely used for extracting worthwhile information from ring datasets, but they typically address two-way matrices, such as ring variables × trees or ring variables × time. Here, we explore the potential of the partial triadic analysis (PTA), a multivariate method dedicated to the analysis of three-way datasets, to apprehend the space-time structure of tree-ring datasets. We analyzed a set of 11 tree-ring descriptors measured in 149 georeferenced individuals of European larch (Larix decidua Miller) during the period of 1967-2007. The processing of densitometry profiles led to a set of ring descriptors for each tree and for each year from 1967-2007. The resulting three-way data table was subjected to two distinct analyses in order to explore i) the temporal evolution of spatial structures and ii) the spatial structure of temporal dynamics. We report the presence of a spatial structure common to the different years, highlighting the inter-individual variability of the ring descriptors at the stand scale. We found a temporal trajectory common to the trees that could be separated into a high and low frequency signal, corresponding to inter-annual variations possibly related to defoliation events and a long-term trend possibly related to climate change. We conclude that PTA is a powerful tool to unravel and hierarchize the different sources of variation within tree-ring datasets.
... At the arctic tree line of Quebec-Labrador, tree density has decreased since 3000 cal. yr B.P. in the forest-tundra as a result of the combined effects of forest fires and climatic cooling (Payette & Morneau 1993;Arseneault & Payette 1997a, 1997b. In Newfoundland, this climate deterioration occurred between 3000 and 2500 cal. ...
The object of this study is to document how the Inuit on the northern coast of Labrador, Canada used terrestrial resources such as peat and wood during the Little Ice Age (LIA; A.D. 1500–1870). Paleoecological investigations consisting of pollen and macrofossil analyses were undertaken in conjunction with archaeological excavations at the Inuit winter settlement sites of Oakes Bay 1, located in the Nain region of north-central Labrador. Our data indicate that the major changes in terrestrial ecosystems of this coastal region were triggered by climate change. From ca. 5700 to 3000 cal. yr B.P., climatic conditions were relatively warm and moist. At ca. 3000 cal. yr B.P. conditions became significantly drier and colder, which corresponds to broader climatic trends during the Neoglacial period. At ca. 1000 cal. yr B.P., the reappearance of hygrophilic species and the establishment of Larix laricina provide evidence of a return to more humid conditions that in turn triggered the onset of the paludification of sandy terraces in the Dog Island region. Peat accumulation persisted after ca. 580 cal. yr B.P. likely due to the elevation of the frost table during the LIA. Elevated frost tables contributed to water saturation of the surface during the spring, creating conditions that were conducive to the preservation of organic material. Natural resources such as trees and peat were therefore readily available and more abundant during the LIA and extensively used by the Inuit for house construction and heating in the Dog Island region.
... Bien que jugée inappropriée par Fritts (1976in Cook et Kairiukstis, 1990, cette méthode a été appliquée pour la première fois à la dendroclimatologie par Briffa et al. (1992). Depuis, elle est couramment utilisée pour la standardisation des séries dendrochronologiques à des fins de reconstitution climatique (Arseneault et Payette, 1997;Esper, Cook et Schweingruber, 2002;Helama et al., 2002, Büntgen et al., 2005 puis alignées selon l'âge cambial des cernes (Figure 6b). Le premier cerne de la série, le plus proche de la moelle, est aligné sur la valeur 1 de l'abscisse (1 an d'âge), plutôt que sur l'année calendaire de sa formation. ...
... The higher water tables after 1600 cal a BP correspond to a period of warmer sea-surface temperature (SST) in the Gulf of St. Lawrence (Dhahri, 2010). In north-western Quebec, a warmer and/or moister period was also documented after $1100 cal a BP with improved tree growth (Arseneault and Payette, 1997) and low eolian activity (Filion, 1984; Filion et al., 1991). In eastern Canada, the warming trend linked to the Medieval Climate Anomaly (MCA) was interrupted by a major climate deterioration after 600 cal a BP (LIA) notably documented by permafrost growth in some peatlands of the lower North Shore (Dionne and Richard, 2006) and forest retreat in the Labrador plateau (Payette, 2007). ...
This study presents paleohydrological reconstructions from ombrotrophic peatlands (bogs) along the north shore of the Estuary and the Gulf of St. Lawrence in eastern Canada. Past water table depths were reconstructed based on testate amoebae analyses within four peatlands from two maritime ecoclimatic regions (boreal and subarctic) using a new transfer function. The comparison of multiple peat-based paleohydrological records was used to distinguish climate-driven changes from variations related to site-specific factors. Coherence between the water table reconstructions at the regional scale suggests a common climatic influence on bog paleohydrology but there are inconsistencies which also suggest an influence of non-climatic factors (e.g. internal peatland processes and feedbacks). The surface drying and increased hydrological variability after 3000 cal a BP in the studied bogs coincide with the transition from the Holocene Climatic Optimum to the Neoglacial cooling documented by proxy climate records in eastern Canada. The bogs of Havre St-Pierre have experienced major drying during the late Holocene, indicating important annual-to-centennial water deficits at the peatland surface. Regional differences in the magnitude of the hydrological fluctuations may result from distinctive climatic conditions or could indicate that bog surface wetness in the Gulf of St. Lawrence was more sensitive to past climate changes.
... Tree rings are a critically important proxy for reconstructing the high resolution climate of the past millennium because they exhibit common features of pre-cise dating to the calendar year, widespread geographical coverage, and decisive climate signals [6][7][8]. A considerable body of tree-ring research has been applied successfully regarding many aspects of treelines (e.g., climate-tree growth relationships, historical reconstructions of treeline position and structure, and the potential trajectory for treeline evolution in landscape effects of future climate change) [9][10][11][12][13][14][15][16][17][18]. Most tree-ring data at the alpine ecotone come from tree species at the treeline, whereas dwarf shrubs that are beyond the alpine treeline have attracted little research interest in dendroclimatic studies [19][20][21]. ...
Rhododendron przewalskii is an important dwarf shrub species in alpine environments of western Sichuan that offers a unique opportunity to expand current dendrochronological networks into extreme environments beyond the survival limit of trees. Our objectives in this study are to evaluate the dendroclimatological potentials of R. przewalskii and determine the major limiting climate factor for the species’ growth rings. We sampled 25 cross-sections of R. przewalskii at an elevation of 4050 m, about 150 m above treeline, on Zhegu Mountain of Miyaluo in western Sichuan of southeastern Tibetan Plateau. R. przewalskii has well-defined growth rings such that most stem sections could be cross-dated. The resulting 61-year long standard chronology (A.D. 1949–2009) was derived from 38 series from 19 cross-sections. Response analysis revealed that radial growth of the Zhegu Mountain R. przewalskii is significantly and negatively correlated to late winter temperature (January–February), mainly driven by maximum temperatures. This correlation indicates that colder daytime temperatures during late-winter lead to improved growth the following growing season. Minimum winter temperatures do not appear important for radial growth of this population. R. przewalskii ring widths are also strongly correlated to late-winter maximum temperatures over the southeastern Tibetan Plateau region. April maximum and July minimum temperatures are positively and significantly correlated to radial growth suggesting that warm April days and warm July nights promote current-season radial growth. In contrast, radial growth is only negatively correlated to April precipitation, indicating that wet soil conditions inhibit total radial increment. The main differences of R. przewalskii compared to tree species living at high-altitude nearby regions is that R. przewalskii has a weaker positive growth response to summer temperature and winter minimum temperature, but a much stronger negative response to winter temperature. Due to the strong climatic signal recorded in the growth curves of R. przewalskii, this dwarf shrub should be useful for climate–growth studies in alpine regions where no forests are present.
... In the Ordesa site, microclimate and the spatial position of seedlings with respect to krummholz become decisive for establishment. The spatial location of trees in the forest±grassland ecotone might modify external climatic constraints and become a new source of inertia (Lloyd, 1996; Arseneault and Payette, 1997 ). In addition, the change from krummholz patches to patch forest in response to improved climatic conditions positively affects regeneration (Weisberg and Baker, 1995b). ...
We describe the spatial structure of two contrasting subalpine Pinus uncinata forest-alpine grassland ecotones located in the Central Pyrenees (Ordesa and Tessó sites) as a preliminary step to infer the processes that produced their spatial patterns. All trees were mapped and measured within 4200 m2 rectangular plots parallel to the maximum slope and encompassing timberline and treeline. The spatial description of the ecotones was accomplished using several methodologies. Point pattern analysis (Ripley’s K) was first used to quantify the spatial pattern of trees using each stem x–y coordinates. Then, surface pattern analyses (Moran and Mantel spatial correlograms) were used to quantify the spatial pattern of tree characteristics across the ecotone (size, growth-form, estimated age). In the Ordesa site, krummholz individuals showed significant and positive spatial interaction with seedlings. In this site, P. uncinata individuals evolved from shrubby to vertical growth-forms abruptly, producing a steep spatial gradient. In the Tessó site, regeneration was concentrated near the treeline and the spatial gradient was gradual. Both ecotones formed ∼45 m long zones of influence along the slope based on different variables. Wind and snow avalanches seem to be the main controlling factors of the spatial pattern of trees in the sites Ordesa and Tessó, respectively. Our results point out potential different responses of treeline populations to environmental changes according to the spatial pattern.
... They are very widespread among subarctic conifers and are associated with cold summers. They are now recognized as the best markers for cross-dating spruce samples in the study region in northeastern Canada (Arseneault and Payette, 1997). However, light rings do not appear in compression wood sequences of tilted trees, even on callous margins of scoured tree sections. ...
The dates of extreme water levels of two large lakes in northern Quebec have been recorded over the last century by ice scars on shoreline trees and sequences of reaction wood in shore trees tilted by wave erosion. Ice-scar chronologies indicate high water levels in spring, whereas tree-tilting by waves is caused by summer high waters. A major increase in both the amplitude and frequency of ice floods occurred in the 1930s. No such change was indicated by the tree-tilting chronologies, but wave erosion occurred in exceptionally rainy years. According to the modern record, spring lake-level rise is due to increased snowfalls since the 1930s. However, the absence of erosional marks in a large number of years since 1930 suggests a high frequency of low-water-level years resulting from dry conditions. Intercalary years with very large numbers of marked trees (e.g., 1935) indicate that the interannual range of summer lake levels has increased since the 1930s. Increased lake-flood frequency is postulated to be related to a slower expansion of arctic anticyclones, favoring the passage of cyclonic air masses over the area and resulting in abundant snowfall in early winter. Conditions in summer are due to the rate of weakening of the anticyclones controlling the position of the arctic front in summer. This position influences the path of the cyclonic air masses, which control summer precipitation and, consequently, summer lake levels in the area.
... and goat willow, Salix caprea L., have more epicormic shoots in the surroundings of two non-ferrous smelters than in unpolluted forests (Zvereva and Kozlov, 2001). These changes in growth habit are usually attributed to wind-driven snow abrasion (Arseneault and Payette, 1997), which may be enhanced by pollutants (Alexeyev, 1990), or to an increase in light availability caused by forest decline (Zvereva and Kozlov, 2001). However, we are not aware of any study that quantitatively describes the crown architecture of trees persisting in habitats that are modified by the long-term impacts of severe pollution. ...
Although trees in polluted areas often exhibit modified growth habits, the immediate causes of changes in crown architecture and their consequences for persistence of plant populations in disturbed habitats are not well understood. We compared individuals of mountain birch, Betula pubescens ssp. czerepanovii, growing in severely disturbed habitats (industrial barrens) surrounding a nickel-copper smelter in north-western Russia, with birches growing in unpolluted habitats. They were found to have shorter heights, a shrubby growth habit, lower depth/width and surface/foliar mass ratios of the crown, higher numbers of dead branches and twisted trunks and higher branching resulting from increased numbers of long shoots and more densely spaced buds than individuals in unpolluted forests. The increased production of long shoots was enabled by their formation not only from the axillary buds of previous-year long shoots but also from the apical buds of short shoots. These latter long shoots develop in the inner part of the crown, thus increasing the crown density. Additionally, birches from industrial barrens better compensated for mechanical damage, such as trunk/shoot removal, compared to birches from unpolluted forest and mountain tundra habitats, presumably due to the larger number of buds formed annually. The specific crown architecture of these birches can be explained by the direct effects of pollution combined with changes in microclimate due to pollution-induced forest decline. The seed progenies of birches from an industrial barren reared in a benign environment produced higher numbers of long shoots than seedlings from other habitats, suggesting that adaptive changes in crown architecture are partially shaped by the selection imposed by long-term pollution impacts. Nearly spherical and compact crowns minimise the impacts of unfavourable environmental conditions on trees and are therefore adaptive. We concluded that the development of specific crown architecture allows mountain birch to dominate in habitats that are severely disturbed by pollution.
... Light rings are caused by low summer temperatures induced by delayed springs, cool summers, or early-ending autumns (Yamaguchi, Filion & Savage, 1993; Wang, Payette & Bégin, 2000 ). Two lightring chronologies were used: a solid chronology covering 690 AD to present and a floating chronology, radiocarbon dated between 178 BC and 785 AD (Lavoie & Payette, 1997; Arseneault & Payette, 1997;. Samples from the present study allowed us to fill the gap between the chronologies, yielding a 2184-y chronology of black spruce growth at tree line (Asselin et al., in prep.). ...
The basic unit of the forest-tundra landscape is a toposequence extending from a wet, forested valley to a xeric, deforested hilltop; the contact zone between these two environments being called a subarctic tree line. Dendrochronological analysis of living, dead, and subfossil black spruce, and radiocarbon dating of peat sampleswere used to reconstruct the dynamics of a subarctic tree line since its post-fire origin about 1000 y ago. Fire is not the sole disturbance to have influenced the dynamics of the toposequence. A regional-scale flooding event ca. 1120 AD killed many black spruce trees, growth of permafrost during the Little Ice Age, and its subsequent degradation in the 20th century, also had major consequences. The climate was favourable to black spruce growth between ca. 300 and 1100 AD, as evidenced by large growth rings and tree growth forms. Ring widths then decreased markedly between the 12th and 19th centuries and trees were replaced by stunted growth forms. Although climate warming during the 20th century resulted in increased ring widths, black spruces have still not produced tree growth forms, a necessary condition for viable seed production and eventual re-colonization of deforested hilltops.
L'unité de base du paysage de la toundra forestière est une toposéquence incluant un bas de pente humide et boisé et un sommet de colline sec et déboisé. La zone de contact entre ces deux milieux constitue une limite d'arbres subarctique. L'analyse dendrochronologique d'épinettes noires vivantes, mortes et subfossiles et la datation au radiocarbone d'échantillons de tourbe ont été utilisées afin de reconstituer la dynamique d'une limite d'arbres subarctique depuis son origine post-incendie il y a environ 1000 ans. Le feu n'est pas la seule perturbation à avoir affecté la dynamique de la toposéquence. Une inondation d'ampleur régionale survenue vers 1120 après J.-C. a causé la mort de plusieurs épinettes noires. La formation du pergélisol durant le Petit Âge Glaciaire et sa dégradation subséquente au XXe siècle ont également eu des conséquences majeures. Le climat a été favorable à la croissance de l'épinette noire entre environ 300 et 1100 après J.-C., tel que démontré par les cernes de croissance larges et les formes de croissance érigées. La largeur des cernes de croissance a ensuite diminué de façon appréciable entre les XIIe et XIXe siècles et les arbres ont été remplacés par des formes de croissance prostrées. Même si le réchauffement climatique du XXe siècle a résulté en une augmentation de la largeur des cernes de croissance, les épinettes noires n'ont toujours pas produit de forme de croissance érigée, une condition sine qua non à la production de graines viables et à la re-colonisation éventuelle des sommets de collines déboisés.
The boreal forest covers about one-third of the world’s forested area. Ecosystem creation and ecosystem collapse are common components of the long-term evolution of the boreal forest fueled by the ever-changing climatic and geophysical conditions since the last glacial period. After the late Pleistocene characterized by abrupt climatic changes, ecosystem building of the boreal biota prevailed from early- to mid-Holocene times (from 10,000 to 4000 years before present), including forest and peatland growth and expansion from south to north towards the Arctic tundra. Large-scale collapse of the dominant terrestrial ecosystems occurred after the mid-Holocene caused by the progressive decrease of orbitally forced solar radiation. Post-fire deforestation during the late Holocene, especially during the Little Ice Age (particularly from the end of the sixteenth century to the mid-nineteenth century), occurred in the northern part of the boreal forest with the creation of subarctic tundra covering the exposed summits and snow-patch environments on downwind side of ridges. Permafrost also expanded in wetlands with the creation of palsas and extensive peat plateaus. Human-induced post-Little Ice Age warming is unparalleled at the scale of the Holocene and is now significantly impacting the cold-prone ecosystems of the northern part of the boreal forest. Their changes and replacement by novel ecosystems adapted to the new climate are already detectable. Variable tree line advances and shrubification of forest-tundra and Arctic tundra environments across North America and Eurasia are presently the main ecosystem responses, with large-scale permafrost decay in subarctic and Arctic wetlands, to the continuous and increased warming of the twentieth and the twenty-first centuries. Fire, the principal disturbance factor of ecosystems after climate is bound to affect profoundly the integrity of most mesic and dry plant communities. It is expected that the fire regime of the northern part and southern part of the boreal forest will cause extensive changes and severe damage to northern ecosystems, more so if precipitation amounts are reduced during this century. Greater damage to the forest is anticipated if this change of the fire regime is accompanied by an increase of human impact.
Long-term climate reconstructions are frequently based on tree-ring high-resolution proxies extracted from subfossil peatland trees. Peatlands are peculiar ecosystems characterized by high moisture in the upper soil part which creates a harsh living environment for trees. The climate mostly indirectly influences tree growth determining seasonal variations in the water table level. Within this framework, the aim of this study was to investigate climate responses of trees (Pinus sylvestris L.) growing inside and outside a Southern Alpine peat bog, by using tree-ring and wood anatomical traits (e.g. tracheid number and dimension, cell-wall thickness). Our results showed differences in the xylem structure and climate signal recorded by peatland and mineral soil trees. Peatland trees were characterized by narrow rings and tracheids with thinner cell wall. Summer temperature and precipitation were the major drivers of xylem formation in peatland trees. At intra-annual level wood anatomical traits revealed a complex within-ring signal during the growing season. The multi-parameters approach together with the high-resolution gained by using tree-ring sectors allowed us to obtain new detailed information on the xylem development of peatland trees and climate drivers that influenced it.
La forêt boréale forme le plus vaste biome de la planète et assure des services économiques, sociaux et écologiques indispensables au développement de la biosphère. Les changements climatiques et l'exploitation forestière à grande échelle sont des menaces potentielles à la capacité du biome à procurer ces services. Il apparaît alors pertinent d'évaluer comment les écosystèmes de la forêt boréal réagiront à ces perturbations. L'identification des mécanismes de résilience opérant à travers les échelles de temps et d'espace est l'étape initiale vers une meilleure compréhension des réactions du système. La résilience d'un écosystème peut-être définie comme une propriété lui permettant de se réorganiser dans le temps lorsqu'il fait face à des perturbations. Dans cette synthèse, j'examine comment se manifeste la résilience des écosystèmes forestiers boréaux en fonction des échelles de temps (saison, succession, long terme) et d'espace (arbre, peuplement, paysage, biome). Pour ce faire, j'évalue la résilience des principaux écosystèmes forestiers du système boréal (pessières, pinèdes et sapinières) en réponse aux perturbations naturelles majeures que sont les feux et les épidémies de tordeuses des bourgeons de l'épinette (TBE). Les pratiques forestières (coupe, suppression des feux) et leurs interactions avec le régime des perturbations naturelles sont également étudiées afin d'évaluer leurs conséquences sur la résilience. L'étude permet d'évaluer que la résilience spatio-temporelle des écosystèmes dominés par l'épinette noire et le pin gris dépend étroitement du régime des feux et des conditions climatiques. Les sapinières sont très résiliente grâce à leur mécanisme de régénération et au contrôle des infestations exercé par la faune avienne. La diversité des groupes fonctionnels à l'intérieur de la forêt boréale est faible et la rend vulnérable aux 4 changements si des espèces disparaissent du système. L'influence anthropique a des impacts importants sur la résilience de la forêt boréale. Les perturbations en rafale comme celles formées par l'interaction entre les coupes et les perturbations naturelles érodent la résilience et peuvent entraîner le basculement du système vers des états alternatifs appauvris écologiquement. La suppression des feux pourrait provoquer des pertes importantes de résilience dans la forêt boréale exploitée. Finalement, l'utilisation de nouvelles approches de gestion, comme l'aménagement écosystémique, permettra de favoriser la résilience et fournira un tampon contre les perturbations extrêmes à venir. La résilience permettra de conserver les services procurés par la forêt boréale de même que le développement durable des systèmes socio-économiques de la biosphère.
Report on the current results of Younger Dryas and Early Holocene tree-ring and paleoenvironmental research in the Lake Ontario region of upstate New York. The research will ultimately provide a new tree-ring 14C record for the YD-EH, plus more understanding about climate change and its dynamics over that period.
Macrofossil data allowed the reconstruction of the developmental history of a paludified dunefield site in the Central St. Lawrence Lowland, Québec. Peat accumulation started between dunes around 7000 years BP after a period of free drainage reflected by podzolized sands. In interdune depressions, the vegetation developed from aquatic communities to damp forests, fen and bog communities. Between ca 7000-4100 years BP, slow peat accumulation reflected effective plant decomposition in the depressions while free drainage prevailed on the dune ridges. After 4100 years BP, wetness increased, and beaver occupied the site until ca 3800-3750 years BP. After 3750 years BP, the contrast between dunes and swales was attenuated by the vertical expansion and lateral spread of peat and the overall ombrotrophication of the site. Peat accumulation resulted from regional climatic changes toward moister conditions after 7000 (peat initiation), 4100 (beaver occupation), 3000 (larch establishment on slopes) and 1500 (ombrotrophication) years BP.
Because alpine timberline is a kind of ecotones which is highly sensitive to disturbances and environmental changes, it has become a major focus of global climate change research. Larix chinensis is a timberline trees species on Mt. Taibai in the Qinling Mountains. It is densely distributed on Mt. Taibai and plays an important role in environmental protection in the high altitude zone. The forest near timberline was divided into three transects, i. e. , below timberline, at timberline and at tree line. Regression equations between DBH(diameter at breast height)and age were established in the three transects, and ages of other trees were calculated. The results indicated that: (1) There were significant differences in most of investigated characteristics, such as average diameter at stem base, average DBH and average height, between different altitudes near timberline (P < 0. 05 ). These characteristics decreased with the increase of altitude. (2) The regression equations between age and DBH of trees in the three transects were statistically significant (P <0.05) , but the relationships between age and DBH of height of tree were poor. (3) Multi-modal age distribution was found in the three transects. We propose that limiting environmental factors, e. g. temperature, play an important role in determining the age structure of L. chinensis populations near timberline ; (4) Most individuals of different age classes had a clumped distribution at different scales. With the increase of population age, the distribution pattern had a trend from clustering to random, and the clustering intensity decreased somewhat.
Lake sediments are natural archives that record the response of a lake to both in-lake processes and catchment disturbance (over a range of timescales). The response (or lack of) of a lake to external forcing is a function of the severity of the disturbance (and its areal extent) but is also strongly mediated by catchment characteristics, such as slope and hydrological connectivity. Many studies of recent lake disturbance have focussed on anthropogenic disruption of geochemical cycles, e.g. acidification and eutrophication, which because of their “ecological” power appear to over-ride catchment filters. Lake sediments also record the variability of natural disturbance regimes themselves (fire, volcanic ash falls, species invasions, and climate) and the consequent lake response but this aspect has received much less attention. The possibility of using lake sediments to address long-term (102 – 103 yr) interactions between e.g., climate, catchment ontogeny and disturbance events (and their synergies) has not been fully explored, largely because of an over-riding emphasis on climate as a direct driver of ecological change. Traditionally, limnology has also focussed on a few key sites (sentinels) but in response to the development of landscape ecology has progressively embraced a regional approach to understanding how lakes respond to external forcing (climate) and disturbance (“lakes in the landscape”). Although the interaction of a lake with its catchment is implicit, i.e. via hydrological and nutrient loading, palaeolimnologists rarely take an explicitly spatial view of this interaction nor have they considered spatial location (i.e. response of a number of lakes within a lake district). Arguably, the inherent spatial variability of terrestrial disturbance has been ignored by palaeolimnologists, largely as a result of focussing on one core from a single lake. This paper reviews the impact of terrestrial disturbance on lakes but also argues for explicit consideration of space and location in determining the resultant temporal variability of the ecological response. The importance of within-lake spatial heterogeneity is also high-lighted (i.e. the major contribution of the littoral zone to both diversity and production). However, any attempt to determine spatially replicated (i.e. at a regional scale), holistic (i.e. whole lake) responses to disturbance will encounter considerable problems associated with dating, loss of temporal resolution and among site comparison. Despite this, it is clear that recent methodological developments in the area of biomarkers, compound specific stable isotopes coupled with progress in dating (age-models), ecological modelling and statistical analysis offer the possibility of undertaking regionally-replicated studies of lake response to natural disturbance, thereby contributing and expanding our understanding of ecosystem dynamics at a range of spatial and temporal scales.
Fire events are common disturbances in many ecosystems. The role of fire in ecosystems is encompassed in the concept of ‘fire regime’, which includes both temporal and spatial aspects as well as the physical characteristics of the fire. Vegetation recovery mechanisms determine ecosystem resilience, or the capacity to return to pre-fire conditions. In general, ecosystems submitted to frequent fires tend to be more resilient. The new environment produced by fire often determines successional trajectories. Thus, species that were not dominant in the pre-fire conditions will eventually be replaced by shadetolerant species. However, this successional scheme is not universal. In some forests, such as fire-prone Mediterranean forests, direct regeneration occurs resulting in post-fire communities that are similar to pre-fire communities. This case supports the fire regime-resilience coupled model. However, under certain conditions, this coupling is disrupted and results in decreased resilience to fire. For example, new climatic conditions, change in the species pool (planted species or invasive species moving into an area), or policies modifying fire regimes, may result in vegetation dynamics that move toward new communities.
Когда зародилась идея работ по бассейну р. Хайдун, первоначальный коллектив исполнителей был меньше, а тематический диапазон исследований значительно уже. В процессе реализации исследований появился ряд новых направлений, отчего, на наш взгляд, работа только выиграла. Объединить ученых со столь разнообразными интересами по основному профилю исследований в рамках проекта оказалось не так сложно. Сложнее было сконцентрировать их внимание на решение новых задач. На наш взгляд, это удалось.
Отметим наиболее интересные результаты проделанной работы:
Полевые исследования и полученные радиоуглеродные датировки показали хорошие возможности имитационного моделирования баланса горных ледников для расчета их планового положения как на максимум последнего оледенения, так и на отдельные отрезки голоцена.
В результате ландшафтного картографирования и последующего сравнительного анализа ландшафтных структур моренных комплексов различных фаз Исторической стадии и стадии Актру в верховьях долины р. Хайдун выявлено, что динамика оледенения в позднем голоцене на фоне незначительных короткопериодных колебаний значений метеопараметров во многом определялась позиционно-географическими особенностями и саморазвитием гляциально-нивальных и смежных с ними геосистем. Сравнительный анализ положения позднеголоценовых моренных комплексов показывает, что изначально более крупный ледник в долине р. Хайдун деградировал интенсивнее, чем ледник в долине притока. Основные причины этого– «неудачная» ориентировка долины р. Хайдун и ее значительная ширина, в связи с чем долина лучше инсолируется и продувается. Кроме этого, не обнаруживают соответствия дробность и разнообразие ландшафтных структур разновозрастных морен.
Современная высотно-поясная дифференциация растительного покрова позднеголоценовых морен северного макросклона хр. Холзун хорошо коррелирует с фазами и стадиями оледенений. В результате уменьшения масштабов каждой из последующих подвижек ледников в позднем голоцене первичное зарастание морен сменялось вековой сукцессией, вызванной поэтапным повышением положения границ высотных полос и поясов. Такого рода направленные временные изменения имеют аналоги в пространстве в виде современного растительного покрова позднеголоценовых морен различных фаз и стадий.
Во время наступления и стационирования ледников перигляциальная зона, расположенная над и перед ними, не была абсолютно лишена растительности. Благоприятные по крутизне, в первую очередь световые, склоны занимали петрофитные, тундровые и луговые группировки и сообщества, которые представляли собой банк семян для заселения освободившихся от ледника поверхностей при его отступании. В частности, водоразделы представляли собой исходную систему или модель растительного покрова, которая могла быть перенесена на освободившееся ото льда пространство, но в каждом случае процесс переноса был преломлен эволюцией ландшафтной структуры
соответствующей троговой долины.
Выявлена связь между температурой воздуха теплого периода и радиальным приростом кедра сибирского в высокогорье хребта Холзун. В целом, сопоставив нашу хронологию с результатами предшественников, можно четко выделить несколько общих периодов спада приростов, приравниваемых нами к периодам похолодания: ~1630, ~1700, ~1780 гг. С некоторой натяжкой можно упомянуть временные промежутки около1850 и 1910 гг.
Наконец, удалось обнаружить связь между подвижками ледников на Алтае и в других горных системах, в частности в Альпах.
При этом мы отдаем себе отчет в том, что на большинство поднятых в монографии вопросов не могут быть даны исчерпывающие ответы. Впрочем, авторы и не надеялись на это, понимая, насколько сложной является реконструкция палеогеографических условий даже в отдельно взятом бассейне. Более того, наши исследования в бассейне р. Хайдун поднимают ряд известных вопросов, относимых как к теории горной палеогеографии, так и региональным реконструкциям. Это и синхронность(или несинхронность) реакции ледников на глобальные и региональные изменения климатических условий, и масштабы воздействия ледника на растительный покров, и характер воздействия наступающего ледника на встречающиеся на его пути рыхлые отложения и др. Без решения этих
вопросов на конкретном региональном материале убедительной схемы эволюции ландшафтов в голоцене не построить.
When the idea to work in the Khaidunriver basin appeared, the original team of investigators was less while the thematic range ofresearch was much narrower. Already in the process of implementing a number of new researchtrends evolved, that in our opinion, made the work more interesting. To bring the scientists with diverse interests within the framework of research project was not so difficult. It was more difficult to focus their attention on new challenges. In our view, we managed to do this.
Among the most interesting results are the following.
Field studies and the radiocarbon dating obtained showed a good opportunity for simulation of mountain glaciers balance to calculate their planned position as for the maximum of the last glaciation, as for the individual periods of the Holocene.
As a result of landscape mapping and subsequent comparative analysis of landscape structures of moraine complexes of different phasesof Historical and Aktru stages in the head of the Khaidunriver valley, it was found that the dynamics of glaciation in the Late Holocene on the background of minor short-period fluctuationsof meteorological parameters were largely determined by the position, geographical features and self-development of glacial-nival and the adjacent geosystems. The comparative analysis of position of Late Holocene moraine complexes shows that the initially larger glacier in the Khaidunriver valley degraded more intensively than the one in the tributary valley. This is explained by the "bad" orientation of the Khaidunriver valley and its wide width, so the valley is betterinsolated and blown. In addition, the granularity
and a diversity of landscape structures of all-aged moraines do not show the compliance.
Modern high-altitude belt differentiation of vegetation of Late Holocene moraines on the northern slope of Kholzun ridge correlates well with the phases and stages of glaciation. Due to the reducing of the following surge of glaciers in the late Holocene the primary overgrowth of moraines was replaced by circular succession caused by a gradual elevation of boundaries of altitudinal strips and belts. Suchkind of directed temporal changes have counterparts in space in the form of the modern vegetation of Late Holocene moraines of different phases and stages.
During the advance and stabilization of glaciers, the periglacial zone, located above and in front of them, was not absolutely free of vegetation. Steep, primarily light slopes were covered by petrophytic, tundra and meadow groups and communities, which represented a seed bank for colonization the glacier free area after its receding. In particular, the watersheds were the original system or model of vegetation that could be transferred to the ice free space, but in each case the transfer process was defracted by the evolution of the landscape structure of the corresponding trough valley.
A relationship between the air temperature of the warm period and the radial growth of Siberian pine in highlands of Kholzunridge. In general,if we compare our chronology with the results of predecessors, one clearly identify several common periods of growth decrease coincided with the periods of cooling: ~ 1630, ~ 1700, ~ 1780. With some reserve, we can mention periods around the years 1850 and 1910.
Finally, it was possible to find a relationship between the surge of glaciers in the Altai and in other mountain systems,the Alps, in particular.
At the same time, we are aware of the fact that the majority of the issues raised in the monograph cannot be answered abundantly. However, the authors did not hope for it, realizing how complex the reconstruction of paleogeographic conditions is, even in a single basin. Furthermore, our studies in the Khaidunriver basin raise a number of the known issues, which refer to both the theory of mountain paleogeography and regional reconstructions. These are synchroneity (or asynchronism) of glaciers reaction to global and regional climate changes, and the scale of the glacier impact on vegetation, and the impact of advancing glacier on the loose deposits occurring on its way, etc. Without addressing these issues using the specific regional material, the real-world scheme of landscape evolution in the Holocene cannot be constructed.
A cyclic behaviour in the intensity of maritime conditions which varies with the periodic behaviour of the North Atlantic oscillation has recently become apparent in the climatic record of northern Europe. Periodic increases in oceanicity are usually viewed as having a positive effect on plant survival, as milder winters, reduction of temperature extremes, low risk of exposure to frost, and freedom from drought reduce many aspects of environmental stress. However, warmer winters in maritime environments may also have a powerful influence in creating habitats that are unfavourable for many species. The dangers of long periods of soil saturation for overwintering plants, soaking injury to germinating seeds, premature bud burst in spring, depression of treelines by increased cloud cover and high lapse rates, as well as the constant leaching of soils, are all negative aspects of the maritime environment. Woody species in which root dormancy is delayed by mild winters are particularly vulnerable to the consequences of winter flooding. Subsequent re-exposure to oxygen as water tables fall in spring can aggravate flooding damage through post-anoxic injury, leading to severe dieback of anchoring roots. Soil leaching, particularly after human disturbance, can induce nutrient deficiencies and the establishment of oligotrophic communities with reduced productivity. Podzolization and iron pan formation have in the past facilitated the processes of paludification in oceanic regions. The resulting spread of bogs and acid moorlands can further reduce the potential for productivity in both agriculture and natural plant communities. Given the probability that current climatic trends may increase the degree of oceanicity in western and northern regions of Europe, the potentially negative consequences of such a climatic change need to be considered in relation to future ecological changes and their consequences for conservation and land use.
The sampling of 1335 tree remains buried in three peatlands from subarctic Quebec allowed us to extend by more than 600 years the light ring chronology of black spruce (Picea mariana [Mill.] BSP.) from A.D. 706 to 1339. In total, 257 light rings have been identified over the A.D. 706-1675 period, among which 124 formed before A.D. 1339. Light ringsofA.D.709,713,759,772,774,810, 885, 931, 945, 973, 998, 1056, 1067, 1104, 1126, 1157, 1162, 1170, 1215, 1256, 1258 and 1309 occurred in more than 75 % of all sampled stems. Diagnostic light rings are well distributed in time, except between A.D. 811 and 872 with no light ring occurring in more than 25 % of all sampled tree subfossils. Several unfrequent light-ring years were identified because of the large number of sampled subfossil trees. In addition, light ring frequency was highly correlated between the studied sites, indicating that local site conditions have only minor influence on the formation of light ring compared to climatic conditions. Several diagnostic tree- ring sequences, including an alternation of light rings and narrow rings, are also useful for the crossdating of subfossil spruce.
Landscape ecology offers a spatially explicit perspective on the relationships between ecological patterns and processes that can be applied across a range of scales. Concepts derived from landscape ecology now permeate ecological research across most levels of ecological organization and many scales. Landscape ecology developed rapidly after ideas that originated in Europe were introduced to scientists in North America. Key research questions put forth in the early 1980s that catalyzed landscape-level research focused on the formative processes that produce spatial pattern; effects of spatial hetero- geneity on the spread of disturbance and fluxes of organisms, material, and energy; and potential applications of landscape ecology in natural resource management. This article describes the development of landscape ecology in North America, discusses current ques- tions and new insights that have emerged, and comments on future directions that are likely to produce new ecological insights. Ecology faces a broad array of challenging questions that require a plurality of approaches and creative insights. Landscape ecology should continue to push the limits of understanding of the reciprocal interactions between spatial patterns and ecological processes and seek opportunities to test the generality of its concepts across systems and scales.
We review the 1000-year climatic and environmental history of the Earth contained in various proxy records. As indicators, the proxies duly represent local climate. Questions on the validity of locality paradigm will become sharper as climatic changes on timescales of 50-100 years or longer are being pursued. This is because the thermal and dynamical constraints imposed by local geography will become increasingly important as the air-sea-land interaction timescale increases. Because the nature of the proxy climate indicators are so different, the results cannot be combined or compared into a hemispheric or global quantitative composite. However, considered as an ensemble of individual expert opinions, the assemblage of local representations of climate establishes both the Little Ice Age and Medieval Warm Period as climatic anomalies with world-wide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous intervening research efforts. Furthermore, the individual proxies can be used to address the question of whether the 20th century is the warmest of the 2nd millennium locally. Across the world, many records reveal that the 20th century is probably not the warmest nor an uniquely extreme climatic period of the last millennium, although it is clear that human activity had impacted many microscopic realms of the climate and environment.
We identified about 2,500 data sources that report impacts of point polluters on terrestrial biota and fit criteria listed
in Section 5.1.2. We think that our collection is sufficiently representative and covers at least a half of existing publications.
Although we have started a systematic review of these publications, we estimate that the completed meta-analyses (Ruotsalainen
& Kozlov 2006; Zvereva et al. 2008; Zvereva & Kozlov 2009; Roitto et al. 2009) still cover less than 25% of the available
information. The aim of Section 9.1 is to briefly evaluate knowledge of changes of landscapes, ecosystems, communities, populations
and individual organisms in impact zones of industrial enterprises in order to reveal critical shortcomings and data gaps.
Numerous exposures of Pleistocene sediments occur in the Noatak basin, which extends for 130 km along the Noatak River in northwestern Alaska. Nk-37, an extensive bluff exposure near the west end of the basin, contains a record of at least three glacial advances separated by interglacial and interstadial deposits. An ancient river-cut bluff and associated debris apron is exposed in profile through the central part of Nk-37. The debris apron contains a rich biotic record and represents part of an interglaciation that is probably assignable to marine-isotope stage 5. Pollen spectra from the lower part of the debris apron closely resemble modern samples taken from the Noatak floodplain in spruce gallery forest, and macrofossils of spruce are also present at this level. Fossil bark beetles and carpenter ants occur higher in the debris apron. Mutual Climatic Range (MCR) estimates from the fossil beetles suggest temperatures similar to or warmer than today. Together, these fossils indicate the presence of an interglacial spruce forest in the western part of the Noatak Basin, which lies about 80 km upstream of the modern limit of spruce forest.
The temporal variations in distribution pattern of Scots pines (Pinus sylvestris L.) have been used as an annual resolution record of past lake-level changes. Logs, preserved for thousands of years in bog and lakes (subfossil wood), were sampled from small lakes of the Scandinavian Mountains in west-central Sweden to construct a tree-ring chronology from an area where pines are sensitive to growth season (ie, summer) temperature. The chronology spans from AD 2006 to 4868 BC, with two minor gaps at 1600 BC and AD 900 and one larger gap at 2900 BC. The dendrochronological approach can provide a high quality long-term perspective on lake level fluctuations, which possibly can be coupled to changes in humidity. Submerged trunks were found in situ, embedded in sediments, and because trees from the earliest periods were not necessarily found at the deepest levels, this shows clearly that lake levels must have been lower than present at the time of tree-growth. The lake levels must have fluctuated, creating alternating conditions of pine regeneration and mortality. The fluctuating lake levels recorded are suggested to be a result of regional humidity increases and decreases, mainly governed by precipitation changes. Periods of lower lake levels were inferred at 4900—4800 BC, 3800—3600 BC, 3400—3250 BC, 2400—2200 BC, 2100—1800 BC, 1500—1100 BC, 900—800 BC, 400—100 BC, AD 50—300, AD 400—600, AD 900—1100, AD 1350—1500 and AD 1700—1800. Periods of higher lake levels are tentatively encountered at 3600—3400 BC, 3200—2900 BC, 2200—2100 BC, 1700—1500 BC, 1100—900 BC, 800—400 BC, 100 BC—AD 50, AD 300—400, AD 750—900, AD 1100—1250 and AD 1550—1700.
Living, dead and subfossil trees of black spiruce (Picea mariina [Mill.] BSP were used to build a 1300-year chronology based on ring width and wood density. All density sariables (maximcum, minimum, earlywood and latewood densities) among the three types of trees were simlilar, whereas ring width was significantly Higher in liviing trees thani in deacd and subfossil trees. Correlition of the indexed series from liviing and dead trees and from dead and subfossil trees that grew during the samiie periods were higher for Maximum density (r = 0.70, 0.63) and mean latewood density (r = 0.65, 0.66) than for Minimum density (r = 0.16. 0.35 and ring with (r=0.15, 0.49). respctively. Maximum density and mean latewood density were significantly correlated with all temperature variables: mean annual (January-December) and growing season (May September) temperatures, sum of degree days and frost-free days. Accordingly, Maximum and latewood density in tree-rings of spruce stem at tree-linec can be conisidered as a function Of summner-temnperature distributions and different types of trees can be combined for the reconstruction of long-term climatic trends due to theii synchronous variations.
Six tree-ring chronologies from Sweden were analyzed to assess if Scots pine (Pinus sylvestris L.) growing on peatlands are useful as annually resolved climate indicators. Also, climate–growth relationships were compared with those of pines growing on nearby dry sites to evaluate if pines from both environments may be combined to yield climate information. While temperatures in spring and summer had positive influences on peatland pine growth, precipitation responses ranged from negative in the north to positive in the south. Climate – growth response patterns differed between peatland and neighboring dry sites, where climatic information in peatland pines was weaker. Added to the direct effect of growth-year climate, is the response of peatland pines to water table variations, a function of climate over several years, likely causing annual growth to reflect a synthesis of climate over a long period. Scots pine climate – growth responses, in both environments, changed throughout the 20th century, corresponding to changes in temperature and precipitation patterns in Sweden. Decreasing growth trends since the late 1970s may be a result of late 20th century change to a warmer and wetter climate, possibly related to a strengthening of the North Atlantic Oscillation (NAO) in recent decades.
Twentieth-century elevational tree-limit (TL) and species-limit histories have been reconstructed for Pinus sylvestris, Picea abies and Betula pubescens ssp. tortuosa within a permanent belt transect in the southern Swedish Scandes. Upward TL shifts of 100-130 m between c. 1915 and 1999 appear related to a summer warming of c. 0.7°C, and increased winter temperatures. Rates of tree recruitment and individual growth have increased substantially since 1974, notably during the 1990s, apparently mainly in response to some warm summers and a sequence of exceptionally mild winters. The distributional trend during the 20th century constitutes a fundamental and unexpected break in a monotonic, century-scale TL descent throughout most of the Holocene (exemplified by Pinus sylvestris), which was particularly accentuated during the 'Little Ice Age' of the past several centuries. Assuming that the TL stabilizes at the new, higher level for several decades, its elevation will be higher than at any time during at least the past c. 4000 radiocarbon years. This recent history is relevant as one possible model of the ecological consequences of anthropogenic climate forcing, and the TL may be an appropriate and sensitive system for monitoring the ecological effects of future climate change and variability.
The 1000 yr climatic and environmental history of the Earth contained in various proxy records is reviewed. As indicators, the proxies duly represent local climate. Because each is of a different nature, the results from the proxy indicators cannot be combined into a hemispheric or global quantitative composite. However, considered as an ensemble of individual expert opinions, the assemblage of local representations of climate establishes both the Little Ice Age and Medieval Warm Period as climatic anomalies with worldwide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous intervening research efforts. Furthermore, the individual proxies can be used to address the question of whether the 20th century is the warmest of the 2nd millennium locally. Across the world, many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climatic period of the last millennium.
Paleoecological analyses were carried out from two sediment cores from lake sites at the alpine treeline on Pyhäkero mountain, the northernmost peak of an isolated mountain range, Pallastunturi-Ounastunturi, south of the polar forestline in western Finnish Lapland. Plant macrofossils, supported by pollen data, show presence of pine (Pinus sylvestris) and birch (Betula pubescens) on the top of the mountain from 9500 cal yr BP (birch) and 8300 cal yr BP (pine) up to 2000 cal yr BP (pine) and 1500 cal yr BP (birch). However, the major floristic elements of the modern alpine vegetation, as evidenced by plant macrofossils of a number of typical alpine taxa, including Salix cf. polaris, show persistence through the warmer periods of the Holocene, and indicate that the scattered pine-birch forest never completely replaced the low-alpine plant communities. The current low-alpine vegetation on the mountain top has therefore long continuity from the time of regional deglaciation. Dispersal of these alpine species took place primarily during the deglaciation phase and it is unnecessary to invoke dispersal of the arctic-alpine plants from the arctic regions of Fennoscandia during the mid- or late-Holocene to explain the origin of the alpine flora of the mountain range.
The objective of this study was to compare juvenile (012 years) height growth pattern of dominant mature trees from two virgin black spruce (Picea mariana (Mill.) BSP) forests established during the 19th century (1870) to that of young dominant black spruce seedlings newly regenerated following a 1983 fire on the same sites. The pattern was reconstructed by measuring the distance between terminal bud scars on young seedlings, and by precise counting of growth rings by cross-dating from the collar, which was identified by anatomical features, for mature trees. New seedlings growth was significantly higher than that of mature trees. Seedlings were almost twice as high as mature trees after 12 years of juvenile growth. Assuming that mature trees were dominant during their juvenile growth phase, we put forward the hypothesis that juvenile height growth of mature trees would have been affected by the combined action of spruce budworm (Choristoneura fumiferana (Clem)) and colder climatic conditions than those presently observed. Also, young seedlings juvenile height growth may have been favored by nitrogen soil enrichment along with more favourable climatic conditions.
A l’aide de macrofossiles ligneux récoltés dans les fens, situés de part et d’autre de la limite moderne des forêts, nous avons reconstitué les déplacements de la limite des forêts au cours de l’Holocène, dans la région de la rivière aux Feuilles, Nouveau-Québec (ca 58°15' N, 72° O). Cette reconstitution des fluctuations de la limite des forêts est basée sur une récolte exhaustive de plus de 700 macrofossiles ligneux, dont 67 spécimens ont été datés par la méthode du ¹⁴ C. Les mélèzes étaient présents dans la région il y a 4500 ans BP. Depuis cette date, les mélèzes ont transgressé au moins à deux reprises la limite moderne des forêts, soit entre 3500 et 2700 ans BP, où s’est faite l’expansion maximale de la limite des forêts vers le nord, et entre 2000 et 1600 ans BP, où une seconde remontée, de moins grande ampleur, a été notée. Très peu de macrofossiles datant des périodes de 2700-2400, 2100-2000, 1600-1300 ans BP ont été récoltés, ce qui suggère une régression des populations arborescentes durant ces périodes. Après 1300 ans BP, aucun déplacement de la limite des forêts vers le nord n’a été enregistré, bien qu'une expansion locale des populations de mélèzes se soit fait entre 1000 et 600 ans BP, ainsi qu’une régression entre 550 et 250 ans BP. Malgré une recherche intensive, aucun macrofossile de mélèze n'a été récolté à plus de 5 km au nord de la limite moderne des forêts.
This study examines the influence of fire-induced deforestation in well-drained conifer sites on the development of adjacent, surviving spruce stands in peatlands at the arctic tree line (northern Quebec). Tree-ring and growth-form patterns of cross-dated black spruce (Picea mariana) remains in peat were used to analyze stand responses in two contrasting topographical settings: a moderately protected valley-bottom peatland, and an exposed hilltop peatland. Dense, old-growth spruce populations occupied the studied peatlands between the 11th and 16th centuries, with stunted and erect spruce dominating the hilltop and the valley sites, respectively. Spruce continued to colonize the peatlands as long as the adjacent well-drained sites were occupied by forest trees able to attenuate the damaging effect of winter snow-drifting conditions. The surviving conifer stands exhibited similar growth responses following deforestation of the well-drained sites by wildfire in AD 1567-1568. These responses included dieback of supranival (above snow) stems and death of trees due to drowning in permafrost-induced ponds. The postfire degradation and disappearance of the conifer stands from the peatlands were the ultimate stage of a positive feedback process triggered by a modification of the snow regime at the landscape scale. Deforestation of the well-drained sites led to reduced snow accumulation and enhanced snow-drift exposure, thus inducing permafrost aggradation and drainage impediment in the nearby peatland sites. While the hilltop conifer stand responded immediately to forest exclusion in the nearby sites, ecosystem changes in the valley peatland were delayed until AD 1580-1590, at the beginning of the Little Ice Age. Our results emphasize the sensitivity and connectedness of adjoining ecosystems to fire and frost disturbances in a changing subarctic landscape mosaic.
Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experiments with white spruce branchlets. During winter, a large proportion of the snowfall was initially blown off the exposed surfaces of Hudson Bay and the coastal tundra regions, into the woodlands. As the woodlands filled up with snow in mid-winter, the rate of snow accumulation in the forest increased from double to triple the snowfall. Variations in the rate of accumulation occurred with wind speed and direction. During May, snowmelt began on exposed tundra, and occurred about three weeks later in the open forest and was characterized there by less runoff, as the water more readily permeated the thawing peat. The late snow beds are characteristic of the forest-tundra woodlands and were usually gone by mid-July. The woodlands were snow free for 1.5-2 months during the year, while the open forest was snow free for about 3 months and the tundra was largely snow free for 6 months or more. -from Authors
Current hypotheses on conifer tree stem degradation at treeline indicate the influence of frost desiccation caused by dehydration of wind-exposed needles above the snowpack because of frozen soil or wind abrasion. Here, we examine, in an exploratory study, the potential of detailed stem analysis to identify other causal factors at a subarctic treeline site using black spruce (Picea mariana (Mill.) BSP.) trees distributed along a wind-exposure gradient and showing various degrees of stem damage, from the normal, conical growth form to the mat growth form. Temporal patterns of stem development revealed a relatively integrated growth system within the tree as long as the normal arborescent form was maintained. With the gradual loss of the normal form owing to defoliation, the growth system of the damaged trees became fragmented into a wind-exposed (west–ast), horizontal component and a vertical (below–bove snowpack) component outlined by the asymmetric development of stem and foliage. Although cool growing seasons reduce tree-ring growth, frost events in July appear also influential, possibly when killing frosts occur during bud break. The impact of such abrupt events may have a long-enduring influence on radial growth, a factor much neglected in dendroclimatological studies of boreal and subarctic environments. Stem degradation may be initiated by the changing position of the snowpack line associated with variable snow precipitation during several consecutive years, thus inducing a shifting erosional zone along the stem most effective when temperatures are below average, i.e., likely the result of mechanical defoliation caused by the synergistic influence of snow and ice abrasion during blizzards and severe windchill conditions on the brittle, cold-exposed needles. Our results suggest that sustained winter defoliation at treeline has an overwhelming influence on subsequent radial growth. More dendroecological studies and experimental field work are needed to test our conclusions. Keywords: stem analysis, treeline, black spruce, winter defoliation, tree rings, subarctic.
A 250-year postfire plant chronosequence in well-drained sites at the northern limit of the Boreal Forest in the Grande rivière de la Baleine area, northern Quebec, was reconstructed from nine sites associated with the development of the lichen–spruce woodland. Most species recorded along the chronosequence reinvaded burned sites within 15 years after fire, whereas important vegetational changes occurred during the first 100 years of postfire recovery, corresponding to full development of the Cladina stellaris – spruce woodland. No vascular plant species replacement was observed during succession, whereas by contrast a well-defined lichen–bryophyte sequence occurred along five successional stages. Species diversity (Shannon index) was low 4 years after fire, but it has reached a maximum about 25 years after fire and then dropped and stabilized at a low value in old-growth woodlands dominated by black spruce (Picea mariana (Mill.) BSP) and C. stellaris. The active period of black spruce sexual regeneration spans about 20–25 years, with maximum regeneration occurring between 5 and 14 years after fire. After this period, seed regeneration is mostly sporadic and sustained layering becomes fairly common at all sites. From 100 to 250 years after fire, no significant changes were observed in vegetation structure, floristic composition, species diversity, and spruce regeneration, suggesting that lichen woodlands are selfperpetuating in absence of fire. In limiting spruce regeneration at the ground surface, the lichen cover seems to be the most important factor controlling the open structure of the lichen–spruce woodland.
FUTURE greenhouse warming is expected to be particularly pronounced in boreal regions1, and consequent changes in vegetation in these regions may in turn affect global climate2–4. It is therefore important to establish how boreal ecosystems might respond to rapid changes in climate. Here we present palaeoecological evidence for changes in terrestrial vegetation and lake characteristics during an episode of climate warming that occurred between 5,000 and 4,000 years ago at the boreal treeline in central Canada. The initial transformation — from tundra to forest-tundra on land, which coincided with increases in lake productivity, pH and ratio of inflow to evaporation — took only 150 years, which is roughly equivalent to the time period often used in modelling the response of boreal forests to climate warming5,6. The timing of the treeline advance did not coincide with the maximum in high-latitude summer insolation predicted by Milankovitch theory7, suggesting that northern Canada experienced regionally asynchronous middle-to-late Holocene shifts in the summer position of the Arctic front. Such Holocene climate events may provide a better analogue for the impact of future global change on northern ecosystems than the transition from glacial to nonglacial conditions.
Environmental changes associated with a fire-induced shift from old-growth lichen-spruce krummholz to lichen-tundra vegetation were evaluated at a tree line site in N Quebec. Tree ring and growth form patterns of black spruce Picea mariana remains in a lichen-tundra community were used to reconstruct the structure of a conifer stand at the time of the burn (around AD 1750). The prefire spruces were the last members of a long regenerative sequence leading to maintenance of a lichen-spruce stand formed after a burn c1700 yr BP. At this time postfire recruitment was most likely facilitated by favorable climatic conditions. Before the 1750 fire event the krummholz was predominantly maintained by layering, due to severe climatic conditions at least since the beginning of the Little Ice Age (AD 1580). The site was deforested by the 1750 fire, because of the limited regenerative potential of stunted spruce. Postfire spruce exclusion has been responsible for major environmental changes associated with a thinner snow cover. Along the border of the deforested site snow cover depth was controlled by living spruces, whereas postfire shrubs (mostly dwarf birch Betula glandulosa) of the lichen-tundra stand were unable to trap drifting snow. A significant decrease in thickness of the snow cover following deforestation was inferred. Results support the hypothesis that several lichen stands of the forest-tundra are postfire communities succeeding from degraded conifer stands during cold periods of the late Holocene. -from Authors
It has frequently been suggested that the period encompassing the ninth to the fourteenth centuries A.D. experienced a climate warmer than that prevailing around the turn of the twentieth century. This epoch has become known as the Medieval Warm Period, since it coincides with the Middle Ages in Europe. In this review a number of lines of evidence are considered, (including climate-sensitive tree rings, documentary sources, and montane glaciers) in order to evaluate whether it is reasonable to conclude that climate in medieval times was, indeed, warmer than the climate of more recent times. Our review indicates that for some areas of the globe (for example, Scandinavia, China, the Sierra Nevada in California, the Canadian Rockies and Tasmania), temperatures, particularly in summer, appear to have been higher during some parts of this period than those that were to prevail until the most recent decades of the twentieth century. These warmer regional episodes were not strongly synchronous. Evidence from other regions (for example, the Southeast United States, southern Europe along the Mediterranean, and parts of South America) indicates that the climate during that time was little different to that of later times, or that warming, if it occurred, was recorded at a later time than has been assumed. Taken together, the available evidence does not support a global Medieval Warm Period, although more support for such a phenomenon could be drawn from high-elevation records than from low-elevation records.
The available data exhibit significant decadal to century scale variability throughout the last millennium. A comparison of 30-year averages for various climate indices places recent decades in a longer term perspective.
A total of 64 subfossil pine samples from the pine tree limit zone or beyond the present limit of pine in Finnish Lapland have been ¹⁴C dated. Ten new dates from the Enontekiö region in northwestern Finnish Lapland are reported in the present paper. The majority of the dates from that region falls between approx. 6000 and 4000 B. P., showing that the pine area reached its maximum extent during that period. The dates from Inari and Utsjoki in the northernmost part of Finland are scattered between a few hundred and about 7000 years B.P., indicating that the retreat of the pine limit was a gradual process. Pine reached its maximum distribution during the postglacial climatic optimum, when the outermost occurrences were far beyond its present limit. The slow climatic deterioration since about 5000 B.P. has been the main reason for the decline of northernmost pine forests. The present material does not allow oscillations in the pine limit to be detected, but presumbly some variations did occur within the general long-term cooling trend.
Tree-ring data contribute to a better understanding of the nature of past climatic variations. Annual ring records several thousand years long can be constructed for a few areas, but interpretation of them requires the development of new approaches. For example, a single record of average ring width in the upper tree line environment provides a guide to past temperature fluctuations. However, comparison of this record with another, that of the arid lower forest border, from the same area permits characterization of associated precipitation and temperature anomalies that may, in turn, be linked to features of the general circulation. Other approaches that promise to be very fruitful include study of the variation of ring-width statistics through time, investigation of the physical and chemical properties of wood, and combined multivariate analysis of data for a variety of paleoclimatic indicators.
Literature on the retreat of the pine forest in northern Fennoscandia covers the period from the early 19th century onwards, ie. since the first reliable observations were made. These data are compared with radiocarbon datings of 44 samples from subfossil pines found at or beyond the present limit of pine forest. Although the dates are scattered over a wide period of time, from about 7000 BP up to recent times, there is a concentration around 4000-6000 BP, sufficient to suggest that pine forest grew beyond the present limit during the Holocene climatic optimum. The results fit well with pollen records. -from Author
From palynological studies it appears that northernmost dwarf spruces of the tundra and parts of the forest-tundra boundary may be relicts from times of prior warmth, and if felled might not regenerate. This disequilibrium may help explain the partial incongruence of modern climatic limits with the present forest edge. Seedlings established as a result of recent warming should therefore be found within the northernmost woodlands rather than in the southern tundra.
RÉSUMÉ
Les krummholz situés près de la limite des arbres dans le Nord québécois constituent généralement des formations âgées, non perturbées par les feux naturels depuis plusieurs siècles. L'analyse de l'un de ces krummholz révèle la présence d'un nombre élevé d'épinettes noires (Picea mariana (Mill.) BSP.) subfossiles qui ont pu être utilisées pour la reconstitution dendroclimatique du site. Ce krummholz empétroïde est composé d'épinettes vivantes croissant principalement au ras du sol (moins de 30 cm de hauteur) et d'épinettes mortes se présentant sous forme fruticoïde (mais de moins de 1 m de hauteur) et subarborescente (entre 1 et 3 m de hauteur). Grâce à l'interdatation des épinettes, l'histoire dendroclimatique du krummholz a été reconstituée pour les 500 dernières années. Les subfossiles subarborescents ont vécu entre la fin du XVe siècle et le début du XVIIe siècle et leur présence indique une période climatique clémente entre ca 1500 et 1570 A.D. Ces petits arbres ont donné naissance par graines à une partie importante d'une population d'épinettes fruticoïdes qui a marqué l'essentiel de la période froide du Petit Âge glaciaire (ca 1570-1880 A.D.). La majorité de ces tiges fruticoïdes sont mortes avant le récent réchauffement climatique (XXe siècle); elles ont été remplacées par les branches de la partie inférieure de ces troncs qui ont formé des marcottes empétroïdes physiologiquement indépendantes. L'absence de régénération sexuée au cours du XXe siècle est liée au manque d'épinettes de grande taille capables de produire des semences viables. Une courbe dendrochronologique couvrant les 500 dernières années a été confectionnée à partir de ces spéciments.
1 Black spruce (Picea mariana) experienced increased radial growth and stem height over the last 100 years at treeline in subarctic Quebec, suggesting warmer and snowier conditions. We tested if recent climate change also induced a shift from forest to krummholz during the Little Ice Age and stand reversion from krummholz to forest during the 1900s. Whether the shifts caused measurable displacements of the forest limit were also examined. 2 Growth forms of living and dead spruces were compared in five lichen-spruce stands located 0, 2, 4, 6 and 8 km south of the krummholz limit. The age structure of supranival shoots (stems standing above the snow cover) at each site was also determined. 3 A reversion from lichen-spruce forest to krummholz probably occurred during the mid-1800s. Since the late 1800s, the forest limit moved 4 km northward most likely in response to milder winter conditions. Krummholz changed progressively to forest as spruce height and frequency of the tree growth form increased. Thus the northward advance of the forest limit resulted from structural changes of pre-established spruces, whereas there was no evidence for a recent spruce establishment in the tundra.
This paper presents preliminary data on the reproductive capacity of the northern Canadian trees (Larix laricina, Picea glauca, and Picea mariana) in Keewatin, N.W.T. A 140-km transect of nine study sites was set up from the northern boreal forest (Kasba Lake) through the forest-tundra ecotone (northern end of Ennadai Lake). The hypothesis that the northern trees are historical (i.e., relicts), established during a warmer climate and out of phase with today's climate, is supported by (1) a pollen viability of 0%, (2) a seed viability of 0% at tree line and in the frost-tundra ecotone, (3) the lack of a buried conifer seed store for ecotonal tree stands, and (4) an almost total lack of either sexually or asexually produced tree juveniles in the ecotone.
Picea mariana predominates in even-aged stands of fire origin at the forest-tundra ecotone near Inuvik, Northwest Territories, Canada. Growth rates of P. mariana decrease with increasing latitude and density although growth rates do not change significantly along the ecotone. Field measurements of water relations, when compared with laboratory determinations of leaf water potentials vs. net photosynthesis, suggest no water stress related photosynthetic reduction for mature trees during 1976. Maximum and minimum leaf water potentials for mature trees were -0.6 and -2.1 MPa on 4 August and 1 July, respectively. Winter measurements of water relations also suggest no water stress. In contrast, 1st-yr seedlings demonstrated sensitivity to water stress with high rates of mortality at low levels of water loss. A macro-mesoclimatic gradient of 4@?C was found across the tree line (135-km study transect). Picea mariana seed production and germination are limited by this environmental gradient to @?40 km south of modern forest line. Germination and survivorship are also affected by microenvironments within a well-developed soil hummock terrain. The lower cardinal germination temperature of 15@?C was the determinant of germination timing and success. Seed production estimates suggested little effect on stand reproduction with burn intervals of 100-200 yr. Shorter or longer burn intervals are believed to reduce stand reproduction. Reduced seed longevity, complete stand destruction by wild-fire, and apparent destruction of seed in the soil restrict establishment to 1-8 yr after fire. This may cause local extermination of the species or out-of-equilibrium tree line positions under conditions of deteriorating environment.
A total of 64 subfossil pine samples from the pine tree limit zone or beyond the present limit of pine in Finnish Lapland have been 14C dated. Ten new dates from the Enontekiö region in northwestern Finnish Lapland are reported in the present paper. The majority of the dates from that region falls between approx. 6000 and 4000 B.P., showing that the pine area reached its maximum extent during that period. The dates from Inari and Utsjoki in the northernmost part of Finland are scattered between a few hundred and about 7000 years B.P., indicating that the retreat of the pine limit was a gradual process. Pine reached its maximum distribution during the postglacial climatic optimum, when the outermost occurrences were far beyond its present limit. The slow climatic deterioration since about 5000 B.P. has been the main reason for the decline of northernmost pine forests. The present material does not allow oscillations in the pine limit to be detected, but presumbly some variations did occur within the general long-term cooling trend.
Using black spruce (Picea mariana) macrofossil remains at the forest-tundra to tundra transition in norther Québec, Canada, we address the question: did the boreal forest limit shift in unison with climate change during the Holocene? Logs buried in peatlands, charcoal in the organic soil layers on well-drained sites, and dry peat underlying stunted clonal spruce were sampled across a 60-km transect south and north of the modern forest limit. Of 146 spruce logs sampled south of the modern forest limit, 34 were 14C dated between 4580 and 1540 yr BP, and 34 additional logs were cross-dated with 14C-dated spruces (3010-1540 yr BP) using diagnostic tree rings. South of the forest limit, charcoal with abundant spruce fragments was widespread in dry-mesic sites and dated between 1920 and 1140 yr BP. No spruce logs or charcoal were found in all the surveyed sites north of the forest limit. The residence time of humified peat confined to the area occupied by tundra clonal spruces ranged between 3040 and 630 yr BP. The absence of spruce macrofossils in the tundra zone, together with the geographic coincidence of the charcoal limit and forest limit, strongly suggest that the forest limit remained stable during the last 2000-3000 yr BP. Old-aged peat produced by tundra spruces is additional evidence for the stability of the forest limit. Because only spruce is able to accumulate such a thick organic layer, the 14C dates of basal peat indicate that spruce clones developed continuously in the absence of external disturbance at least over the last 3000 yr BP. The stability of the forest limit during war (≈ 2000 yr BP, medieval times, and this century) and cold (≈ 3000 and 1300 yr BP, and the Little Ice Age) periods of the late Holocene demonstrates that mechanisms allowing forest limit advance or retreat are not easily triggered by climatic change.
1 Two subfossil pine chronologies relating to the third and sixth millennia BC have been completed from sites in the north of Ireland. 2 The chronologies are replicated by trees from several sites and the more recent chronology is dated to 3451-2569 BC by cross dating with the long north of Ireland oak chronology. 3 The bog pines show higher autocorrelation and lower levels of cross-dating than commonly found in bog oaks. 4 Dendrochronological dating of bog pines may be used to throw light on the timing and interpretation of the pine decline in Britain and Ireland.
Wind exposure strongly affected needle water relations and mortality in Picea engelmannii and Abies lasiocarpa krummholz during winter at timberline in Wyoming. Windward needles had lower water contents, xylem pressure potentials, and viability, compared to leeward needles. Water content and viability increased for windward needles with height above the snow level. Below the snow surface, needle dehydration and mortality were minimal. Both absolute and relative water content were highly correlated with needle viability (a needle viability of near 50% corresponded to a water content of about 60% of dry weight). Air temperatures did not drop below -29°C in 1980/81, and needle mortality was not correlated with low nighttime needle temperatures. However, dehydration and mortality of wind-exposed needles were associated with low daytime needle temperatures and low cuticular resistance to water vapor loss. A preliminary experiment with excised shoots indicated that summer pre-disposition probably was not the primary cause of winter needle death, and that dehydration and death could have resulted from winter wind exposure alone, perhaps through cuticle abrasion.
The recent fire history of N Quebec biomes (54 000 km2) was documented by examining size and dates of 20th century wildfires using tree ring techniques. Fire frequency per biome decreased S→N from closed forest (0.7 fire/yr) to Shrub Tundra (0.4 fire/yr). Average fire size decreased south-north by 100-fold from ≃8000 ha in the N Boreal Forest to 80 ha in the Shrub Tundra, while modal fire size was <50 ha in each of the 4 biomes. Most fires (>80%) of the N Forest-Tundra and the Shrub Tundra were <100 ha, and fires >100 000 ha occurred only in the N Boreal Forest and the S Forest-Tundra. Less than 35% of all mapped fires in the Boreal Forest were <50 ha, but >30% were >1000 ha. From S→N, the fire-free interval per biome was, respectively, c2.6, 0.6, 0.6, and 2.2 yr, the Boreal Forest data being overestimated. The fire rotation period per biome, based on the percentage of burned areas during 1920-1984 (or 1930-1984 in Tundra), increased S→N by 100-fold from 100 yr in the northern Boreal Forest to 9320 yr in the Shrub Tundra. The fire rotation period around the tree line, ie 20 km south and north of the present tree line, was estimated to be >7800 yr. Biome boundaries have developed and are maintained in response to fire by the ability of spruce Picea spp to seed and regenerate. Stability of northernmost conifer sites is maintained by 1) the inability of patchy shrub and conifer cover in the northern Forest-Tundra and Shrub Tundra to carry fire and 2) failure of trees to produce viable seeds in these 2 biomes. -from Authors
We present direct evidence for the relict nature of lichenspruce woodlands thriving at treeline, using 14C-dated stands growing in the humid climate of eastern Hudson Bay in northern Québec. Black spruce, Picea mariana (Mill.) BSP, forms two groups of relict stands: a Neoglacial group of postfire origin dating between 2000 and 900 yr B.P. and a pre-Neoglacial group most likely established between 4500 and 3200 yr B.P. The latter group shows no evidence of fire (absence of charcoal under topsoil organic horizon). The stands are exceptional because they probably are the direct lineages of the primeval forests that colonized the land sometime after deglaciation. The structure of the relict lichen-spruce communities gives strong support to the current thesis that considers lichen-spruce woodland as a self-perpetuating open forest, typical of the subarctic environment.
The large 1950s fires that burned > 5,500 km² of land across a south-to-north climatic gradient in northern Quebec provide an opportunity to evaluate the role of fire in forest-tundra development on a demographic basis. The tree population density before and â 30 yr after fire was estimated by censusing trees in plots of 400 m² located in upland and lowland within four representative ecoregions of northern Quebec. The analysis of tree recruitment before and after fire, in 410 randomly selected sites along a transect crossing the upper boreal forest and forest-tundra zones, indicated that wildfires induced substantial depletion of tree populations. Taken as a whole, fires have significantly reduced the density of black spruce populations in forest-tundra uplands, but not in the lowlands. Sustained reduction of tree population density after several destructive fires appears as one of the main deforestation processed in the subarctic zone. This leads to the patchy distribution of forest stands and scattered tree populations typical of the forest-tundra biome. Comparisons with paleoecological data suggest that the impact of the 1950s fires contributed to the expansion of the forest tundra into the upper boreal forest. The ecological impact of these fires was probably similar to those fires responsible for development of the forest tundra during the Holocene. It is suggested that the fire-climate interaction should be considered in order to predict the ecological impact of warming climate on high-latitude forest ecosystems.
The environmental conditions prevailing at treeline in subarctic Quebec have been reconstructed over the past 400 yr through a comparative analysis of tree rings and growth forms of black spruce (Picea mariana (Mill.) B.S.P.). Because black spruce growth forms are closely associated with the winter environment, they are a direct response to conditions of low temperature and windblown snow abrasion affecting living tissues at the snow-air interface. The age structure of supranival shoot populations was closely associated with periods of higher stem survival in winter most likely under snowier and windless conditions. Spruce growth on slopes and in the valley revealed periods of low tree-ring growth between 1601 and 1663 and between 1700 and 1904, respectively. A long-lasting period of low radial growth 1697 and 1939 prevailed in the hilltop site. During the 20th century, spruce height increased from 0.8 to 1.6 m on slopes and in the valley, while the basal level of abrasion from windblown snow increased from 0.1 to 0.5 m, suggesting an increasing trend towards warmer and snowier conditions. Abraded spruces growing during the Little Ice Age (1570-1880) were replaced by symmetrical trees during the 20th century. Supranival skirted and whorled spruces which dominated on the hilltop site during the 16th century reverted to infranival cushion and mat growth forms during the Little Ice Age. These stunted spruces were unable to recover during the recent warming because of their inability to catch enough drifting snow to allow vertical growth.
Interspecific correlations between plant height and basal stem diameter (the allometry of height) and between stem length distal to where diameter is measured (the allometry of stem taper) were determined for a total of 265 species with self-supporting stems and for the shoots of five conifers and 15 angiosperm vine species. The allometric equations obtained for these data are proposed as a method to predict the heights of fossil plants for which basal stem diameters are either known or inferred, and to predict the missing lengths of fragmented stems based on the most proximal stem diameters observed.
IN the current debate on the magnitude of modern-day climate change,
there is a growing appreciation of the importance of long, high-resolution proxies
of past climate13. Such records provide an indication of natural
(pre-anthropogenic) climate variability, either singly at specific geographical
locations or in combination on continental and perhaps even hemispheric
scales4. There are, however, relatively few records that are well
dated, of high resolution and of verifiable fidelity in terms of climate response,
and conspicuously few that extend over a thousand years or more5.
Here we report a tree-ring-based reconstruction of mean summer temperatures over the
northern Urals since AD 914. This record shows that the mean temperature of the
twentieth century (190190) is higher than during any similar period since AD
914.
Reconstructions of climate in the Holocene rely heavily on palaeoclimatic indicators such as altitudinal and latitudinal tree-line movements1 inferred either from direct (macrofossil), or indirect (pollen), evidence of sites more or less distant from the present treelines2-8. Long-term trends in tree regeneration on well-drained sites of the forest-tundra-the transition zone between the Boreal forest and the Arctic tundra zones-may also be used as ecological indicators of Holocene climatic changes. Charcoal found in soils of treeless or forest vegetation in the transition zone indicates respectively, failure or success in the post-fire regeneration of trees; as regeneration is influenced by climate, radiocarbon-dated charcoal can be used as a record of palaeoclimate. We suggest here that the widespread occurrence of treeless sites is the result of late Holocene deforestation involving climate-fire interactions and that disjunct lichen-forest sites are the outcome of successful regeneration sometime during the last 1,000 yr. This climatically induced process is acting at the site and the species levels, south of the present tree line.
Numerous subfossil trees from the 13th, 14th, and 15th centuries sampled in the Rivière Boniface area (east of Hudson Bay) made it possible to extend to A.D. 1221 a previously published light-ring chronology. The spatial distribution of these diagnostic rings was studied along a south-north and a west–east transect in several sites extending from the northern limit of the Boreal Forest Region to the tree line. Data showed an increase in the number and frequency of light-ring years among populations along the south–north transect but little variations along the west–east transect.
Dendroclimatic analysis from a krummholz site near the tree-line, Lake Bush, northern Québec. Many krummholz sites found around the tree-line in northern Québec are composed of old- growth vegetation, free of fire disturbance for several centuries. The study of one of these krummholz indicated that the stand was dominated by prostrate black spruce (Picea mariana ( Mill. )BSP. ) ( height of less than 30 cm) living between erect-shrubby (height less than 1 m) and tree-like (minimum height ranging from 1 to 3 m) dead spruce. Using cross-dating and tree-ring characteristics, the krummholz dendrocli matic sequence has been reconstructed for the last 500 years. The tree-like subfossils lived between the end of the XVth century and the beginning of the XVIIth century, an indication of a milder climatic period between ca 1500 and 1570 A. D. These small trees acted as seed-bearers in the establishment of an important proportion of the erect-shrubby spruce population that spanned the Little Ice Age period (ca 1570-1880). Most of these individuals died before the XXth century climatic warming ; they were then replaced by low-growth layers orig inating from branches connected to the dying spruces. Sexual re generation during the XXth century did not occur at this site because of the absence of spruce able to produce viable seeds. A krummholz dendrochronological curve covering about the last 500 years was built using both living and dead spruce.
Past and recent changes in the position of the arctic tree line and spatial structure of nearby forests are reviewed. The advancing tree line in the arctic tundra in the first half of the Holocene was promoted by sexual regeneration because of warmer climate. The northernmost tree-line positions were reached in early Holocene in western North America and northern Eurasia, and during or after mid-Holocene in central and eastern North America. The position of the arctic tree line remained rather stable across the northern hemisphere from 3500 years BP to the present. General cooling during the late Holocene did not induce significant retreat of the tree line at sites not affected by catastrophic disturbances (fire) because trees and forests were able to survive through vegetative regeneration. Minor displacements deduced from dendroecological data were recorded during the last 500 years, whereas more important structural changes occurred in the nearby northernmost forests and tree groves through the shifting dominance of growth forms in preestablished tree and forest populations. The spatiotemporal pattern of tree-line and forest changes along the arctic border suggests that both equilibrium and nonequilibrium conditions prevailed during the Holocene.Key words: tree line, Arctic, climate change, paleoecology, Holocene.
The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cross-dated ring-width series is reviewed, with an emphasis on preserving very low-frequency signals potentially due to climate. In so doing, a fundamental statistical problem coined the 'segment length curse' is introduced. This 'curse' is related to the fact that the maximum wavelength of recoverable climatic information is ordinarily related to the lengths of the individual tree-ring series used to construct the millennia-long chronology. Simple experiments with sine waves are used to illustrate this fact. This is followed by more realistic experiments using a long bristlecone pine series that is randomly cut into a number of 1000-, 500- and 200-year segments and standardized using three very conservative methods. When compared against the original, uncut series, the resulting 'chronologies' show the effects of segment length even when the most conservative and noncommittal method of tree-ring standardiza tion is applied (i.e., a horizontal line through the mean). Alternative schemes of chronology development are described that seek to exorcise the segment length curse. While they show some promise, none is universal in its applicability and this problem still remains largely unsolved.
The climate of the Northern Hemisphere changed during recent centuries, as shown by the Little Ice Age episode1 and the warming trend of the past 100 years2,3. The ecological impacts of these changes have yet to be evaluated in several terrestrial ecosystems, incorporating direct evidence such as detailed botanical field observations4. We report here results of the analysis of a ~600-year response of a lichen–spruce woodland to this long-term trend, which are thought to be the first extensive illustration of these impacts in the Subarctic. It suggests that tree-line vegetation is in a dynamic equilibrium with climate in the absence of other external disturbances; this is emphasized by spruce reaction through phenotypic adaptation—a shift from stunted individuals (krummholz) to normal trees (forest)—and differential regeneration. This study produces evidence that marginal northern forests can persist through time and that successional processes, in the absence of fire, perpetuate the original lichen–spruce facies. The longest tree-ring chronology (AD 1398–1982) yet available in eastern North America was constructed from living and dead spruces found in the lichen woodland of our study.
KNOWLEDGE of the vegetation response to climate change is necessary to assess and predict realistic ecosystem development in the anticipated, CO2-induced warmer world, particularly at high latitudes where greater warming is expected1–3. Reconstruction of vegetation development over the past 1,000 years may be helpful in this respect, because this period was characterized by contrasting climatic conditions4–9. Here we report the reconstruction of wind-exposed, tree-line vegetation associated with long-term climate change in northern Canada, using tree-ring and growth-form analyses of spruce subfossils. Three major types of growth form within the exposed, but stable, lichen–spruce community successively predominated in response to climate forcing: high krummholz (dwarf spruce, <2-m high) with scarce small (<2-m high) trees (AD 1305–1435, cool period), trees >2–3 m high) and high krummholz (AD 1435–1570, warm period) and low krummholz ( ≲50 cm) (little ice age to present: AD 1570 onwards, cold period and present climate, respectively). Whereas the expansion of a marginal lichen–spruce woodland climaxed during the late Middle Ages (AD 1435–1570), present development of a low-krummholz vegetation at these sites seems to be out of phase with the twentieth century warming. This suggests that ecosystem recovery to global warming is not straightforward, depending on the nature of vegetation structure present at the time climate change occurred. The implications of such ecosystem resilience for the detection and monitoring of the expected CO2-induced warming is discussed, particularly for the climate-sensitive arctic and subarctic regions.
Oxygen isotope analysis of anew ice core from the crest of the Greenland ice sheet reveals a climatic record of the past 1,420 years. Climatic changes of medium frequencies are in phase with corresponding changes in Iceland and England, whilst long-term changes at mid Atlantic longitudes are out of phase with Europe and North America. Reconciliation with Norse history suggests a strong climatic impact, and a parallel is drawn to the present critical situation of the human society.
Statistical comparisons between tree-ring width sequences and climatic records provide a means of identifying climatic limitations on tree growth and allow the reconstruction of past climates. This information is especially important in the North American sub-Arctic where climate–growth relationships are poorly understood and instrumental weather records are very short, typically less than 75 yr. Dendroclimatic reconstructions before 1900 are essential for estimating a realistic range of high latitude climatic variation, because twentieth century climate is now thought to be somewhat anomalous1. While some dendroclimatic studies have been carried out in the sub-Arctic2–8, none has made full use of current multivariate statistical techniques. This study, as part of a multidisciplinary investigation of treeline fluctuations in the Brooks Range of Alaska, uses ring-width sequences of white spruce (Picea glauca [Moench] Voss) to define climatic limitations on radial growth at treeline and to reconstruct past climatic variables for Fairbanks, Alaska. Multiple regression techniques were used to analyse climate–growth relationships and the results suggest important modifications to the theory that growth at treeline is limited primarily by summer temperature2,8,9 . For example, we have found that radial growth is directly related to summer and autumn temperature and precipitation during certain months, but is inversely related to winter–spring temperature. The same ring-width sequences were used to reconstruct average May–July temperature at Fairbanks, Alaska, for the period 1829–1930. This reconstruction, which more than doubles the length of the existing climatic record, was verified by statistically comparing it to independent instrumental data. It indicates that the Fairbanks area has been warmer in the twentieth century than in the nineteenth century during these months. As the longest annual record of temperature from northern Alaska, this reconstruction provides quantitative evidence for a climatic warming similar to that occurring throughout large portions of the Northern Hemisphere during the past 100 yr (refs 10–12).
Kullman, Leif 1987 03 01: Sequences of Holocene forest history in the Scandes, inferred from megafossil Pinus sylvestris. Boreas, Vol. 16, pp. 21–26. Oslo. ISSN 0300–9483.
Altogether 54 14C-datings of megafossil Pinus sylvestris L. from above the present tree-limit in the southern Swedish Scandes were evaluated. The samples were discussed in relation to the present-day tree-limit of pine. Pine reached almost 200 m higher than at present shortly after deglaciation and remained at very high altitudes until at least 4,000–3,500 14C years B.P. The conclusion was that the precise biological and climatological interpretation of these regionally sampled data is still obscure. It is impossible to judge whether clusters of megafossil pine represent periods relatively favourable for pine growth and reproduction or merely periods advantageous for production of dead wood and its long-term preservation. In terms of climate, these alternatives reasonably implicate widely different conditions.
Quantitative estimates of 1480 years of summer temperatures in northern Fennoscandia have previously been derived from continuous treering records from northern Sweden. Here we show the results of spectral analyses of these data. Only a few peaks in the spectra are consistently significant when the data are analyzed over a number of sub-periods. Relatively timestable peaks are apparent at periods of 2.1, 2.5, 3.1, 3.6, 4.8, 32–33 and for a range between 55–100 years. These results offer no strong evidence for solar-related forcing of summer temperatures in these regions. Our previously published reconstruction was limited in its ability to represent long-timescale temperature change because of the method used to standardize the original tree-ring data. Here we employ an alternative standardization technique which enables us to capture temperature change on longer timescales. Considerable variance is now reconstructed on timescales of several centuries. In comparison with modern normals (1951–70) generally extended periods when cool conditions prevailed, prior to the start of the instrumental record, include 500–700, 790–870, 1110–1150, 1190–1360, 1570–1750 (A.D.) with the most significant cold troughs centred on about 660, 800, 1140, 1580–1620 and 1640. Predominantly warm conditions occurred in 720–790, 870–1110 and 1360–1570 with peaks of warmth around 750, 930, 990, 1060, 1090, 1160, 1410, 1430, 1760 and 1820.
Microclimate and photosynthesis of krummholz mat growth forms of Picea engelmanii (Parry) and Abies lasiocarpa [Hook.] Nutt. were investigated to determine structural features which may aid survival in alpine environments. The structure of krummholz mats was described in terms of the vertical distribution of leaf area index and leaf area density, which exceeded 50 m-1 (based on total leaf surface area) near the canopy surface and approached zero below 30 cm from the surface in both species. Photosynthetic photon flux density (PPFD, 0.4–0.7 m wavelengths) and wind decreased by an average of 6 and 50-fold, respectively, between 1 m above and 10 cm below mat surfaces in both species. Needle temperatures on a P. engelmannii krummholz mat during July averaged about 2C above air temperature during the day, with a maximum overtemperature of greater than 20C above T
air during one sunlit period. At night, needle temperatures averaged 3–4C below T
air.Net photosynthesis in year-old P. engelmannii shoots reached a maximum at 15–20C during July and August. Surface shoots were light saturated at near 1200 moles m-2s-1 PPFD, and had higher photosynthetic rates than subsurface, predominantly shaded shoots above 800 moles m-2s-1. Shade shoots had higher photosynthetic rates when PPFD was below 600 moles m-2s-1, and at 250 moles m-2s-1 shade shoots maintained about 50% of the net photosynthetic rate of sun shoots at light saturation. Shade shoots appeared capable of benefitting photosynthetically from elevated temperatures within krummholz mats despite relatively low light levels. Especially rapid photosynthesis may occur when canopy needles are illuminated by sunflecks and needle temperatures rise by 10 C or more.Snow cover appears crucial for the survival of needles during winter. Snow accumulated within krummholz needle canopies before the sub-canopy zone of unfoliated branches became filled. The concentrated needle growth in the krummholz canopy captured snow in early autumn without support from ground-level snowpack. Early snow cover in both species prevented cuticle abrasion and resulted in high winter needle water contents and viabilities for subsurface compared to surface needles which became abraded, severely dehydrated, and had high mortality between December and February, especially on windward sides of shoots.Extremely high concentrations of needles within krummholz mat canopies created an aerodynamic structure which elevated needle temperatures to more optimal photosynthetic levels in summer and resulted in more efficient snow accumulation in winter. These factors appear crucial for winter needle survival. Thus, krummholz mats appear to be an important adaptation in growth form which provides survival benefits in both summer and winter.
The influence of climate on the population dynamics of trees must be inferred from indirect sources of information because the long lifespans of trees preclude direct observation of population growth and decline. Important insights about these processes come from 1) observations of the life histories and ecologies of trees in contemporary forests, 2) evidence of recent treeline movements in remote areas unaffected by human disturbance, and 3) results of experiments performed on forest simulation models. Each line of evidence indicates that tree population responses are influenced by many factors: including lifespans, seed productivity and dispersibility, phenotypic plasticity, genetic variability, competition, and disturbance. Some population characteristics should allow rapid changes in population sizes, while others should confer stability in times of environmental fluctuation. Interactions between controlling factors should result in a wide array of possible responses to climatic change. Interpretations of late-Quaternary forest dynamics must be based on an understanding of the biological processes involved in population responses to environmental variations.
The palynological records of arctic tree-lines in North America give clear evidence of large scale northward displacement of the forest limit during the early Holocene. However, small scale or local changes in forests that occurred during the mid or late Holocene have been more difficult to detect using pollen analysis. A grid of sites from the region to the east of the Makenzie Delta, N.W.T., provides a good temporal and spatial record of tree-line (forest) movements. Detailed pollen and macrofossil analyses at thre sites, Reindeer Lake, Sleet lake and Bluffer's Pingo, which lie 50, 75, and 100 km north of the modern forest limit, respectively, provide a detailed paleoecological record. The evidence indicates that the northward displacement of forests in the early Holocene form 10,000 to 8400 yr BP was not simply a northward shift of trees but that a complex pattern of vegetation developed with white spruce populations growing north of open poplar stands. Open woodlands with black spruce grew as far north as Sleet Lake from 8400 to 3500 yr BP. These woodlands gradually retreated to just south of Reindeer Lake during the late Holocene.
Data on glacier, tree-line, tree-ring, pollen, and ice-core variations in North America, Greenland, and Europe during the last 2000 yr (up to A.D. 1800) are compared in detail on the century time scale. Only data that may be indicative of summer temperature changes are included, since these comprise most of the available paleoclimatic information, although some variations (especially of glaciers) may have been in response to precipitation changes instead. Radiocarbon dates and 14C-dated records are converted to calendar (dendrochronological) years using the calibration of M. Stuiver (1982, Radiocarbon24, 1–26). Despite the basic uncertainties in dating, interpretation, response times, and “noise level” of proxy climatic data, there is at times good agreement among different kinds of evidence from within a region to suggest an episode of generally warmer or cooler summers. Three previously identified episodes find expression in records from all of the regions considered: the “Little Ice Age” of the last few centuries, a “Medieval Warm Period” around the 12th century A.D., and an earlier cold period some time between the 8th and 10th centuries. However, the timing of minima and maxima within these episodes seems to have varied from region to region (although the evidence is consistent within regions). In the 15th century, summers were warm in the eastern Canadian Arctic and southern Greenland while there was a cold episode in Europe and western North America.
Tree-ring data from subalpine conifers in the southern Sierra Nevada were used to reconstruct temperature and precipitation back to A.D. 800. Tree growth of foxtail pine (Pinus balfouriana) and western juniper (Juniperus occidentalis ssp. australis) is influenced by nonlinear interactions between summer temperature and winter precipitation. Reconstruction of the separate histories of temperature and precipitation is feasible by explicitly modeling species and site differences in climatic response using response surfaces. The summer temperature reconstruction shows fluctuations on centennial and longer time scales including a period with temperatures exceeding late 20th-century values from ca. 1100 to 1375 A.D., corresponding to the Medieval Warm Period identified in other proxy data sources, and a period of cold temperatures from ca. 1450 to 1850, corresponding to the Little Ice Age. Precipitation variation is dominated by shorter period, decadal-scale oscillations. The long-term record presented here indicates that the 20th century is anomalous with respect to precipitation variation. A tabulation of 20- and 50-yr means indicates that precipitation equaling or exceeding 20th-century levels occurred infrequently in the 1000+-yr record.
Evidence has been accumulating in many fields of investigation pointing to a notably warm climate in many parts of the world, that lasted a few centuries around A.D. 1000–1200, and was followed by a decline of temperature levels till between 1500 and 1700 the coldest phase since the last ice age occurred. There has been some controversy as to whether this climatic variation was geeat enough to be significant in connection with the balance of Nature or the economy of Man. It is time to marshal the evidence and attempt a numerical assessment of the climatic values involved in some area for which there are enough data to permit it. This is attempted here and provides an illustration of how data from the realms of botany, historical document research and meteorology may be used to confirm, correct and amplify each other.Changes of prevailing temperature and rainfall in England between periods of 50–150 years duration around 1200 and around 1600 are found which, on all the evidence at present available, probably amounted to 1.2–1.4°C and 10% respectively. Changes in some reasons of the year may have exceeded these ranges of the annual mean. The changes indicated are small enough to account for earlier impressions in some quarters that there has been no significant change of climate in the last 2,500 years, yet they are big enough to be surprising in terms of previous meteorological knowledge and undoubtedly upsetting for the human economies of those times (and perhaps of any time).It is by quantitative assessment of climatic values for epochs, such as the one here treated, for which some sort of extreme character is indicated by reliably dated evidence, that palaeoclimatology may hope to progress. The time-scale of the epoch here studied appears similar to that of several earluer climatic oscillations of known importance to the history of the European fauna and flora, which for obvious reasons cannot be submitted to equally close meteorological analysis. It is essential that the geographical distribution of climatic values arrived at, should be examined (as is done here) for consistency and the possibility of a reasonable interpretation in terms of the condition of the wind and ocean circulations.
Light rings are characterized by one or a very few latewood-cell layers, an indication of shortened growing seasons, and are particularly frequent in black spruce (Picea mariana (Mill.) BSP) at the treeline in Québec. The construction of a light-ring chronology spanning the period AD 1398–1982 showed that the highest frequency (>25%) of light rings among 160 trees and krummholz occurred in 1593, 1620, 1634, 1784, 1816, 1817, 1853, 1969, and 1972. These diagnostic rings be a useful cross-dating tool for dendroecologists working with living and dead krummholz with a low-growth variability. About two-thirds of the 65 light-ring years coincide with years (or triads) of major volcanic eruptions. The climatic conditions (low temperature) occurring at the end of the growing season, in part induced by the climatic effect of volcanism, seem to initiate light rings.