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Development of black spruce growth forms at treeline
Abstract
Most treeline populations in northeastern Canada are monospecific stands of black spruce (Picea mariana [Mill.] B.S.P.), a hardy, cold-tolerant species able to withstand harsh climatic conditions under different growth forms. In the forest tundra, black spruce thrives in protected areas and exhibits a normal arborescent growth form, but in exposed sites, upright stems are damaged above the snowpack by snow abrasion and wind. In this study, the development of damaged growth forms was examined in a moderately exposed habitat. Five developmental stages were identified and described using detailed stem analysis of 13 spruce trees. Four different types of damaged growth forms were identified according to variations in supra-nival (above snow) stem height and number. At the site scale, the age structure of supra-nival shoots, based on a larger sample of 256 stems, was unimodal, suggesting a synchronous development of the spruce stand in which 46% of the shoots were initiated during the 1960s and 1970s. Subfossil trunks on the ground were all depressed trees, indicating that the former vegetation was a krummholz, not a forest. This indicates the recent development of the small-tree stand above the snowpack, probably triggered by recent milder conditions associated with snowier winters in the last decades.
... They can be described along the following two pattern dimensions: a) Abruptness of the decline in maximum woody-plant height across the ecotone (Fig. 2b, x-axis): most abrupt if tall trees change directly to low-stature alpine vegetation or krummholz, most gradual if woody plants slowly decrease in size across the ecotone. b) Deformation of the tree shape (Fig. 2b, (Pereg and Payette 1998). A species turnover into shrubs does not constitute deformation, but if a belt of tall shrub species exists that changes into small prostrate shrubs (i.e. ...
... Abrupt height declines from tall forest into alpine vegetation or krummholz indicate that positive feedback occurs, either between tree presence and seedling survival or between tree size and biomass maintenance. In the first case, this implies a positive feedback switch (Wilson and Agnew 1992) and a high temporal stability of the treeline (Armand 1992, Bader et al. 2008, while in the second case the escape of krummholz to upright trees is much more likely to occur and can lead to a fast treeline change through a rapid vertical growth of already established individuals (Pereg and Payette 1998, Gamache and Payette 2004, Holtmeier and Broll 2005, Devi et al. 2008, Kullman and Oberg 2009. Similar to island shape, the directionality (anisotropy) of deformations can be an indication of the causal (secondlevel) processes. ...
... In most cases, a detailed analysis of damage and regrowth patterns, including not only directionality but also e.g. the heights above the ground where damage starts and ends (possibly related to e.g. snow-depths or the reach of local browsers), will give strong indications of the second-level processes at hand (Pereg and Payette 1998). ...
Globally, treeline ecotones vary from abrupt lines to extended zones of increasingly small, stunted and/or dispersed trees. These spatial patterns contain information about the processes that control treeline dynamics. Describing these patterns consistently along ecologically meaningful dimensions is needed for generalizing hypotheses and knowledge about controlling processes and expected treeline shifts globally. However, existing spatial categorizations of treelines are very loosely defined, leading to ambiguities in their use and interpretation. To help better understand treeline‐forming processes, we present a new framework for describing alpine treeline ecotones, focusing on hillside‐scale patterns, using pattern dimensions with distinct indicative values: 1) the spatial pattern in the x‐y plane: a) decline in tree cover, and b) change in the level of clustering. Variation along these dimensions results in more or less ‘discrete' or ‘diffuse' treelines with or without islands. These patterns mainly indicate demographic processes: establishment and mortality. 2) Changes in tree stature: a) decline in tree height, and b) change in tree shape. Variation along these dimensions results in more or less ‘abrupt' or ‘gradual' treelines with or without the formation of environmental krummholz. These patterns mainly indicate growth and dieback processes.
Additionally, tree population structure can help distinguish alternative hypotheses about pattern formation, while analysing the functional composition of the ecotonal vegetation is essential to understand community‐level processes, controlled by species‐specific demographic processes.
Our graphical representation of this framework can be used to place any treeline pattern in the proposed multi‐dimensional space to guide hypotheses on underlying processes and associated dynamics. To quantify the dimensions and facilitate comparative research, we advocate a joint effort in gathering and analysing spatial patterns from treelines globally. The improved recognition of treeline patterns should allow more effective comparative research and monitoring and advance our understanding of treeline‐forming processes and vegetation dynamics in response to climate warming.
... Trees there can be exposed to heavy snowpacks that affect growing season length and ring-width patterns (Körner, 2012;Peterson & Peterson, 2001). In addition, heavy snowpacks, ice abrasion, and rime-ice loading can damage plant tissue and cause stem or branch breakage, which in turn can affect tree growth rates, architecture, and wood morphology (Laberge et al., 2001;Pereg & Payette, 1998). Secular changes in tree architecture and growth rates of treeline trees have been used to reconstruct changing snow conditions and winter storminess during the Little Ice Age in Eastern Canada and elsewhere (Kajimoto et al., 2002;Laberge et al., 2001;Lavoie & Payette, 1992;Payette et al., 1989;Payette et al., 1994;Pereg & Payette, 1998;Tremblay & Bégin, 2005;Wooldridge et al., 1996). ...
... In addition, heavy snowpacks, ice abrasion, and rime-ice loading can damage plant tissue and cause stem or branch breakage, which in turn can affect tree growth rates, architecture, and wood morphology (Laberge et al., 2001;Pereg & Payette, 1998). Secular changes in tree architecture and growth rates of treeline trees have been used to reconstruct changing snow conditions and winter storminess during the Little Ice Age in Eastern Canada and elsewhere (Kajimoto et al., 2002;Laberge et al., 2001;Lavoie & Payette, 1992;Payette et al., 1989;Payette et al., 1994;Pereg & Payette, 1998;Tremblay & Bégin, 2005;Wooldridge et al., 1996). ...
... The umbel-style architecture of taller trees (>3 m in height) indicates repeated, height-dependent pruning of their canopies by the combined effects of wind and ice-crystal scouring, in addition to breakage by loading under rime ice and wet snow ( Figure 2a). Another common growth form is a small-to medium-sized tree with numerous replacement leaders indicative of height-dependent, leader mortality ( Figure 2b; Hibbs, 1981;Payette et al., 1989;Pereg & Payette, 1998). Elfin or krummholz trees (<3 m in height) growing on and near exposed ridgelines possess "snow skirts" of undamaged branches and foliage near their bases that indicate the presence of a protective snowpack ( Figure 3; Payette et al., 1989). ...
Winter is a critical season for land‐surface feedbacks and ecosystem processes; however, most high‐latitude paleo‐environmental reconstructions are blind to cold season conditions. Here we introduce a winter‐sensitive, paleo‐proxy record that is based on the relative frequency of tangential rows of traumatic resin ducts (TRDs) in the annual growth rings of mountain hemlocks (Tsuga mertensiana) growing near treeline in Southeast Alaska. Hemlocks produce a row of TRDs in the earlywood portion of their annual rings in response to cambial damage incurred during winter. Multidecadal bouts of TRD production were followed by growth‐leader replacement, reaction wood formation, and divergence in radial growth between storm‐damaged trees and less exposed mountain hemlock forests. These patterns are consistent with TRDs being a response to tree damage caused by ice and snowstorms, a conclusion supported by the krummholz tree architecture at these sites. This relationship is further corroborated by significant correlations between our TRD record and the strength of the wintertime Aleutian Low (AL) pressure system that is linked to tree‐damaging agents like wind, precipitation, and ice storm strength in Southeast Alaska. The combined TRD/krummholz architecture record indicates that abrupt shifts between strong and weak AL phases occurred every several decades since CE 1700 and that the 1800s had relatively long AL phases with heavy snowpacks. In addition to describing the magnitude and tempo of wintertime climate change in Northwestern North America, these results suggest that North Pacific Decadal Variability underlies the long‐term dynamics of treeline ecosystems along the northeast Pacific coast.
... Black spruce growing at their northern range limit are extremely sensitive to climatic variations and develop different growth forms depending on summer growth conditions and winter harshness (Bégin 1991;Lavoie and Payette 1992;Pereg and Payette 1998). The morphological development of black spruce follows Rauh's architectural model (Bégin and Filion 1999). ...
... (Bégin and Filion 1999), causing the regression from erect (tree) to shrubby (krummholz) growth forms ( Figure 3a,b,d,f). Eroded black spruce trees can survive for centuries as stunted individuals with a reduced leaf mass (Pereg and Payette 1998). Krummholz can also develop sub-arborescent growth forms (multi-stemmed or single-stemmed) when winter conditions become milder. ...
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.
... There is also at least one example of the formation of a krummholz zone from a forest of upright trees (LaMarche & Mooney, 1972). Several authors (e.g., Holtmeier, 2009;Pereg & Payette, 1998;Smith, Germino, Hancock, & Johnson, 2003) have described processes that allow 'flagged' above krummholz mats, establishment of upright trees in the krummholz zone will also require climatic change that reduces wind and snow transport which cause winter damage. ...
... Most of the natural emergent stems we sampled were 8-15 years old, although it is not clear from our analyses whether this represents directional change due to an increasingly suitable climate (e.g., see Hessl & Baker, 1997) or if emergent stem establishment is episodic in general (e.g., see Millar et al., 2004). Pereg and Payette (1998) found that Picea mariana krummholz at arctic treelines establish upright stems above the winter snowpack, but that these stems eventually degrade because of chronic exposure to wind-transported snow. ...
Understanding the processes that control alpine treelines, the elevational limits of tree growth forms, has been a central question in ecology and is growing in importance with concern over climate change. Cool summer air temperatures are currently thought to be the ultimate limiter of upright tree growth at alpine treelines globally. However, winter damage has long been recognized as a shaping force near alpine treelines. Low‐growing krummholz growth forms provide an opportunity to test hypotheses about the controls of upright growth in environments above current treelines.
To distinguish between effects of growing season temperature, winter damage and their interaction on preventing upright growth in krummholz, we conducted a field experiment on krummholz growth forms of Pinus albicaulis over the summer and winter of 2015–2016 at 10 mountain top sites in the Tobacco Root Mountains, Montana, USA. We experimentally manipulated four factors using a fully crossed design: shoot position (natural low position in the krummholz mat vs. propped up above the krummholz mat), summer warming (warming chamber vs. ambient), winter exposure (shelter cage vs. exposed), and elevation position (local high vs. low krummholz limits). We also conducted an observational study of the climatic conditions associated with recent natural emergent stem establishment from krummholz.
Experimentally propped shoots that were exposed in winter experienced the highest mortality (10%–50%), while propped shoots in shelter cages and shoots located within the krummholz mat, whether caged or not, had low mortality (0%–10%). Summer warming had little influence on shoot mortality. Surviving mat shoots had marginally higher growth rates than surviving propped shoots during the early growing season after treatments were established. Natural emergent stem establishment was associated with warmer than average summer temperatures, but also warmer winter temperatures, lower winter wind speeds, and lower snowpack.
Synthesis. Our results suggest winter damage plays a more important role than does growing season temperature in maintaining the krummholz growth form. While warming may increase opportunities for emergent shoot establishment above krummholz mats, establishment of upright trees in the krummholz zone will also require climatic change that reduces wind and snow transport which cause winter damage.
... Approaching the tree limit, height growth is generally more impaired than diameter growth. Thus, compact growth forms, particularly in mature trees, are very common (e.g., [27][28][29]) ( Figure 3). The uppermost krummholz, however, often displays mat-like growth. ...
... Like chamaephytes (dwarf shrubs), low mats profit from the relatively warm, calm microclimate near the ground and Approaching the tree limit, height growth is generally more impaired than diameter growth. Thus, compact growth forms, particularly in mature trees, are very common (e.g., [27][28][29]) ( Figure 3). The uppermost krummholz, however, often displays mat-like growth. ...
Scales in treeline research depend on the objectives and must match the underlying natural processes. Factors and processes at one scale may not be as important at another scale. In the global view, the number of factors influencing climatic treeline position can be reduced to the effects of heat deficiency. Emphasis, however, should be laid on differentiation of the treeline by their regionally and locally varying physiognomy, diversity, spatial and temporal features, and heterogeneity. An assessment of the relative importance of the factors shaping regional/local treeline physiognomy, spatial patterns, and dynamics should have priority. This can be achieved only by syndisciplinary research. Such studies are indispensable for assessing treeline response to climate change at the regional and landscape scales.
... The growth form of treeline trees is a response to their harsh environment, in particular to winter stress (Lavoie & Payette, 1992;Pereg & Payette, 1998;Holtmeier, 2003;Harsch & Bader, 2011). At the upper limit of tree growth, trees often form clusters where several stems belong to one tree individual. ...
... At the upper limit of tree growth, trees often form clusters where several stems belong to one tree individual. These multistemmed trees result from winter stress such as wind abrasion, snow and ice damage, and winter desiccation (Pereg & Payette, 1998). For the Polar Urals, Devi et al. (2008) showed that these multistemmed trees grew in a creeping form for centuries but that vertical stems emerged when conditions improved at the beginning of the 20th century. ...
High-altitude treelines are temperature-limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we estimated how forest-tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub-Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11 100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade-1), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05 °C decade-1). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind-sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single-stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest-tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.
... Unfortunately, there are no similar data of snowdrift size for the studied Pyrenees treelines. Nevertheless, the snow precipitation in the Ordesa site does not produce so deep snowpacks, as suggested by the estimated snow depth based in the mean height AE1 SD of the krummholz mats (0.74 AE 0.40 cm; see Pereg and Payette, 1998). In fact, the snow cover may be absent some winters, leading to water stress, dessication damage and death of P. uncinata seedlings (Frey, 1983). ...
... The ability of tree and krummholz patches to modify microenvironmental conditions in the forest±grassland ecotone (snow cover, soil moisture and temperature, microclimate, etc.) is well known (Hadley and Smith, 1987; Holtmeier and Broll, 1992; Scott et al., 1993). Snow cover affects tree form and establishment (Earle, 1993; Schaefer and Messier, 1995; Hessl and Baker, 1997; Pereg and Payette, 1998 ). For example, Earle (1993) found that intermediate snow depths (1±1.5 m) were adequate for successful seedling establishment of P. engelmannii and Abies lasiocarpa in SE Wyoming. ...
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.
... Distorted krummholz forms under atmospheric conditions too cold for tree growth, with repeatedly unsuccessful shooting attempts, ending up in crooked growth, forming thickets that remain trapped in the warmer microenvironment they produce by aerodynamic decoupling (Körner 2012a, b;Millar et al. 2020). Once the climate warms above a threshold, upright tree stems emerge from such thickets (Pereg and Payette 1998;Gamache and Payette 2004). In line with our observations, Schickhoff et al. (2023) report a lagging of upper tree limits behind climatic change and high growing season mean soil temperature (7.5 ± 0.6 °C) at 10 cm depth in Rolwaling (Nepal). ...
In the European Alps, mean temperature has risen by 2.5 K since the end of the nineteenth century. A 2 K warming of the growing season has taken place in the last 4 decades only. The 2.5 K warming should rise the position of the climatic treeline by about 400 m. Actual shifts in uppermost tree positions reported here for the Austrian Defereggen Valley and the Swiss Lower Engadine region of the Eastern Alps reach only around 140 m of elevation above the limit of old trees that date back to the nineteenth century. Uppermost Pinus cembra trees of > 2 m height currently occur at c. 2500 m, representing elevation records for the Eastern Alps. In situ temperature records for 2022–2023 revealed seasonal mean temperatures for uppermost trees that are 1–3 K higher than the equilibrium treeline isotherm of c. 6 °C in both regions (corrected for temperature anomalies from long-term records). The 2 K span reflects microhabitat differences and two ways to define the season. Thus, tree advances lag behind the upslope shift of the treeline isotherm, on average, by more than 200 m. The uppermost trees currently grow under quite warm conditions with annual shoot length increments frequently reaching 20 cm. Even without additional future warming, the new steady-state climatic treeline will exceed the Holocene maximum elevation in the Eastern Alps substantially.
... Black spruce (Picea mariana (Mill.) B.S.P.) is the dominant tree species in bogs of the region (Pereg & Payette 1998). Individual trees are typically stunted as their growth is limited by the saturated conditions (poor soil aeration) (Lahde 1969, Mannerkoski 1985, low soil temperatures (Rothwell 1991) and low nutrient availability (Humphrey & Pluth 1996) in peatland systems. ...
The peatland-dominated Hudson Bay Lowland (HBL) is facing increasing pressures from climate change and resource extraction operations. Despite the potential for widespread changes in water availability to occur, information about hydrological and ecological feedbacks in the HBL remains limited. This study, located near the De Beers Victor diamond mine ~90 km west of Attawapiskat (Ontario, Canada), investigates the influence of mine dewatering activities ('pumping') and climatic variability on the radial growth of black spruce (Picea mariana (Mill.) B.S.P.) trees. Tree stem disks were collected from stunted black spruce trees in one reference (n=25) and three mine-affected bogs within the area of dewatering influence (n=41) along a transect of variable underlying aquitard (marine sediment) thickness. Pumping was not found to have influenced annual ring-width indices (RWIs) in mine-affected areas with either thick (6 to >18 m) or thin (< 5m) underlying marine sediment, as these sites showed similar growth patterns to the reference site during the period of mine operation. Analyses of the influence of climate on tree radial growth (1970-2018) using 20-year moving windows showed significant (p < 0.05) positive correlations (Pearson R) between residual RWI and mean monthly air temperature). In addition, for the period during which ground temperature data were available (2011-2018), significant negative correlations were detected between residual RWI and mean monthly soil temperatures in late winter and early spring. The above relationships highlight the importance of both growing and shoulder season conditions for tree growth. As the HBL continues to respond to climate change, the growth response and potential proliferation of black spruce will undoubtedly influence the water balance and hydrological function of bog peatlands in the region.
... These trees did not show any mechanical disturbances (impact scars, tilted stems, apical recoveries) related to slope processes such as rockfalls and snow avalanches. Moreover, these single-stemmed subarborescent trees showed only a very slight asymmetry of stem and foliage indicating a very low wind-exposure gradient (Pereg & Payette, 1998). They were cut at the collar, and their exact position, circumference and microtopography were recorded as well as the diameter at breast height. ...
Overcooled talus slopes are generally described as islands of sporadic permafrost below the lower alpine limit of permafrost. The negative thermal anomaly of the ground is mainly consecutive to the internal ventilation of the deposit, but it is also conditioned by multiple factors as topography, slope aspect and incline, openwork structure and coarseness of the deposit, air temperature, solar radiation and wind regime. Therefore, the study of the spatiotemporal dynamics of ventilation processes allows a better understanding of the phenomenon. At Cannon Cliff, New Hampshire (USA), several field visits and environmental monitoring allowed us to describe the varying nature and significance of the ventilation mechanisms that can be observed at the ground surface and associated with both the intensity and direction of the airflows in a talus debris accumulation/protalus rampart system. The thermal negative anomalies are strong enough to lower the ground temperature to the point of preserving ice during the late spring and summer seasons. The monitoring of the gradient between external (air) and internal (talus) temperatures coupled with several dendroecological and geomorphological analyses provided a complete environmental picture of the impacts, feedback and extent of the phenomenon.
... Ríos [24] found, further south in the Province of Iquique (Chile), that the proportion of life forms changes according to different topographic conditions. Likewise to other species that inhabit extreme ecosystems [43], the different life forms of P. tarapacana show a great phenotypic plasticity (i.e., an ability to respond to changes in their architecture to adapt to different environmental variations throughout their distribution). ...
In the upper vegetation limit of the Andes, trees change to shrub forms or other life forms, such as low scrubs. The diversity of life forms decreases with elevation; tree life forms generally decrease, and communities of shrubs and herbs increase in the Andean highlands. Most of treeline populations in the northwestern Argentina Altiplano are monospecific stands of Polylepis tarapacana, a cold-tolerant evergreen species that is able to withstand harsh climatic conditions under different life forms. There are no studies for P. tarapacana that analyze life forms across environmental and human impact gradients relating them with environmental factors. This study aims to determine the influence of topographic, climatic, geographic and proxies to human uses on the occurrence of life forms in P. tarapacana trees. We worked with 70 plots, and a new proposal of tree life form classification was presented for P. tarapacana (arborescent, dwarf trees, shrubs and brousse tigrée). We describe the forest biometry of each life form and evaluate the frequency of these life forms in relation to the environmental factors and human uses. The results show a consistency in the changes in the different life forms across the studied environmental gradients, where the main changes were related to elevation, slope and temperature.
... We assume a 'tree' to represent an upright woody plant of a minimum height that exposes its canopy to the full strength of atmospheric conditions (aerodynamic coupling to prevailing air temperature, commonly at heights of >2 m), irrespective of whether the stem reaches 'timber'-size. Shrubby individuals (so-called krummholz) do not belong to the life form tree and experience their own microenvironment (Millar et al., 2020;Pereg & Payette, 1998). Tree seedlings are nested in a warmer boundary layer near the ground for the "good" (facilitation) and the "bad" (competition, dehydration), usually amid small stature alpine species, and thus do not instantaneously track treeline position but can expand far above treeline, setting the stage for potential future treeline advances. ...
... Climate research on high-elevation treelines has investigated environmental factors, such as high sunlight [6,7], low temperature [8,9], topography [10,11], water stress [12], fire disturbance [13], snow cover [14]. Species response research has focused specifically on vegetation growth form [15], vegetation growth rate [16,17], soil microbiomes [18,19] and plant physiology [12], as well as seedling recruitment and regeneration [20,21]. ...
Treeline patches are among Earth’s most sensitive and are important model ecosystems for assessing climate change trends. To explore ecological factors that limit the species’ survival in treelines, the treeline patch of Li Mountain National Nature Reserve was selected as the research site. Pinus armandii (P. armandii), Betula albo-sinensis (B. albo-sinensis), and Betula utilis (B. utilis) were selected as research species based on their dominance. Two 50 m × 50 m plots were established separately from the upper and lower limits of the highest treeline patch for point pattern analysis. Five 10 m × 10 m quadrats per plot were sampled to investigate the flora and environmental factors. The results showed that: (1) Slope and community composition at tree layer in quadrates had significant differences between upper and lower limits. Pinus armandii had a greater population size at the upper limit. Seedling recruitment restricted population development for B. albo-sinensis at the lower limit and B. utilis at the upper limit and less regeneration of B. albo-sinensis at the upper limit. (2) More aggregation scales occurred at the upper limit, and P. armandii had more aggregation scales than the other two species at 0–25 m. The heterogeneity caused by density distribution affected P. armandii pattern at the upper limit, and heterogeneity of seed dispersal could explain species patterns in both limits. Distinctness of size difference may have an influence on inter-specific species correlations.
... During the LIA, occasionally beneficial climatic conditions enabled some seeds to spread and colonize above the upper margin of the closed forest [10][11][12]. Although most seeds and seedlings would die under stressful conditions at the sites above treelines, some seedlings would survive and establish as isolated trees, possibly due to strong growth plasticity to habitat adversity [13][14][15][16]. The growth plasticity is an ability in trees to adapt to stresses and disturbances and to recover to a normal state of growth after environmental perturbations [17][18][19]. ...
Trees greater than 150 years old growing in the current treelines were most likely isolated tree outposts above previous treelines of the Little Ice Age (LIA). An intuitive question is, how did these isolated trees grow at such a high elevation in the cold environment? Here, we tackle this question using tree-ring width data of the Northern Hemisphere’s highest treelines at 4900 m a.s.l. (Basu) and 4680 m a.s.l. (Langkazi) on the Tibetan Plateau. The results showed that an age-related exponential growth trend did not exist in most of the ring-width sequences of the sampled trees. The values of ring widths in the isolated trees had a similar pattern of probability distribution during and after the LIA. The coefficients of variation in ring widths of the isolated trees were significantly greater than those of the non-isolated trees in their common growth period. Synchronicity of annual change in radial growth among trees varied in time. These results indicated that the isolated trees in the LIA developed an adaptive ability to slow down radial growth rate and modulate growth synchronicity among individuals in cold stressful environments. Our study highlights growth plasticity in isolated trees above treelines for coping with harsh conditions in the LIA.
... malformations analogous to those observed at the treeline, such as the emergence of multiple small stems (Dy and Payette, 2007;Pereg and Payette, 1998;Payette et al., 1996). Stunted growth and malformations reduce the quantity, the quality and the value of the wood for the industrial use and increase the rotation length, with significant impact on sustainable forest management. ...
Reforestation in the boreal forest is challenging; trees must survive to large daily temperature variations and to the cold environment. Even if local tree species are adapted to withstand these harsh environmental conditions, spruce plantation failure after artificial regeneration occurs frequently, with important impacts on sustainable forest management. We hypothesized that this regeneration problem is caused by recurrent frost events occurring during the growing season. These events would freeze the terminal bud and the newly formed needles of the planted trees, thus limiting photosynthesis capacity and height growth. Our goal was to identify key permanent physical attributes of the landscape (elevation, slope shape and angular slope) and of microsite conditions (hole vs. mound) that best predict tree height and frost damage to foliage. In summer 2016 and 2017, we sampled tree height of 2,943 white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Mill.] B.S.P.) trees in 66 monoculture plantations aged between 6 and 13 years distributed in the Clay Belt region of Quebec (Canada), and environment prone to frequent growing-season frosts. Using linear and binomial mixed regression models, we analyzed the effects of the physical attributes of the landscape and of microsite conditions on tree height (linear) and on frost damage (binomial). Tree height increased with increasing elevation and when seedlings were planted on mounds compared to planted in holes. The impact of microsite conditions on tree height increased as plantations aged, but the importance of elevation on tree height decreased with age. The probability of frost damage to foliage decreased for trees planted on mounds compared to trees planted in holes and from concave to convex slopes. These relations were most important in young plantations, but trees showing growth problems were still shorter by 2 m, even 13 years after planting. We also observed differences between species: white spruce was significantly more damaged by frost and was smaller compared to black spruce. Therefore, growing-season frosts can cause growth suppression problems in white spruce plantations established in the boreal mixedwood region. Since microsite conditions also play a key role in driving plantation success, mechanical site preparation techniques should not only focus on reducing the competition between the planted trees and the competing vegetation but should also focus on limiting frost damage by planting trees on elevated microsites. Our results will support forestry practices limiting plantation failure in boreal mixedwoods.
... Here we define established seedlings as seedlings that have developed true needles and photosynthetic capacity (Brodersen and others 2019). Saplings encompass a broad range of growth forms and ages, as trees grow slowly in the North and occur in a variety growth forms due to windblown snow abrasion (Pereg and Payette 1998;Mamet and Kershaw 2013a). During the initial census in 2013, the height of all seedlings and saplings surveyed in each zone was recorded to help estimate missing years due to boring height in the mature tree surveys (see below). ...
At a global scale, mean air or soil temperatures appear to be drivers of treeline position. However, at finer scales, seed availability and microsite conditions may limit the germination, establishment, and growth of tree seedlings—and therefore the position of treeline. Tree islands are features of many treelines, and they can alter microsite conditions by producing seed, providing shelter, and redistributing snow. Near Churchill, Manitoba, Canada, tree islands have higher seedling establishment than nearby forest and treeline sites and could be hot spots for treeline expansion. However, seedling establishment and tree recruitment events may be stochastic, making predictions based on the relationship between seedling counts and microsite conditions difficult. We asked whether current tree island seedling establishment can be predicted by (1) historical recruitment and/or (2) tree and snowpack characteristics. To answer these questions, we measured tree island tree characteristics and monitored seedling establishment and snowpack in five tree islands at latitudinal treeline near Churchill, Manitoba. We fit hierarchical, generalized linear mixed effect models to assess the influence of our hypothesized predictors. We found that tree recruitment in the past was strongly positively associated with current seedling density. Mean basal area and age of trees were also positively associated with seedling density, whereas tree density was negatively associated with seedling density. We found weak positive effects of snowpack snow water equivalent. Our results provide evidence for positive feedbacks within tree islands and suggest useful factors to include in models for predicting future treeline change.
... Also, the relatively deep onshore water column may additionally benefit the preservation due to lowered possibility of lake-ice activity reaching the sediment column during the cold season (Eronen et al., 1999(Eronen et al., , 2002. In addition, the submerged trunks remain unexposed to damage by snow abrasion and wind (see e.g., Pereg and Payette, 1998) decreasing the preservation on ground unless the snowpack provides them with less weathering, subnivean conditions. Divergent courses of depositional histories were indeed obtained for the different sedimentary settings (Fig. 4a-c). ...
... However, in spring during the transition to the seasonal growth, plants become most vulnerable to temperature fluctuations. Negative effects of winter stress and damage to recruitment, tree survival, and growth appear to contradict the dominance of summer growth control [41][42][43]. ...
Research Highlights: For the first time, the Pinus sibirica Du Tour and Abies sibirica L. conifer forest at the West Sayan ridge timberline has been explored to reveal which species is likely to react to climate change and a shift of the timberline. Such a shift may modify the ecological functions of the forests. Background and Objectives: Long-term climate change has become obvious in the mountains of southern Siberia. Specifically, a half-century rise in annual mean temperatures has been observed, while precipitation remains unchanged. Trees growing at the timberline are likely to strongly react to climate alterations. The objective was to estimate which of the two species sharing the same habitat would benefit from climate alteration and shifting of the timberline. Materials and Methods: At several altitudes (from 1413 to 1724 m a.s.l.), samples of P. sibirica and A. sibirica needles have been collected and contents of chlorophyll a and b as well as carotenoids were measured in June 2019. The temperature of needles of the two species was measured in both cloudy and sunny weather conditions. Results: The studied species have been shown to have different patterns of pigment variations with the growth of altitude. The decline of chlorophylls and carotenoids was more pronounced in P. sibirica (ratio at timberline ca. 2.2) than in A. sibirica (ratio ca. 3.1). Accordingly, the electron transport rate decreased more strongly in P. sibirica at the timberline (ca. 37.2 μmol of electrons/m−2 s−1) than in A. sibirica (56.9 μmol of electrons/m−2 s−1). The temperatures of needles in both cloudy and sunny weather were higher in A. sibirica (10.5 and 43.3 °C, respectively) than in P. sibirica (3.8 and 24.2 °C, respectively). Conclusions: The considered physiological and ecological traits show that P. sibirica is better protected from higher-altitude hazards (excess insolation, rise of temperature etc.) than A. sibirica. P. sibirica may be therefore a more likely winner than A. sibirica in the movement of the mountain timberline under climate warming in the area.
... Dans ces peuplements, les épinettes noires ont adopté une forme plus trapue malgré une compétition pour la lumière quasi inexistante (Pereg et Payette, 1998 (Gagné, 2000;Lavoie, 2001;. Dans l'environnement des pessières à lichens, les petits arbres ne sont pas éliminés par la compétition pour les ressources comme en forêt fermée . ...
Une approche biogéographique, dendroécologique et paléoécologique a été utilisée
pour étudier la répartition, la dynamique et l’origine des pessières à lichens dans la zone de la forêt boréale fermée. À l’intérieur d’un gradient latitudinal de plus de 500 km, la
répartition des pessières à lichens et des principales perturbations visibles sur les
photographies aériennes des années 1950 et des années plus récentes a été cartographiée le long de 19 transects d’inventaire. L’analyse photographique, validée sur le terrain, a permis de mesurer une diminution de 9 % de la superficie totale de la forêt fermée au profit de la pessière à lichens au cours des 50 dernières années. Cette diminution est le résultat d’une faible régénération de la forêt fermée suite aux incendies, principalement au-delà de 51oN de latitude. La croissance des épinettes noires établies dans 54 pessières à lichens a été étudiée et la relation avec les variables climatiques (précipitations et températures mensuelles) a été vérifiée sur une période de 100 ans. Bien que les précipitations exercent une plus grande influence sur la croissance des arbres que les températures mensuelles, très peu de relations significatives ont été obtenues. Même si ces peuplements présentent une faible densité arborescente, la réponse dendroclimatique des arbres est atténuée par les épidémies de la tordeuse des bourgeons de l’épinette dans la plupart des pessières à lichens. Les réductions de la croissance radiale reliées à cet insecte sont importantes dans les pessières à lichens et sont plus marquées dans la partie méridionale de la zone de la forêt fermée. Grâce à l’utilisation de techniques complémentaires, de la radiodatation des charbons de bois, de la datation des cicatrices de feu et de l’observation des photographies aériennes des années 1950, il a été possible de retracer l’époque et les facteurs responsables de la transformation de la forêt de conifères dense en pessière à lichens. Trois séries de
perturbations causent le passage de la forêt fermée en pessière à lichens : 1) les feux légers, 2) les feux successifs et 3) l’incidence d’une épidémie de la tordeuse des bourgeons de l’épinette suivie d’un feu. La majorité des pessières à lichens inventoriées dans cette étude se sont formées au cours des 200 dernières années.
... Natural conifer clones are rare, but they do occur in high stress environments, e.g., in Arctic areas, and at high altitudes in species such as Black spruce (Picea mariana (Mill.) B.S.P.) and Norway spruce (Pereg and Payette 1998;Öberg and Kullman 2011). Vegetative propagation has been exploited for horticultural crops for millennia, e.g., Cabernet Sauvignon, a cultivar of Vitis vinifera L., has been propagated vegetatively since Roman times and existed only in adult form until recent its reappearance in juvenile form due to in vitro propagation (Mullins et al. 1979;Bonga 2016). ...
The aim of this work was to improve the protocol of somatic embryogenesis (SE) and propagation efficiency in Norway spruce (Picea abies (L.) Karst.), which would enable the integration of SE into Finnish breeding programme and the nursery practices applied to seedlings. The studies specifically investigated the following three areas: i) how maturation, cold storage, germination and growing conditions (laboratory–nursery interface) affect the survival and height growth of emblings (Papers I and II); ii) how to improve the efficiency of embling production from genotypes from wide genetic backgrounds (Papers I and II); and iii) how to increase propagation efficiency by rooting cuttings from emblings, and produce field testing material by combining SE and the rooting of cuttings (Papers II and III). To evaluate the possibility of improving the efficiency of SE in the laboratory–nursery interface, a series of experiments were conducted. The cost structure of SE, and the effects of improvements on costs, was estimated.
As a result, the protocol improvements doubled the yield of cotyledonary embryos, nearly doubled embling survival, and increased the height growth of emblings in the nursery by so much that sufficient planting height was reached one year less than before. Emblings were also obtained from 356 genotypes (50% thawed), and embling cuttings rooted well in conditions similar to those used for seedling cuttings. The protocol improvements also reduced embling production costs by 75%. Based on this work, emblings may be grown in nurseries after one week of in vitro germination, without any measures that differ from seedlings after transplanting. Propagation efficiency may be further increased by rooting embling cuttings. Furthermore, large-scale clone testing can be initiated with 5–12 emblings acting as cutting donors.
... The krummholz zone begins at the tree-form line at approximately 600-620 m a.s.l. Increased exposure to icy wind abrasion, frost desiccation, and heavy snow packs results in conifers adopting suppressed, slow-growing, clonal krummholz growth forms (Pereg and Payette 1998;Trant, Jameson, and Hermanutz 2011). Krummholz-zone individuals range from stunted miniatures of the tree's true growth form to prostrate, layering mats extending just below the average snow-pack level. ...
Cone and seed production at the forest-tundra ecotone, or treeline, depend on species-specific tolerances to limiting abiotic and biotic factors. As range expansion via seed dispersal is needed to keep pace with climate change, reproductive limitations act as a bottleneck for treeline advance. The treeline in the Mealy Mountains, central Labrador, was comprised of four codominant species: black spruce (Picea mariana [Mill.] B.S.P.), white spruce (Picea glauca [Moench] Voss), eastern larch (Larix laricina [Du Roi] K. Koch), and balsam fir (Abies balsamea [L.] Mill.). Conifer stem surveys from three treeline zones (forest, forest-tundra transition, krummholz) were used to assess patterns of altitudinal distributions, tree densities, and cone production to provide insight into overall reproductive potential. The altitudinal limit of the spruce species was 39 m a.s.l. higher than the altitudinal limit of black spruce cone production. Black spruce had the highest densities of cone-bearing trees across treeline with eastern larch values being comparable in the forest-tundra transition zone, although overall cone production was low and highly variable in all species. Compared to the other treeline species, black spruce has the greatest reproductive potential for upslope advance.
... There is a growing body of evidence that points towards fire playing a major role in retarding forest regeneration and increasing forest fragmentation (Payette and Gagnon, 1985;Sirois and Payette, 1991;Gajewski et aI., 1993;Lavoie and Payette, 1996). Numerous studies have shown that, despite the cooling climate, the extent of the northern treeline zone did not shift southward, but rather the trees within this transitional zone changed their growth form (from upright to krummolz forms) (Payette and Lavoie, 1994;Lavoie and Payette, 1996;Pereg and Payette, 1998). Trees remained at this latitude, despite the cooling climate, until a disturbance event such as fire removed them from the site and they were not able to successfully regenerate thereafter. ...
Diatoms and other siliceous microfossils were examined from a 386-cm-long peat core, covering the last ca. 7200 yr, from north-central Siberia to gain insights into peatland developmental history, and to explore the potential of diatoms as proxy indicators in arctic peats. Diatom analyses of arctic peatlands are rare, and so one aspect of this study was to examine the sensitivity of diatom taxa in relation to independent paleoindicators already described from this core. Changes in the relative abundances of diatom taxa divided the core into four zones that closely tracked the ontogeny of the peatland from an open water environment (Zone I: benthic, alkaliphilic taxa), followed by fen environments (Zones II and III; epiphytic, acidophilic taxa), and finally to a better-drained, high-centered bog (Zone IV; aerophilic taxa). In addition to the diatom taxa, observations were made on the relative abundances of siliceous protozoan plates, chrysophyte cysts, and phytoliths. Both the diatoms and these other siliceous microfossils appeared to respond to changes in hydrology and moisture, as well as to fire episodes likely triggered by climatic change. This study demonstrates that diatoms and other siliceous microfossils from arctic peat deposits provide an important source of paleoenvironmental information that can strengthen interpretations derived from other commonly used indicators.
... Natural clones of conifers are rare; they occur occasionally in high stress environments, i.e., in arctic areas or at high altitudes near the tree line. Black spruce (Picea mariana), for example, will reproduce as a low bush in arctic northeastern Canada by rooting of low branches (air-layering) when environmental stress limits vertical growth (Pereg and Payette 1998). Similarly, clonal Picea abies is found at high altitudes in Sweden, these clones being thousands of years old and having living stems that show up to 600 growth rings (Ӧberg and Kullman 2011). ...
Clonal propagation of conifers is achieved mostly by rooting of cuttings, organogenesis and somatic embryogenesis (SE). Of these, SE is the most powerful in obtaining genetic gain because SE cultures can be maintained in a juvenile state indefinitely by cryopreservation. This allows for long-term field testing of clonal lines while part of these lines are maintained in a juvenile physiological state until the field test has shown which are the best clonal lines for mass-production of propagules. This makes within family selection possible which is not the case with rooting of cuttings or organogenesis. However, as is explained in this review, one can expect that with advances in culture proceedings and in particular with increasing use of modern DNA analysis, within family selection may become possible for rooting of cuttings and organogenesis as well. Furthermore, issues such as deployment and field performance of clones and the cost of mass cloning are discussed.
... Since the 1960s and 1970s, small-tree stands have emerged above the snowpack, triggered by less harsh conditions associated with snowier winters in the last few decades (Pereg and Payette, 1998).This process decreases the numbers of low, damaged growth-form individuals in the population, and increases the numbers of upright, tree-form stems.The temperature increases projected by the ACIA-designated models are likely to be sufficient to provide the final requirement of summer warmth that would result in viable seed production at treeline in northeastern Canada (see Fig. 14.8 for summer temperature scenarios at Goose Bay, Labrador).The initial effect of warming is very likely to induce viable seed production within the forest-tundra zone, resulting in infilling of the patchy forest-tundra border. It is possible that production of viable seeds at the absolute tree limit would begin seed rain onto the tundra, a process that probably has not occurred in appreciable amounts for thousands of years. ...
... Une variation aussi minime que quelques pourcents d'écart sous les prévisions engendre des pertes économiques considérables. Malgré cet épisode récent de bas niveau d'eau dans les réservoirs du Nord, la majorité des recherches portant sur la reconstitution historique des précipitations indiquent une tendance séculaire à la hausse [1] [2] [3] [4] . Cette augmentation conforte notamment la décision du Québec qui a opté pour le développement hydroélectrique il y a une trentaine d'années. ...
In order to retrace the evolution of the hydrological regime parameters with time, we have reconstructed the main hydroclimatological variables used in forecast models. The study was conducted in the region of the James Bay hydroelectric complex of La Grande. Trees occupying sites sensitive to water availability variations (xeric, mesic and hydric sites) and those sensitive to thermal conditions (depending on exposition) give the opportunity to reconstruct climate variability between years. A calibration period with instrumental data is essential and it is also important to verify the models relating rings to climate with independent data. More than one hundred tree-ring series covering more than 180 years, tens series of more than 250 years and one series covering one thousand years have been reconstructed. These tree-ring series cover an area of 320 000 km2 (800 km in longitude × 400 km in latitude). Summer temperatures, snow precipitation and seasonal water supply were reconstructed over the past 200 years by means of various tree-ring proxies: ring width, density (9 derived variables) and ratios of stable isotopes of oxygen and carbon.
... Après cette période de croissance lente, le passage de l'interface neige-air ne semble pas avoir touché l'arbre A, au contraire de l'arbre B qui montre une élongation de l'axe principal particulièrement lente, jusqu'à ce qu'il ait atteint une hauteur d'environ 80 cm. Les deux arbres présentent des cernes larges sur une bonne partie de la tige entre 1940 et 1970, ce qui est conforme à d'autres observations ailleurs dans la zone subarctique (Pereg et Payette, 1998 ;Boivin et Bégin, 1997). L'arbre B accuse une mortalité de cime en 1960, dont la dominance est reprise par une branche périapicale préexistante. ...
Le developpement et la croissance d'une population cotiere d'epinette blanche ( Picea glauca [Moench] Voss) ont ete etudies sur le littoral en emersion du detroit de Manitounuk situe sur la cote est de la baie d'Hudson. Cette region subit un relevement glacio-isostatique a un taux des plus eleves au monde (1,2 a 1,5 m/siecle). La recherche vise a determiner la relation entre le developpement d'une frange d'arbustes et l'expansion de l'epinette blanche et a mettre en evidence les changements environnementaux survenus au cours du dernier siecle et menant a une population forestiere ouverte. La colonisation des rivages par l'epinette blanche est sous l'etroite dependance de l'etablissement prealable d'une frange arbustive. Les arbustes forment une bande etroite dans la zone suprariveraine encore humide, alimentee par le drainage hypodermique des terres avoisinantes. En hiver, ils retiennent la neige poudree par le vent et protegent ainsi du froid les semis d'epinette blanche qui s'y etablissent en abondance. Avec l'emersion des terres, la nappe phreatique s'abaisse et la frange d'arbustes hygrophiles se deplace vers le plan d'eau. La degradation des arbustes change le regime d'accumulation de la neige au sol a l'emplacement des epinettes preetablies. Seules les epinettes les plus developpees, qui avaient initialement une croissance rapide, survivent. Il en resulte une population clairsemee en milieu tres expose, qui n'a de possibilite d'expansion que derriere les buttes de pergelisol et les plages soulevees ou s'accumule la neige. Les etapes du processus de colonisation sont mises en evidence par la distribution des arbustes et des arbres selon leur âge, par l'elongation et l'accroissement radial de l'epinette blanche et les macrofossiles identifiables laisses depuis la fin du XIXe siecle.
... Whether there is a carbon-balance problem for taller trees is unclear, but photoinhibition has been observed in older trees as well (Grace et al. 2002, Ö quist andHuner 2003), though effects on actual growth remain unresolved. Significantly greater tissue damage (needle loss due to frost and wind exposure) has been reported for some exposed treeline sites (van Gardingen et al. 1991, Pereg and Payette 1998, Sveinbjö rnsson 2000. Such mechanical damages can reduce the potential for assimilation, but are not ubiquitously occuring at all treelines, including the Alps (Turner 1968). ...
... Long-lived clonal patches at the tree line have been used to reconstruct post-glacial migration patterns (e.g., Kullman, 2006), but the importance of ramets has been little discussed. In North America, the widely distributed black spruce (Picea mariana) is the dominant species forming these clonal patches (Pereg and Payette, 1998), making these trees ideal for exploring ideas of persistence. ...
While old trees have long been of interest, their significant role in responding to climate change at northern tree lines has been overlooked. Long-lived black spruces at the tree line in Labrador show a radial growth response that is synchronous with recent climate warming. The ability of individuals to persist with suppressed radial growth rates during adverse growing conditions may have significant implications for the rate at which these trees are able to respond when conditions become favourable.
... Trees within this ecotone demonstrate speciesspecific response to these physical factors. The most widespread adaptation is a change in growth form of individual trees [5][6][7][8][9]. ...
... There is a growing body of evidence that points towards fire playing a major role in retarding forest regeneration and increasing forest fragmentation (Payette and Gagnon, 1985;Sirois and Payette, 1991;Gajewski et al., 1993;Lavoie and Payette, 1996). Numerous studies have shown that, despite the cooling climate, the extent of the northern treeline zone did not shift southward, but rather the trees within this transitional zone changed their growth form (from upright to krummolz forms) (Payette and Lavoie, 1994;Lavoie and Payette, 1996;Pereg and Payette, 1998). Trees remained at this latitude, despite the cooling climate, until a disturbance event such as fire removed them from the site and they were not able to successfully regenerate thereafter. ...
Diatoms and other siliceous microfossils were examined from a 386-cm-long peat core, covering the last ca. 7200 yr, from north-central Siberia to gain insights into peatland developmental history, and to explore the potential of diatoms as proxy indicators in arctic peats. Diatom analyses of arctic peatlands are rare, and so one aspect of this study was to examine the sensitivity of diatom taxa in relation to independent paleoindicators already described from this core. Changes in the relative abundances of diatom taxa divided the core into four zones that closely tracked the ontogeny of the peatland from an open water environment (Zone I: benthic, alkaliphilic taxa), followed by fen environments (Zones II and III; epiphytic, acidophilic taxa), and finally to a better-drained, high-centered bog (Zone IV; aerophilic taxa). In addition to the diatom taxa, observations were made on the relative abundances of siliceous protozoan plates, chrysophyte cysts, and phytoliths. Both the diatoms and these other siliceous microfossils appeared to respond to changes in hydrology and moisture, as well as to fire episodes likely triggered by climatic change. This study demonstrates that diatoms and other siliceous microfossils from arctic peat deposits provide an important source of paleoenvironmental information that can strengthen interpretations derived from other commonly used indicators.
... Compared to trees of closed forest stands, lichen woodland trees produce shorter stems but larger tree rings (Pollock & Payette, 2010). Woodland spruce tend to adopt a stout growth form even if there is no competition for light (Pereg & Payette, 1998). Overall, however, volume growth of woodland trees was similar than that of closed-crown forest trees. ...
Lichen—spruce woodlands occur in the closed-crown forest zone as a divergent type of the spruce—moss forest because of regeneration failure caused by compounded disturbances (fire, insect outbreaks, and logging). From the southern limit of distribution of lichen woodlands (47° 30′ N) to the northern limit of the closed-crown forest zone (52° 40′ N), 53 lichen woodlands were sampled for a detailed dendroecological analysis. Radial, height, and volumetric growth of black spruce (Picea mariana) trees among the woodlands varied significantly according to stand age. Growth rates were similar for all even-aged spruce trees, whereas growth rates of trees < 100 y old were significantly greater than those of trees > 100 y old. No significant differences were found in growth rates of spruces distributed along the latitudinal gradient. Spruce trees were generally small in all the woodlands studied, with only a few trees taller than 12 m. Although spruce budworm may have affected all stands, the timing and intensity of the infestations were not necessarily synchronous among the studied stands. Partial least square response (PLS) functions were calculated to evaluate the impact of climate on tree-rings with latitude. Along the 600-km transect, PLS response functions indicated that growth was strongly influenced by current and previous year climatic conditions, particularly precipitation.
... Above snowpack, the flagged crown is characterized by a winddeformed leader above a section free of branches and needles. Below the snowpack, the crown is cushion-shaped (Fig. 1;Tranquillini 1979;Pereg and Payette 1998). Tree form largely depends on the winter snow cover, since shoots projecting above the snow are severely damaged regularly (Tranquillini 1979;Körner 2003). ...
In the flagged crown, which is asymmetric growth formed by severe stresses during winter in alpine regions, needles of evergreen conifers often became brown and died in early spring, but did not in a cushion-shaped crown. Needle browning and death is thought to occur by increasing transpiration due to a thinner cuticle or mechanical damage to the cuticle by wind-born snow and ice particles. To confirm whether the needle browning and death in the flagged crown of Abies mariesii Mast., in the alpine region of Japan conform with this concept, we assessed mechanical damage of the needle cuticle in a timberline ecotone and evaluated the effect of cuticle thickness on cuticular resistance. Mechanical damage on needle cuticles of A. mariesii was not observed. In the cushion-shaped crown, epicuticular wax covered the cuticle and plugged stomatal antechambers. In the flagged crown, epicuticular wax was mostly absent. Cuticular resistance in the flagged crown was lower than that in the cushion-shaped crown. However, the cuticle in the flagged crown was thicker than that in the cushion-shaped crown. The needle browning and death in the flagged crown of A. mariesii occurred even though needle cuticles were not mechanically damaged. The thicker cuticle of the flagged crown may play a role in other stresses. To estimate desiccation stress in relation to the cuticle, we need to elucidate not only cuticular resistance and cuticle thickness, but also cuticle quality and structure.
... Snow is perhaps the single most important variable controlling distribution of biological systems in arctic ecosystems (Walker et al. 1999). Snow affects the length of the plant growing season, soil moisture, soil chemistry, soil temperatures, depth of freezing and heat flux and protects growth from wind damage (Billings and Bliss 1959, Bowman 1992, Walker et al. 1993, Pereg and Payette 1998, Pomeroy et al. 2003. Duration and extent of snow cover are key products of interactions between landscape and climate and the spatial and temporal environmental heterogeneity produced by snowmelt has not yet been adequately captured for modelling at landscape scales. ...
Detailed snowpack observations, meteorology, topography and landcover classification were integrated with multi‐temporal SAR data to assess its capability for landscape scale snowmelt mapping at the forest–tundra ecotone. At three sites along an approximately 8° latitudinal gradient in the Fennoscandian mountain range, 16 multi‐temporal spaceborne ERS‐2 synthetic aperture radar (SAR) were used for mapping snowmelt.Comparison of field measurements and backscatter values demonstrates the difficulty of interpreting observed backscatter response because of complex changes in snow properties on diurnal and seasonal temporal scales. Diurnal and seasonal melt–freeze effects in the snowpack, relative to the timing of ERS‐2 SAR image acquisition, effectively reduce the temporal resolution of such data for snow mapping, even at high latitudes.The integration of diverse data sources did reveal significant associations between vegetation, topography and snowmelt. Several problems with the application of thresholding for the automatic identification of snowmelt were encountered. These largely related to changes in backscattering from vegetation in the late stages of snowmelt. Due to the impact of environmental heterogeneity in vegetation at the forest–tundra ecotone, we suggest that the potential to map snow cover using single polarization C‐band SAR at the forest–tundra ecotone may be limited to tundra areas.
... The phenomenon of vertical tree stem development from pre - established krummholz was reported for northern and alpine tree lines ( Lescop - Sinclair & Payette , 1995 ; Dereg & Payette , 1998 ; Luckman & Kavanagh , 2000 ) . It was also observed that about 5Á15% of ' ' post - prostrate ' ' trees have a dead ( or substitute ) top , showing that the observing warming is a not straightforward process . ...
A warming climate provides competitive advantages to Siberian pine (Pinus sibirica Du Tour) in areas with sufficient precipitation. The warmer temperatures observed in central Siberia over the past three decades appear to have had a noticeable effect on growth of Siberian pine and larch (Larix sibirica Ledeb.) in the south Siberian Mountain forest-tundra ecotone. Larch is more tolerant of harsh climates and exhibits an arboreal growth form, whereas Siberian pine is in krummholz form. Larch also has an advantage at the upper tree limit and in areas with low precipitation. Since the mid-1980s there have been measurable increases in growth increments, stand densification, regeneration propagation into the alpine tundra and transformation of krummholz into arboreal forms. Warming winter temperatures have been sufficient for increased survival of regeneration. Regeneration responded to temperature increase of 1°C by migration to areas 10-40 m higher in elevation. Regeneration has propagated into the alpine tundra at the rate of ~1.0-2.0 m year-1. Siberian pine and larch regeneration surpassed their upper historical limit by 10-80 m in elevation. While increased tree growth and migration into alpine tundra areas affect the regional carbon balance, it will also decrease albedo, which may increase warming at the regional level.
... Authors also try to reconstruct the invasion history [8,25,49] of sites by using core sampling and analysing the installation date of tree species (chronosequence) [30]. Chronosequences have been studied often for: primary succession [25], cyclic dynamics [42,50] fire frequency [8,31], natural gap [26,43], competition [36], serpentine soil [41] or studies of global change [11,45,46]. ...
The aim of this research is to reconstruct the invasion history of sites by using core sampling and analysing the installation date of tree species. A total of 107 sample sites, abandoned since 0 to 45 years ago and distributed in eight valleys of central Corsica, were studied. Results show a high (r2 = 0.86 P < 0.0001) correlation between the abandonment date derived from historic aerial photographs and the age of the oldest woody specimen in the site. For recent abandonment periods, the oldest woody species present in sample plots were grazing tolerant shrubs (short-lived species). When the abandonment period exceeded 20 years, the oldest species in sample plots were long-lived tree species. The use of category (short or long-lived species) and age of oldest woody species of sites allow to create a quantitative estimation of abandonment period available without old aerial photographs. This new chronological variable is relevant for studying intermediate (one to several decades) periods of abandonment in secondary succession processes. The use of such variable allows the analysis of succession of vegetal communities or the establishment of the dynamical response curves of species.
... Since the 1960s and 1970s, small-tree stands have emerged above the snowpack, triggered by less harsh conditions associated with snowier winters in the last few decades (Pereg and Payette, 1998).This process decreases the numbers of low, damaged growth-form individuals in the population, and increases the numbers of upright, tree-form stems.The temperature increases projected by the ACIA-designated models are likely to be sufficient to provide the final requirement of summer warmth that would result in viable seed production at treeline in northeastern Canada (see Fig. 14.8 for summer temperature scenarios at Goose Bay, Labrador). The initial effect of warming is very likely to induce viable seed production within the forest-tundra zone, resulting in infilling of the patchy forest-tundra border. ...
The boreal region covers about 17% of global land area,and the arctic nations together contain about 31% of the global forest (nonboreal and boreal). The boreal forest is affected by and also contributes to climate change through its influence on the carbon cycle and albedo. Boreal forests influence global levels of atmospheric carbon dioxide and other greenhouse gases by taking up carbon dioxide in growth, storing carbon in live and dead plant matter, and releasing carbon through decomposition of dead organic matter, live plant and animal respiration, and combustion during fire. Human management influences on carbon uptake and storage include the rearrangement of forest age classes through timber harvest or wildfire suppression, selection of tree species, fertilization, and thinning regimes.The combined effect of all management actions can either enhance or reduce carbon uptake and storage.
... Whether there is a carbon-balance problem for taller trees is unclear, but photoinhibition has been observed in older trees as well (Grace et al. 2002, Ö quist andHuner 2003), though effects on actual growth remain unresolved. Significantly greater tissue damage (needle loss due to frost and wind exposure) has been reported for some exposed treeline sites (van Gardingen et al. 1991, Pereg and Payette 1998, Sveinbjö rnsson 2000. Such mechanical damages can reduce the potential for assimilation, but are not ubiquitously occuring at all treelines, including the Alps (Turner 1968). ...
Historically, carbon limitation, through a shortage of photoassimilates has been argued to limit the growth of trees at the upper altitudinal treeline. In a three-year free-air CO2 enrichment (FACE) experiment, two species of 30-year-old alpine conifers (Larix decidua and Pinus uncinata) were studied to test this hypothesis in situ in the Swiss Central Alps (2180 m above sea level). CO2 enrichment was combined with foliage removal to test the effect of altered source-sink relationships on tree growth and leaf level responses. Elevated CO2 enhanced photosynthesis and increased nonstructural carbohydrate (NSC) concentrations in the needles of both species. While the deciduous larch trees showed longer needles and a stimulation of shoot growth over all three seasons when grown in situ under elevated CO2, pine trees showed no such responses. Irrespective of CO2 concentration, defoliation in both species stimulated photosynthesis and increased stomatal conductance in remaining current-year needles in the treatment year and reduced leaf nitrogen concentration in the year following defoliation. Defoliated larch trees had fewer and shorter needles with reduced NSC concentrations in the year following defoliation and showed no stimulation in shoot elongation when exposed to elevated CO2. In contrast, defoliation of evergreen pine trees had no effect on needle NSC concentrations, but stimulated shoot elongation when defoliated trees were exposed to elevated CO2. After three years, our results suggest that deciduous larch is carbon limited at treeline, while evergreen pine is not. However, as indicated by the defoliation treatment, the carbon economy of these trees can clearly be modified by extreme events. The expected changes in growth of these treeline trees with improving carbon availability as atmospheric CO2 continues to increase will thus depend on both the interplay between biotic and abiotic processes, and the species or tree functional types involved.
... Lescop-Sinclair and Payette 1995Lavoie and Payette 1996Gamache and Payette 2004Lavoie and Payette 1992, 1994Pereg and Payette 1998 Masek 2001 ...
Nowhere are the consequences of climate change greater than in the Arctic. The Arctic Climate Impact Assessment (ACIA, 2004) synthesized the potential impacts of climate change to Arctic ecosystems, including vegetation changes projected for 2100
using the BIOME4 dynamic vegetation model. In my research I synthesized empirical data from 30 studies of Arctic vegetation responses to climate change, and compared these data to the ACIA projected vegetation distribution for 2100. A general agreement between observed and projected changes was found, with exceptions due to regional variability and geographic clustering of the empirical data. There exist large areas of Siberia east of the Taymyr Peninsula and the Arctic Peninsula without empirical data, but are projected to undergo expansive change. Likewise, geographically limited empirical data have been published for large areas with no projected change in central Siberia and in the Yukon and Northwest Territories, Canada.
The change of water phase around 0 °C has considerable impacts on wildlife ecology because liquid and solid water strongly differ in their insulating capability, mechanical resistance, and light reflectance. Freeze and melt events thus have strong ecological relevance, particularly in the Arctic where snow and ice are omnipresent and their conditions are changing due to climate warming. We first review the mechanisms linking water phase transitions to wildlife ecology, with emphasis on seven key processes. These processes are illustrated with examples or detailed case studies, such as snowmelt and icing events affecting herbivore populations, thaw-induced collapse of structures used by wildlife for reproduction, and thermal erosion of ice wedges reducing waterfowl habitat. We infer that water phase transitions generate some critical places and critical times that play a disproportionate role in the ecology of tundra wildlife. We map these critical places and times to help structure future research on the effects of climate change on tundra wildlife in a context where changing permafrost and snow conditions might trigger abrupt ecological responses in the Arctic tundra.
Despite its wide geographic distribution and important role in boreal forest fire regimes, little is known about the climate-growth relationships of black spruce (Picea mariana [Mill.] B.S.P.). We used site- and tree-level analyses to evaluate the radial growth responses to climate of black spruce growing on a north-facing toposequence in interior Alaska for the period A.D. 1949-2010. At the site level, correlations between growth and climate were negative for temperature and positive for precipitation. The signs and strengths of these correlations varied seasonally and over time. These site-level differences probably arise from tree interactions with non-climatic factors that vary with topography and include active layer thickness, soil temperature, solar radiation, microsite, and tree architecture. We infer that black spruce suffers from drought stress during warm, dry summers and that the causes of this moisture stress relate to topography and the seasonality of drought. Tree-level analyses reveal that divergent inter-tree growth responses among individual trees at the same site also occur, with the lower slope positions having the greatest frequency of mixed responses. The overall complexity of black spruce's climate-growth relationships reflects the plastic growth strategy that enables this species to tolerate harsh, high-latitude conditions across a transcontinental range.
Zum Leidwesen vieler Praktiker und einer Reihe von Wissenschaftlem ist das Recht der „Multimediadienste“ bzw. — in juristischer Terminologie — der Informations- und Kommunikationsdienste sowie des Rundfunks auf ein Bundesgesetz und zwei Länderstaatsverträge aufgeteilt worden1, deren Begriffiichkeiten und Regelungsinhalte nur unzureichend aufeinander abgestimmt worden sind2, Es sind dies das Teledienstegesetz des Bundes (TOG)3, welches jüngst durch das Elektronischer Geschäftsverkehr-Gesetz (EGG)4 novelliert worden ist, sowie der Mediendienstestaatsvertrag (MOStV)5 und der Rundfunkstaatsvertrag (RfStV)6 der Länder.
Do large hydroelectric reservoirs in northern Quebec have an influence on regional climate? In the absence of any climatic record covering the period prior to and after damming, the influence of a reservoir was studied by means of tree rings. The objective of this research was to determine the reaction of black spruce to local climate change. The study was carried out on an island in the second-largest reservoir of the La Grande complex in the James Bay area, the Robert Bourassa Reservoir (2835 km2), which is also the oldest (dammed in 1978 and completely flooded in September 1979). Trees (n = 14) about 100 years old were selected by means of two perpendicular transects on the island. Cross sections were cut at 1-m intervals at the base of trees, and at 10-cm intervals in the top section of the stem which developed during the last 30 years. Densitometric analysis was performed on each section. Results were compared with two reference chronologies using the same tree-ring parameters. Reference stands were selected outside the area of climatic influence of the reservoir. The identified tree-ring indicators of reservoir local climate were (i) absence of frost rings, despite their abundance outside the reservoir's area of influence, due to a delayed growing season caused by local cool conditions; (ii) high frequency of light rings related to cooler summer conditions; (iii) large numbers of trees showing compression wood sequences indicating destabilization due to windier conditions associated with the increased fetch; (iv) lower annual wood production; (v) degradation of tree growth forms marked by massive foliage loss and branch mortality; and (vi) reduced wood density, despite increased proportion of latewood. The number of trees that showed these six tree-ring indicators was a function of distance to water and stand density.
The interactions between aeolian and biotic processes are discussed, including the ability of vegetation to control aeolian transport, the impact of aeolian transport on soils and vegetation (including soil nutrient content), feedbacks between vegetation and aeolian transport, and aeolian transport in managed (agriculture and rangeland), vegetated systems.
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.
Aim
Comparisons of how different species respond to changing climatic conditions offer insight into future community composition and the potential formation of novel communities. This study investigated changes at a subarctic forest–tundra ecotone, or ‘tree line’. Our objectives were: (1) to explore species‐specific growth forms; (2) to identify temporal patterns of establishment and stand density; and (3) to explore relationships between climate and recruitment/survival amongst co‐dominant tree species, with the expectation that climate change will affect species differentially.
Location
The Mealy Mountains in the High Subarctic Tundra ecoregion in central Labrador, Canada.
Methods
We examined tree line dynamics for four tree species over the past two centuries. Using ecological and age‐structure data, we compared diameter/height relationships across the tree line and generated static age structures from which changes in stand density through time were compared. In addition, model residuals were used to quantify relationships between multi‐decadal windows of temperature/palaeotemperature/Palmer Drought Severity Index and decadal tree recruitment.
Results
Trees were more stunted as elevation increased, except for white spruce ( Picea glauca ) for which tree islands became the dominant growth form. The only tree seedlings found at the tree line were of larch ( Larix laricina ) and to a lesser extent black spruce ( Picea mariana ). From the age structure of trees (height > 2.0 m), only black spruce showed evidence of an advancing tree line. Larch and balsam fir ( Abies balsamea ) have become established at the tree line most recently and have undergone greater increases in density over the past few decades. Variability in recruitment increased with elevation: larch recruitment was positively correlated with temperature and negatively correlated with drought at low elevations but negatively correlated with temperature and positively correlated with drought at high elevations, whereas black spruce recruitment was consistently positively correlated with temperature and drought.
Main conclusions
The multispecies approach provides evidence that species are responding differentially to climate. With continued climate change, we expect density increases and advances of larch and black spruce, giving rise to novel tree line communities.
Several environmental indicators show that climate changed dramatically at the turn of the 19th century, with global warming throughout the 20th century. Among the most used proxies to evaluate long-term changes in climate are trees located at their northern range limit. Several studies have shown enhanced tree regeneration at treeline caused by recent warming, but no data are available on height growth performance of forest trees at treeline before and during the 20th century warmth. In this study, we examined the long-term growth performance of black spruce (Picea mariana (Mill.) B.S.P.) trees located in a lichen-spruce woodland, near the arctic treeline in eastern Canada. The study woodland is an old-growth stand that has escaped fire approximately during the past two millennia. We performed stem analysis on normally developed trees which grew in the woodland over the last centuries. The sampled trees are ramets from long-lived, self-regenerating black spruce clones that are forming small tree islands within the woodland. The co- occurrence of living and dead tree stems inside clones gives the opportunity to evaluate the growth performance of the same genets through time. Height and radial growth of 60 tree stems from 15 old-growth clones were evaluated for a period spanning the last 400 yr. Sampling included the two tallest living stems and the two tallest dead stems of randomly selected clones. No differences in height and radial growth were found among the 30 living stems nor among the 30 dead stems within clones. Living stems were 2 m taller and radial growth was 1.6 times greater than dead stems. Vertical development of stems was divided into two parts according to position of snow cover. Growth of dead and living stems was similar below the snowpack line. However, significant differences were found for growth above snowpack for the two types of stems. Growth above snow cover for all dead stems occurred between the 17th and 19th centuries. In contrast, growth of living stems above snow cover started during the late 19th century. Our data indicate that stems of the same genotype responded directly to climate change, in conjunction with climatic conditions prevailing at the time when they were protruding above the snowpack. Compared to extant trees, significantly smaller trees grew in the woodland during the Little Ice Age. Potential causal factors of differential growth performance through time are discussed.
In southern Quebec (Charlevoix, Canada), reconstructions of the subalpine insect assemblages were made in environments that were transformed by fire during the late Holocene (since 4500 BP). The ecological requirements of beetle species likely to be found as fossils were studied by collecting living beetles in pitfall traps in forest, suibalpine and alpine environments. The late-Holocene changes of insect assemblages were reconstructed by recovering fossil insects from 13 peat sections. Modem beetle assemblages are valuable indicators of tree cover. Well-developed podzol soils under peat, as well as numerous spruce charcoal remains overlaying the podzol in all sampling sites, suggest that a dense spruce cover was present before the beginning of the accumulation of peat, and was eliminated by fire (4570-800 BP). Ground beetle and bark beetle assemblages at the base of the peat sections contained a mixture of forest and open-environment species, indicating that the postfire environment was a subalpine one with a tree cover of 30-50%. The most recent fire in the area (AD 1915) increased the openness of the landscape (tree cover
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.
This paper is an up-to-date review of instrumentally-recorded, seasonal, surface temperature change across the land and marine regions of the world during the twentieth century. This is the first part of a two part series. The second part will deal with the interpretation of proxy-climate data in terms of large-scale hemispheric or global-scale temperature averages for the Holocene.
In Part 1, we review the uncertainties associated with combining land and marine instrumental records to produce regional-average series. The surface air temperature of the world has warmed 0.5°C since the middle of the nineteenth century. The warming in the Northern Hemisphere only occurred in winter, spring and autumn. Summers are now no warmer than in the 1860s and 1870s. The same half-degree warming is seen in all seasons in the Southern Hemisphere.
Spatial patterns of temperature anomalies during two warm decades, the 1930s and 1980s, all vary from season to season. Temperatures during the 1980s were by far the warmest in the last 140 years. Though most areas of the world experienced above normal temperatures, the variability from season to season was notable in the Northern Hemisphere where much of the warmth was in winter and spring over northern and central Asia and northwestern North America. Almost all of the Southern Hemisphere was warmer during these years.
The recent history of Piceaabies (L.) Karst. at its altitudinal tree limit has been studied in the southern Swedish Scandes. Altitudinal transects (131) were evenly distributed over a tract of mountains of ca. 40 × 200 km. The age of spruces growing at the tree limit and downhill were estimated by annual ring counts. The spruce tree limit had risen (on average by ca. 50 m altitudinally) in ca. 70% of the studied transects as a result of the subsequent growth in height of old, established, formerly stunted individuals. Their growth in height accelerated during the 1930's, in response to the general climatic warming. A rise in the tree limit because of the establishment of new individuals (after 1915) was noted in only 7% of the studied transects. Most of the spruces growing in the tree-limit ecotone established around the 1860's and the 1940's, which were epochs with relatively snowy winters. After 1860, spruce establishment was not correlated with the summer mean temperature. Successful regeneration of spruce at the tree limit is dependent of a deep and stable snow cover and the requisite balance between precipitation–meltwater and evaporation being maintained in the early summer. The importance of air temperatures in May for successful growth and natural regeneration was evident. High air temperature in May is detrimental, since it promotes a too early initiation of growth and a consequent increased risk of frost damage. The spruce populations at the tree limit are recruited both from local seed parents and from long distance dispersal of seed from trees growing at lower altitudes.
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.
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
[From Introduction:] This paper describes dendrochronological techniques
that can be applied to tree-ring samples to measure
radial growth during past and present budworm
outbreaks. The techniques will be illustrated with
examples from a radial-growth study of trees infested
by western spruce budworm (C. occidentalis
Freeman) in northern New Mexico.
Most techniques described here have been standard
for many years in dendroclimatic and dendroecologic
studies (Anon. 1977, Fritts 1971) and have many
advantages over other types of tree-ring analyses for
evaluating effects of insects on radial growth. In
particular, the rigorous use of cross-dating and
standardizing can provide more precise measurements
and improved understanding of the effects of insects.
climate, and other environmental factors on tree
growth.
Describes the architectural model as being the growth pattern which determines the successive architectural phases. This model defines the way by which a plant elaborates its form and is an inherent growth strategy. The architectural unit is interpreted as the specific expression of the model. Reiteration is interpreted as a morphogenetic process which allows total or partial duplication of the architectural unit; the process is called the reiterated complex. Trauma can be the cause; otherwise the phenomenon is known as adaptive reiteration. -S.J.Yates
Dans les regions hemi-arctiques, les patrons d'enneigement sont causes par l'arrangement spatial des unites de relief et de vegetation. A Poste-de-la-Baleine, l'etude de la distribution du couvert de neige en fonction des unites de relief demontre que les depressions et les versants affectes d'un microrelief accuse ou de fortes denivellations sont les sites privilegies d'accumulation. Les structures vegetales hautes exercent un meilleur controle des conditions d'enneigement que les structures vegetales basses. Dans les milieux forestiers, la relation entre la densite et la profondeur de neige est lineaire: a une augmentation de la profondeur correspond une hausse de la densite. Ces variations s'expriment le long d'un gradient altitudinal. Dans les milieux ouverts domines par des formations basses, l'influence du vent est determinante: elle se traduit par des profondeurs de neige tres variables et des densites uniformes et generalement elevees. La saturation topographique est un concept utile pour caracteriser les conditions d'enneigement de ces espaces ouverts : l'accumulation de neige a tendance a attenuer le relief et a regulariser le profil des versants. D'une annee a l'autre, les patrons d'enneigement semblent constants.
Tree-ring analysis was used to relate the pattern of eastern larch Larix laricina growth to insect defoliation over the last three centuries. Four sampling sites were selected along a moisture gradient near Lake Bienville, in an area where larch is currently recovering from a recent outbreak of the larch sawfly Pristiphora erichsonii. In the two dry sites tree-ring series clearly showed eight periods of defoliation-induced growth suppression since 1782. The study suggests that larch sawfly outbreaks had a considerable long-term impact. The long-term effect may be a gradual substitution of the mixed black spruce Picea mariana - eastern larch forest by a monospecific black spruce forest. -from Authors
Options for overwintering success involve migration, hibernation and the evolution of means of resisting the harsh conditions. Changes in the physical nature of snow as it falls, settles, freezes and melts are shown. Plants need the means to acclimate to the cold, survive physiological drought (non-availability of free water), and to deal with a number of mechanical stresses. The evergreen habit is shown to be an important option. For animals, the basics of energy exchange are noted, and means of maintaining a warm body in a cold environment are examined, with consideration of physical versus physiological thermoregulation. Examination of the problem of appendages leads to evaluation of Bergmann's and Gloger's rules of size and colour, respectively; is it better to be large and white in a cold climate? Life under ice is described. A number of plant-animal interactions are noted, eg herbivore preferences and morphological and chemical plant resonses to winter browsing, and CO 2 build-up under snow and effects on subnivean animal activity. Finally, the response by humans to winter stress is outlined.-P.J.Jarvis
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.
The extent and causes of winter desiccation were studied in three tree-line species on Mt. Washington, New Hampshire. We did not find excessively low relative water contents in intact foliage of balsam fir (Abies balsamea [L.] Mill.) or black spruce (Picea mariana [Mill.] BSP.) during the winter. Relative water contents were lowest following damage by ice abrasion and in intact stems of paper birch (Betula papyrifera Marsh.). Winter transpiration in conifer foliage is very low perhaps due to several dormancy-related mechanisms. Transpiration is further reduced by high wind speeds which maintain thermal equilibrium between leaf and air. Thus winter desiccation is not universal at tree line and is probably related to factors other than the high-wind regime.
1 The very cold, wind-exposed coastal environment along the eastern Hudson Bay coast (northern Quebec, Canada) has resulted in a treeline which runs parallel to the coast (i.e. north-south). We have measured changes in the longitudinal position of the black spruce (Picea mariana (Mill.) B.S.P.) treeline during the last centuries, using tree ring analysis of spruce stem development. 2 Treeline trees ($\geqslant$ 2.5 m high) were sampled along transects in valley sites of four different rivers draining towards Hudson Bay, from the present treeline to the forest limit. The year of initiation of supranival stem growth (i.e. when stems first developed above the snowpack) was determined for trees, whorled growth forms and dead shrubs. Supranival tree stems developed synchronously in the valleys, from the late 1800s to the mid 1900s, in the area presently occupied by treeline trees, indicating that only krummholz (shrubby spruces) were present at these sites during the Little Ice Age. 3 A corresponding displacement of the treeline of about 12 km towards Hudson Bay has occurred in the area since the late 1800s, most likely as a result of recent warming. The recent treeline shift also resulted from the development of vertical tree stems from pre-established krummholz, suggesting that stem growth was a direct response to favourable summer growth conditions and reduced wind-driven snow abrasion and supercold wind-chill. The absence of treeline trees originating from seeds indicates that the recent warming has not been of sufficient magnitude or duration to promote sexual reproduction.
In northern Quebec, black spruce (Picea mariana [Mill.] B.S.P.) occurs throughout the hemiarctic zone along with white spruce and larch. Black spruce reproduces mostly vegetatively by layering. Population study of a clonal stand located in the Lake Minto area, northern Quebec, was undertaken to provide a better understanding of these populations. The smooth straight line trend of age, height, and diameter distributions indicated a stable population. The inverse J-shaped depletion model suggested by the population structure was similar to those described for southern stable tree populations. The inverse exponential model which best described the population implied a constant death rate through time. Clonal population growth was little disturbed by climatic change. Small variations (statistically not significant) that were still apparent in the growth curve could be related to more or less favorable growth periods. In the forest tundra, where disturbances are mostly linked to climatic changes, the opportunistic use of both vegetative and seed reproduction constituted an important asset for black spruce populations. Spatial growth pattern of the black spruce clone was determined using cartographic data related to above- and belowground components of the population. Because it reproduced mainly by layering, the black spruce clonal stand displayed a radial development. Older individuals were found near the middle of the stand while regeneration was most active on the border. Few living ramets occurred in the immediate center of the clonal stand while rotten roots and stems were found buried in the soil organic layer. These rotten structures were probably the remains of individuals that gave birth to the clone. Spatial distribution of above and belowground structure showed three units that resulted from the decay of branch connection between layerings. If the population were to expand, these units would be expected to develop either towards stability or towards extinction depending on topography and space available.
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.
Loss of leaf surface wax during winter could contribute to the extreme desiccation and leaf mortality previously observed for timberline conifers. For Picea engelmannii, leaf surface wax declined most rapidly for leaves above the snowpack as winter progressed, until there was approximately 60% less surface wax than for leaves protected by snow cover. Near the upper limit of the timberline ecotone (3400 m), leaves of P. engelmannii had less surface wax compared to individuals near the lower ecotone limit (3200 m), as well as smaller differences between windward versus leeward sides of the same shoot. Abies lasiocarpa leaves at 3400 m had the least amount of wax, while leaves of Pinus contorta at a lower, wind-exposed site (3100 m) had the greatest amount. Only small differences in wax quantities occurred on windward versus leeward leaves on P. contorta saplings, despite much higher mortality for windward leaves near the snow surface. From measurements on artificial wax cylinders placed in the field, an increase from 20 cm to 4 m in height above the snowpack would result in an estimated 90% decrease in surface wax erosion. Also, wax erosion as a function of height above the snow surface was similar to a typical vertical profile of the kinetic energy flux of blowing snow. A similar estimate using wax erosion from actual leaves showed that a similar decrease in cuticle erosion would result for an approximate decrease in elevation of about 230 m. Leaf surface wax erosion via blowing snow may be a primary force influencing the distribution of evergreen plants in the timberline ecotone of the central Rocky Mountains.
In the lower timberline ecotone in SE Wyoming, needle mortality was primarily due to winter wind and cuticle abrasion; death was frequent only in wind-exposed needles of flagged trees and surface needles of krummoholz mats. At higher elevations where only krummholz mats and a few flagged trees exist, mortality averaged >75% for needles unprotected by snow, regardless of wind exposure. Snow-covered needles had low mortality throughout the timberline ecotone. Winter death of naturally wind-exposed needles of Picea engelmannii occurred at <-4.5MPa water potential (<60% relative water content).-from Authors
Les, espèces actuelles de Conifères se développent conformément aux modèles de RAUH, MASSART, ATTIMS et MANGENOT. Cependant, les modalités, particulièrement nuancées, de l'expression de la plagiotropie conduisent souvent à des architectures intermédiaires entre plusieurs modèles. Cette originalité est peut-être liée au caractère archaïque de ce taxon.
The northernmost black spruce (Picea mariana) outpost in the Ungava Peninsula (arctic Quebec) is about 80 km north of the treeline. It is a confined (440 m2) low-growth krummholz stand with a fragmented cover of creeping ramets about 50 ± 15 cm high. The spruce patches probably belong to the same clone and resulted from lateral spread as suggested by the widespread distribution of connecting ramets. The krummholz stand developed in a sedge-shrub fen influenced by sheet flow promoting higher soil temperatures than nearby exposed permafrost soils. Stem analysis of the two largest ramets indicated slow horizontal, creeping growth during the 19th century, and relatively rapid upslope growth from 1880 to 1900 and vertical growth since the 1930s, most likely associated with recent warming. Periods of relatively fast lateral and vertical stem growth were recorded in the oblique sequences by larger annual rings below the stem apex; narrow and uniform annual rings developed during periods of slow stem growth. Because the outlier is at the arctic spruce line and far north from the treeline, the species probably established from long-distance seed transport.
Five crown forms of treeline white spruce are used to define open forest, forest-tundra, and tundra regions at Churchill, Manitoba, Canada. Shoot elongation of 25-, 50-, and 100-yr-old open forest trees was studied on a transect through an area of forest succession during 1983. During 1984, shoot elongation of different crown forms was examined relative to ground temperatures, air temperatures, and precipitation. Stem and branch elongation of open forest trees was greatest and occurred over the longest period of time, followed by forest-tundra trees, and tundra trees. The increasing density of spruce in the zone of invading open forest is associated with moss development that moderates the ground temperature, increasing the lower root zone temperature early in the growing season and maintaining lower temperatures in the upper root zone later in the season. Continued moss accumulation coincides with permafrost formation into the root base. On the forest-tundra and tundra, moss development is patchy. Early warming of the peat surface results in drying that impedes heat influx to the lower root zone. The temperatures in the lower root zone appear limiting to forest-tundra trees and restricting to tundra trees at the start of the growing season. Shoot elongation is over before the upper root zone becomes very hot. Treeline crown forms are grouped according to similar patterns of elongation and common types of development. The wind-abraded crown forms are caused by slow growth that leaves a tree exposed to wind effects for a longer period of time.
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.
Because of its ability to layer and to produce different phenotypes, black spruce (Picea mariana [Mill.] BSP.) develops a complex clonal structure ensuring its survival and longevity. Here we report tree stem development and demise of a black spruce clone at treeline over the last 500 yr. Since the 16th century, the apical meristems of the clonal spruce experienced three periods of stem development associated with brief warmings and two periods of stem decline corresponding to known cold spells of the Little Ice Age. Ortet development was particularly vigorous in the 16th century, while the two layered stems slowly developed in the late 17th century and in the 20th century, respectively. Stem decline appeared as a progressive process lasting for several decades in the form of a basipetal death-gradient along the bole amplified by the above/below snow-pack position. Stem elongation was possibly facilitated by lesser winter-snow abrasion and/or thicker snowpack. Clonal stem development may have important implications for spruce spread in the arctic tundra in a warmer world. 26 refs. 2 figs.
Studies on recruitment of white spruce from 1785 to present document the open forest succession at Churchill, Manitoba. Advance of trees onto land emerging from Hudson Bay due to isostatic uplift and permafrost intrusion has resulted in the development of two distinct systems. Open forest and forest-tundra are distinguished by seedling establishment, crown forms, and the growth patterns of the trees. Forest-tundra plots which demonstrate transition to open forest are documented. Where the two systems were established prior to the major climatic warming, they are resistant to invasion and there is little resultant change in the position of the treeline. Once started, population growth within the open forest system tends to be self generating and is little affected by subsequent climatic cooling. The mature open forest established prior to 1800 has shown a decline in seedling establishment during the climatic warming. These sites are characterized by extensive lichen cover.
This paper is the second of a two-part series analysing details of regional, hemispheric and global temperature change during the twentieth century. Based on the grid box data described in Part 1 we present global maps of the strength of regional temperature coherence measured in terms of the correlation decay length for both annual and seasonal mean data. Correlation decay lengths are generally higher for annual rather than seasonal data; higher in the Southern compared to the Northern Hemisphere; and consistently higher over the oceans, particularly the Indian and central north Pacific oceans. Spatial coherence is relatively low in all seasons over the mid to high latitudes of the Northern Hemisphere and especially low in summer over the northern North Atlantic region.
We also describe selected regional temperature series and examine the similarities between these and hemispheric mean data, placing emphasis on the nature of the relationships in different seasons. The Equatorial Indian and Atlantic oceans appear to be the best regional proxies for representing global mean temperatures. Temperature change in regions bordering the northern North Atlantic are less representative of hemisphere mean data, particularly in summer.
The implications of these results for the large-scale interpretation of high-frequency palaeoclimate proxies are discussed. Strictly, given the length of the instrumental data, our results apply to temperature variability on timescales up to, at most, 50 years. Traditional inferences on global mean temperature change based on regional proxies originating around the margins of the northern North Atlantic must be viewed with some caution. Information on temperature change over the oceans, especially the tropical oceans, is an important prerequisite for accurate portrayal of global mean change. Care must be taken when integrating the evidence of different climate proxies to respect the seasonality of the response. Warm-season temperatures are most atypical of the various seasonal averages, especially in the Northern Hemisphere. Inferring annual temperature change on the basis of summer-responsive data is highly questionable.
Recent homogenized near-surface temperature data over the land and
oceans of both hemispheres during the past 130 years are combined to
produce the first comprehensive estimates of global mean temperature.
The results show little trend in the nineteenth century, marked warming
to 1940, relatively steady conditions to the mid-1970s and a subsequent
rapid warming. The warmest 3 years have all occurred in the 1980s.
The Krummholz zone is at the upper limit of trees where prostrate and flag shaped forms are all that can survive. Snow and vegetation patterns are intimately related, while snow is itself shaped by wind. Wind snow vegetation interactions are reviewed.-K.Clayton
. Repeated crown condition surveys, 1974–94, of subalpine clonal groups of Norway spruce (Picea abies (L.) Karst.) were carried out in the Swedish Scandes. Complementary analyses concerned radial and vertical growth, sexual regeneration and range limit responses of other plant species. Significant defoliation of spruce progressed linearly over the period of study, reaching cumulative values of about 85%. It is inferred that defoliation was preconditioned by decreasing radial growth since the thermal climax in 1937 and was proximately initiated by the extremely cold winter of 1965/66 and paralleled by consistently declining radial growth and staggering vertical increase. It appears that severe and prolonged ground freezing invoked winter desiccation (xylem cavitation), extensive needle loss and reduced radial growth. Hypothetically, from circumstantial evidence, these processes are interrelated in a positive feedback system, implying increasing sensitivity to climatic stress and decreasing ability to take advantage of positive climatic anomalies. Thus, the total demise of the supranival stems is cautiously predicted, by linear regression of the 20-yr defoliation pattern, to be less than a decade ahead. The recession of P. abies, clearly relevant in a landscape perspective, conforms with analogous responses of Pinus sylvestris L. and Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman and a significant altitudinal range-limit retraction of certain silvine field-layer species. The structural development examined in this study concurs with long-term climate cooling and cold events and strongly contrasts with simulations of the performance of this system in response to a putative enhanced‘greenhouse’effect.
Trees were sampled from 26 black spruce (Piceamariana (Mill.) B.S.P.) stands across New Brunswick and within the boreal forest of Quebec to test the validity of the pipe model theory for sampling purposes. Trees were sampled from a wide range of ages, site qualities, densities, and social classes for determination of foliage biomass and sapwood cross-sectional areas at breast height and at the base of the live crown. A strong relationship was found between sapwood cross-sectional area and foliage biomass across the range of conditions. The y-intercepts for these regression equations, based on the untransformed data, were not significantly different from zero (P < 0.05). Partial correlation analysis indicated no significant relationship between the foliage biomass and tree age, site quality, or stand density but confirmed the strong relationship with sapwood cross-sectional area. Suppressed and intermediate social classes followed a similar relationship as dominant trees. Regression equations developed from this study were similar to those determined for spruce species and supported the use of sapwood cross-sectional area as a predictor of foliage biomass for black spruce.
Book review of the intergovernmental panel on climate change report on global warming and the greenhouse effect. Covers the scientific basis for knowledge of the future climate. Presents chemistry of greenhouse gases and mathematical modelling of the climate system. The book is primarily for government policy makers.
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.
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.
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.
Un tableau plié. Thèse (Ph.D.) -- Université Laval, 1991. Bibliogr. : f. 152-159. Microfiches du ms dactylographié.
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