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Water relations of adult Norway spruce (Picea abies L.) Karst) under soil drought in the Vosges mountains: Whole-tree hydraulic conductance, xylem embolism and water loss regulation
Abstract
Drought-induced changes in whole-tree hydraulic conductances (gL) were monitored throughout a growing season in a 30-year-old stand of Picea abies. gL was derived from concurrent measurements of leaf water potentials and sap flux densities through the trunk. Soil water deficits clearly reduced gL, the reduction being most likely located in the soil-root compartment of the soil-plant sap pathway. The decreases in gL did not result in large decreases in midday leaf water potentials because midday sap flux densities were reduced proportionally to gL. We therefore hypothesized that maximum sap flux densities in Picea abies are adjusted under dry conditions according to changes in whole-tree hydraulic conductances with effect of maintaining midday water potentials above the point of xylem dysfunction caused by water stress-induced cavitations. Relations hydriques chez l'épicéa commun (Picea abies (L) Karst) soumis à une sécheresse édaphique dans les Vosges : conductance hydraulique totale, embolie du xylème et régulation des pertes en eau. Les variations de conductances hydrauliques totales induites par une sécheresse ont été suivies tout au long d'une saison de végétation dans une parcelle de Picea abies âgés de 30 ans. gL a été calculé à partir de mesures simultanées de potentiels hydriques foliaires et de densités de flux de sève dans les troncs. Le déficit hydrique dans le sol a réduit nettement gL, cette réduction étant probablement localisée dans le compartiment sol-racine. La chute de gL n'a pas induit de diminution importante du potentiel hydrique minimum parce que les densités de flux de sève maximales ont été réduites proportionnellement à gL. Nous faisons l'hypothèse que les valeurs maximales journalières de flux de sève chez l'épicéa sont ajustées en fonction de la conductance hydraulique totale, ceci ayant pour effet de maintenir le potentiel hydrique minimum au dessus du point de dysfonctionnement xylémique causé par la cavitation des trachéïdes.
... However, Ψ MD has been found to vary largely between sites and years across European P. abies populations, suggesting that the stringency of leaf water potential regulation may depend partly on local environmental conditions. We measured in P. abies Ψ MD minima of − 2.35 and − 2.40 MPa in 2018 and 2019, which compares well with the minimum of − 2.50 MPa recorded by Lu et al. (1995) in the Vosges Mountains, France. In contrast, Schuldt et al. (2020) reported Ψ Leaf minima up to − 4.20 MPa in this species in the very dry summer of 2018 in certain Central European stands, which later succumbed to death. ...
... P. abies maintained in both years a larger HSM than the other species, partly due to its stringent stomatal regulation. It closed its stomata in 2018 already 0.5 MPa above the P 12 threshold, which agrees with results of other studies reporting a water-saving, cavitation-avoiding strategy in P. abies (Anfodillo et al. 1998;Lu et al. 1995;Sellin 2000). In our and some other studies, Ψ MD did not drop below − 2.5 MPa (e.g., Lu et al. 1995;Cochard 1992), thus avoiding critical xylem tensions even in a drought as severe as in 2018. ...
... It closed its stomata in 2018 already 0.5 MPa above the P 12 threshold, which agrees with results of other studies reporting a water-saving, cavitation-avoiding strategy in P. abies (Anfodillo et al. 1998;Lu et al. 1995;Sellin 2000). In our and some other studies, Ψ MD did not drop below − 2.5 MPa (e.g., Lu et al. 1995;Cochard 1992), thus avoiding critical xylem tensions even in a drought as severe as in 2018. However, P. abies may well suffer catastrophic hydraulic failure, as was observed in summer 2018 in regions such as Switzerland, where cavitation triggered P. abies dieback in subsequent weeks (Schuldt et al. 2020;Arend et al. 2021). ...
Key message
Norway spruce operates with larger hydraulic safety margins (HSM) than beech and Douglas-fir despite the known drought sensitivity of spruce, questioning a pivotal role of HSM in drought tolerance.
Abstract
The exceptional 2018/2019 drought exposed Central Europe’s forests to severe stress, highlighting the need to better understand stomatal regulation strategies and their relationship to xylem safety under extreme drought. We studied diurnal, seasonal, and inter-annual variation in stomatal conductance (gs) and leaf water potential (ΨLeaf) in co-occurring European beech (F. sylvatica), Norway spruce (P. abies), and Douglas-fir (P. menziesii) trees in the two summers and related them to hydraulic traits characterizing drought resistance. In 2018, F. sylvatica exhibited a continuous ΨLeaf decline from June to September, as is characteristic for an anisohydric strategy, while P. abies closed stomata early and reached the least negative ΨLeaf-values at the end of summer. P. menziesii showed low ΨLeaf-values close to P12 (the xylem pressure at onset of embolism) already in July. Both conifers closed stomata when approaching P12 and maintained low gs-levels throughout summer, indicative for isohydric regulation. In 2019, all three species showed a linear decline in ΨLeaf, but F. sylvatica crossed P12 in contrast to the conifers. The three species exhibited similar water potentials at turgor loss point (− 2.44 to − 2.51 MPa) and branch P50 (xylem pressure at 50% loss of hydraulic conductance; − 3.3 to − 3.8 MPa). Yet, F. sylvatica and P. menziesii operated with smaller hydraulic safety margins (HSM means: 0.79 and 0.77 MPa) than P. abies (1.28 MPa). F. sylvatica reduced leaf size and specific leaf area in 2019 and increased Huber value. Our species comparison during extreme drought contradicts the general assumption that conifers operate with larger HSMs than angiosperm trees. Contrary to expectation, P. abies appeared as hydraulically less vulnerable than Douglas-fir.
... The understanding of water transport in plants is based on the cohesive tension theory. Soil water deficits caused by an imbalance of water input (rainfall) and output (transpiration + evaporation) change the cohesion and tension of plant water, resulting in changes in hydraulic characteristics [71,72]. The cohesion tension theory is widely supported as an effective theory, consistent with the preponderance of data on water transport in plants [71]. ...
Seasonal precipitation variance significantly alters soil water content, potentially inducing water stress and affecting plant transpiration in semiarid deserts. This study explored the effects of environmental variables and hydraulic conductance on whole-tree transpiration (ET) in Mongolian pines (Pinus sylvestris var. mongolica) across different forest stages in the semiarid deserts of northern China. We measured ET using sap flow in mature (MMP), half-mature (HMP), and young (YMP) Mongolian pine plantations. Measurements included soil-leaf water potential difference (ΔΨ), atmospheric conditions, and soil moisture contents on sunny days, both in dry and wet periods. Seasonally variable rainfall distinctly affected soil moisture, correlating ET with photosynthetically active radiation (PAR) and vapor pressure deficit (VPD). Notable nocturnal sap flow occurred during the growing season. During the dry periods, both stomatal and hydraulic conductance influenced ET, whereas during the wet periods, stomatal conductance primarily regulated it. Discrepancies between predicted and measured ET were noticed: compared to the predicted ET, measured ET was lower during dry periods while higher during wet periods. Hydraulic conductance (KT) increased with tree height (H) and ΔΨ, although tall trees exhibited lower KT rates, suggesting that hydraulic compensation had occurred. This compensation, observed between 11:00 and 13:00, aligned with increased hydraulic resistance during dry periods. Decreasing hydraulic conductance intensified leaf water stress in dry periods, especially during the time when PAR and VPD were heightened, potentially increasing stomatal sensitivity to drought, promoting water conservation and plant survival. A linear relationship between predawn and midday leaf water potentials was noticed, indicating extreme anisohydric behavior across forest stages during both dry and wet periods. Although both stomatal and hydraulic conductance influenced ET during the dry period, MMP and YMP were more susceptible to drought conditions. Understanding these dynamics could help evaluate semiarid desert ecological functions for water conservation amidst uneven seasonal precipitation in Northern China.
... ANCOVA was used to test for significant differences in m and G cref of R. pseudoacacia between Changwu and Mizhi sites under different soil moisture conditions. The whole-tree hydraulic conductance of the soil to leaf pathway (K S-L , g m − 2 s − 1 MPa − 1 ) was calculated according to the Darcy equation (Cohen and Naor, 2002;Lu et al., 1996;Sperry, 2000): ...
... Norway spruce (Picea abies [L.] Karst.) is one of the most important tree species for forestry in Europe (Lu et al., 1996;Spiecker, 2003). Due to its high productivity and economic value (Klimo and Hager, 2000), spruce was widely cultivated outside its natural range and frequently in unsuitable conditions (Spiecker, 2003;Strělcováet al., 2013). ...
Introduction
Under ongoing climate change, more frequent and severe drought periods accompanied by heat waves are expected in the future. Under these conditions, the tree’s survival is conditioned by fast recovery of functions after drought release. Therefore, in the presented study, we evaluated the effect of long-term water reduction in soil on tree water use and growth dynamics of Norway spruce.
Methods
The experiment was conducted in two young Norway spruce plots located on suboptimal sites at a low altitude of 440 m a.s.l. In the first plot (PE), 25% of precipitation throughfall was excluded since 2007, and the second one represented the control treatment with ambient conditions (PC). Tree sap flow, stem radial increment, and tree water deficit were monitored in two consecutive growing seasons: 2015-2016, with contrasting hydro-climatic conditions.
Results
Trees in both treatments showed relatively isohydric behavior reflected in a strong reduction of sap flow under the exceptional drought of 2015. Nevertheless, trees from PE treatment reduced sap flow faster than PC under decreasing soil water potential, exhibiting faster stomatal response. This led to a significantly lower sap flow of PE, compared to PC in 2015. The maximal sap flow rates were also lower for PE treatment, compared to PC. Both treatments experienced minimal radial growth during the 2015 drought and subsequent recovery of radial growth under the more the humid year of 2016. However, treatments did not differ significantly in stem radial increments within respective years.
Discussion
Precipitation exclusion treatment, therefore, led to water loss adjustment, but did not affect growth response to intense drought and growth recovery in the year after drought.
... Due to their height, large trees tend to be more strongly exposed to radiation, wind and heat. In drought years, this exposure can cause more pronounced hydraulic stress than for their shorter neighbours (Lu et al., 1996;Bennett et al., 2015;Grote et al., 2016). This pattern was true for many of the morphological variables but not for the crown volume. ...
Adaptive silvicultural approaches intend to develop forests that can cope with changing climatic conditions. Just recently, many parts of Germany experienced 3 years of summer drought in a row (2018–2020). This study analysed the effects of this event on beech (Fagus sylvatica L.) in two regions in northern Bavaria, Germany. For this purpose, 990 beech trees were studied on 240 plots in drought-stressed forests. We examined trees of different social position and different size. Their morphology (e.g. tree height, crown volume) was recorded by laser scanning, and drought stress was quantified by tree core sample analyses. In addition to increment analyses, the δ13C signal was determined by year.
Results show that the dominant tree collective was particularly affected by the drought. They still managed to perform well in 2018, but the radial growth decreased significantly in 2019 and 2020, partly resembling the performance values of subordinate trees. Subordinate trees, on the other hand, provide some consistency in growth during drought years. The drought was so severe that the effects of competition on tree growth began to disappear. The difference in growth of two geographically distinct study areas equalized due to drought. With continuing drought, increasing levels of the δ13C signal were detected. Similar patterns at different δ13C levels were found across the social positions of the trees. The influence of tree morphological variables on tree resistance to drought showed no clear pattern. Some trends could be found only by focusing on a data subset.
We conclude that the intensity of the 2018–2020 drought event was so severe that many rules and drivers of forest ecology and forest dynamics (social position, morphology and competition) were overruled. The influence of morphological differences was shown to be very limited. The weakening of dominant trees could potentially be no longer linear and drought events like the one experienced in 2018–2020 have the potential of acting as tipping points for beech forests.
... Site conditions like those in places at low altitudes where spruce has been artificially introduced, adversely affect the physiological functioning of spruce trees and diminish their ability to defend against bark beetles and other harmful organisms, especially in drought years [148][149][150][151][152][153], which seem to have become increasingly common in this area in recent decades [154]. Consequently, during the years 2011-2012, when the Standardized Precipitation Index (SPI-12) averaged at country level was between −1.5 and −1.0 [155], we noted that spruce trees at low altitudes were so dehydrated that they did not release any resin when the beetles penetrated the bark. ...
Ips duplicatus (Sahlberg, 1836), Xylosandrus germanus (Blandford, 1894) and Neoclytus acuminatus (Fabricius, 1775) are invasive species reported in Romania, but their current distribution is poorly known. The research aim was to provide new information on this issue. A survey was conducted over the period 2015–2017 in 82 locations, using flight-interception traps and bottle traps, baited with different attractants. Data obtained in our other unpublished studies were also taken into account. A total of 35,136 I. duplicatus beetles were collected in 30 survey locations. The highest captures were in the log yards of some factories processing logs of Norway spruce (Picea abies (L.) H. Karst.). Considering all known records so far, most of these are in the eastern part of Romania, where an outbreak took place during the years 2005–2014, mainly in spruce stands growing outside their natural range. During the survey, 4259 specimens of X. germanus were collected in 35 locations, but in our other studies the species was found in 13 additional places. It was collected at altitudes of 18–1200 m, and the largest catches were from beech stands, growing at 450–950 m. N. acuminatus was found in only six locations, in the western and southern parts of the country, at low altitudes, in tree stands composed of Fraxinus excelsior L., Quercus spp. and other broadleaf species, as well as in broadleaf log yards. The results suggest that I. duplicatus is established in most parts of the Norway spruce’s range, X. germanus is still spreading in the country, with some areas having quite high populations, while N. acuminatus is present only in the warmest regions of the country.
... e genetic diversity and the population differentiation reported by many studies between distant populations of the Middle and High Atlas of Cedrus atlantica in Morocco are in agreement with this idea [32,33,61,74,75]. e capacity of trees to survive aridity depends on a group of physiological and morphological changes including stomatal closure [76], inhibition of shoot growth [77], and reduction of root longevity [78]. Similarly, EL Amrani and Bendriss Amraoui [79] have recently found that the continuous and partial mechanical barriers combined with the low availability of water reduce the aerial part growth and the main root length of C. atlantica seedlings. ...
Atlas cedar (Cedrus atlantica (Endl.) G. Manetti ex Carrière) is an endemic species in the mountains of North Africa that is attracting international interest in its use in the reforestation of degraded ecosystems. is study aims to investigate and evaluate the morphoanatomical characteristics of needles of four cedar populations localized in the Middle and High Atlas Mountains. Descriptive statistics, analysis of variance (ANOVA), descriptive power, scatter-plot of the discrimination function, scatter-plot of discrimination, and dendrogram of the closest Euclidean distances were made on traits. e results of the linear model of ANOVA nested as population and tree within population suggest the differences statistically significant for the traits measured at a different level. Among these traits, the length of the needle, the width of a vascular bundle including endodermis, and thickness of the wall of hypodermis cell revealed the highest discriminating characters among populations of C. atlantica from the Middle and High Atlas and between the populations of the Middle Atlas. e agglomeration of populations over short Euclidean distances also showed a higher level of differentiation between two ecotypes of C. atlantica not very geographically distant in the Atlas Mountains of Morocco. e ecotype belonging to Aït Oufella and Aït Ayach confers this species a place of choice in the projects of revalorization of the Mediterranean populations, especially in semiarid areas.
Against the background of the ongoing climate change, forest science and Central-European forest management need information about tree species that are suitable of forming resistant and resilient multispecific and multipurpose forest stands. Non-native species are also considered for this purpose. One of these species may be the Douglas fir (Pseudotsuga menziesii), which has been introduced from western North America to Europe in the 19th century. The aim of this review is to compile recent scientific results that are relevant in the context of the use of the Douglas fir in future Central-European forestry to create a foundation for science-based decision-making, and to formulate future research tasks on that basis.
Due to its high growth rates and low susceptibility to needle cast fungi, the Douglas fir's coastal variety (P. menziesii var. menziesii, syn.: var. viridis) is being preferred in Central-European silviculture. It is competitively superior to all indigenous Central-European forest tree species due to rapid height growth and efficient shading of competing plants; high drought tolerance, water use efficiency and resilience after drought stress; and efficient water and nutrient uptake and high nutrient use efficiency. In Central Europe, the Douglas fir is currently less threatened by pests and pathogens than are the indigenous Norway spruce and Scots pine. Its litter is better decomposable compared to native conifers, but increased nitrification, especially at sites with former agricultural use or under anthropogenic nitrogen deposition, or lower nitrate uptake rates due to lower nitrogen demand of the species can result in enhanced soil acidification, aluminum mobilization and leaching of nitrate, “base” cations and aluminum compounds. Mixtures of Douglas fir and native trees may be particularly effective in sequestrating carbon and nitrogen in the soil. Negative effects of the Douglas fir on the plant diversity of a given community seem to be small or even non-existent, but its interactions with the fauna is more ambiguous. The majority of nature conservation organizations recommend avoiding Douglas-fir monocultures and restricting the fraction of Douglas-fir admixtures to stands of native tree species to 30% at maximum in considering current regulations for nature protection. Future research tasks comprise monitoring of the Douglas-fir provenances in cultivation and of introduced pests and pathogens, investigations of responses to consecutive and combined stress factors and of the species' invasiveness at dry sites as well as comparative long-term studies on interactions with animal communities and on matter flux and turnover in the ecosystems.
Black locust (Robinia pseudoacacia) is widely planted throughout semiarid and subhumid regions of the Loess Plateau of China. Determining the changes in transpiration of this species in different climatic areas is important for revealing the acclimation mechanism of black locust and developing suitable forest management practices, particularly in the context of global climate change. Here, sap flow and canopy conductance of black locust plantation trees in semiarid (Yan’an) and subhumid (Yongshou) sites were quantified using Granier-type thermal dissipation probes and concurrent environmental observations from 2012 to 2017. Several physiological parameters were measured throughout the growing season. The results showed that sap flow was correlated with phenological factors across seasons within a year. However, interannual changes in sap flow were affected mainly by the reference evapotranspiration (ET0) at the Yongshou site, and jointly by precipitation (P), soil water content, and P/ET0 at the Yan’an site. Sap flow response to meteorological factors showed less discrepancy between pre- and post-rainfall periods at the Yan’an site. Moreover, canopy conductance fluctuated less with a wider range of vapor pressure deficit (VPD) and the slope of canopy resistance as a function of VPD was lower, indicating relatively lower sensitivity of stomatal conductance to environmental factors in Yan’an site. Physiological parameters, except for predawn leaf water potential, were significantly different between the two sites. The results suggested that black locust tended to reduce transpiration, modify leaf morphology, and improve water use efficiency to enhance its adaptability to the dryer site. The species changes stomatal regulation characteristics and general growth rate to acclimatize to distinct water habitats.
Resistances to the flow of water through young potted apple trees were estimated by measuring the transpiration rate of trees with and without root systems. Root system resistances were obtained by difference. Whole-plant resistances were of the order 10 × 10¹³ Pa s m⁻³ and there was some evidence that root resistances (Rr) varied with transpiration rate; the ratio Rr:Rx (where Rx is resistance to water flow in the stem system) altered from 2:1 at relatively high transpiration rates to 1:1 at lower rates.
The trunk of a 9-year-old orchard tree (trunk diameter ˜7 cm, height ˜2.5 m) was cut under water and estimates of the flow resistances in this tree were obtained. These were much lower than the resistances to flow in the potted trees.
Capacitance (defined as the change in stored water content per unit change in plant water potential) values were calculated for the small trees and the large tree from measurements of weight and water potential changes after the trees were removed from water. They were very similar on a weight basis (approx. 2.0 × 10⁻⁸ kg kg⁻¹ Pa⁻¹). Leaf capacitance values (˜1 × 10⁻⁸ kg Pa⁻¹ m⁻²) were also obtained.
Stomatal conductances decreased with water potential and increased with short-wave radiation, but the relationships were not definitive. Estimates of boundary layer conductance in a greenhouse (very low wind speeds) were of the same order (˜5 mm s⁻¹) as values obtained previously.
Abstract Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m−3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.
The reactions of sessile oak (Quercus petraea (Mattuschka) Liebl.) to drought were studied under natural conditions in a 32-year-old stand near Nancy (northeastern France) during the summers of 1989 (strongly rain deficient) and 1990. A plot of five trees was subjected to imposed water shortage, while a group of irrigated trees was used as a control. Measurements of xylem sap flows and water potential enabled the computation of plot transpiration, canopy conductance, and specific hydraulic conductance in the soil-tree continuum. Stomatal conductance was measured directly with a porometer. Specific hydraulic conductance of our oaks was of the same order of magnitude as that reported for other species. It decreased significantly during spring because of a time lag between cambial growth and leaf area expansion. Measured transpiration was close to potential evapotranspiration, except during days with high vapor pressure deficits, which promoted stomatal closure in the absence of soil water deficits. Imposed drought caused predawn leaf water potentials to reach values as low as -2.0 MPa and a progressive decline in hydraulic conductance, which was probably attributable to modifications in hydraulic properties at the soil-root interface. This gradual decline in conductance was attributed to their deep rooting (1.40 m). This study revealed that Q. petraea may be considered as drought tolerant because of adaptations like deep rooting, efficient and safe xylem sap transport, maintenance of significant stomatal conductance, and significant transpiration, even during strong drought stress.
Changes in pre-dawn water potential, soil-to-leaf hydraulic conductance, and leaf diffusive conductance were well correlated in oak trees during natural drying cycles in the field. Maximum daily leaf conductance was highly correlated with pre-dawn water potential for blackjack oak (Quercus marilandica Muenchh.) and northern red oak (Quercus rubra Lam.), while leaf water potential at the time of measurement of leaf conductance was not significantly correlated with leaf conductance. Soil-to-leaf hydraulic conductance was positively correlated with pre-dawn water potential and with maximum daily leaf conductance in both species. The relationships between leaf conductance, soil-to-leaf hydraulic conductance, and pre-dawn water potential were similar for blackjack oak at three sites along a moisture gradient, despite large differences in those values across sites. Soil-to-leaf hydraulic conductance declined more in co-occurring northern red oak than blackjack oak as pre-dawn water potential in both species declined from near zero to -1.0 MPa. These results suggest that bulk leaf water potential may not be as important a limitation to gas exchange in the field as soil/root water potential and/or vapour pressure gradient.
The present study aims at characterizing plant water status under field conditions on a daily basis, in order to improve operational predictions of plant water stress. Ohm's law analog serves as a basis for establishing daily soil-plant relationships, using experimental data from a water-limited soybean crop: 227-1. The daily transpiration flux, T, is estimated from experimental evapotranspiration data and simulated soil evaporation values. The difference, 227-2, named the effective potential gradient, is derived from i) the midday leaf potential of the uppermost expanded leaves and ii) an effective soil potential accounting for soil potential profile and an effectiveness factor of roots competing for water uptake. This factor is experimentally estimated from field observation of roots. G is an apparent hydraulic conductance of water flow from the soil to the leaves. The value of the lower potential limit for water extraction, required to assess the effective soil potential, is calculated with respect to the plant using the predawn leaf potential. It is found to be equal to –1.2 MPa. It appears that over the range of soil and climatic conditions experienced, the daily effective potential gradient remains constant (1.2 MPa), implying that, on a daily basis, transpiration only depends on the hydraulic conductance. The authors explain this behaviour by diurnal variation of osmotic potential, relying on Morgan's theory (1984). Possible generalization of the results to other crop species is suggested, providing a framework for reasoning plant water behaviour at a daily time step.
The water-cohesion-tension insufficiency hypothesis, which suggests the primary reason for forest decline produced by air pollutants to be an increase of cavitational rupture and embolism of negative pressure water within the water carrying tracheids and vessels of the tree xylem, has been examined in detail. A thermodynamic model of water under tension is presented which is based on the van der Waals equation in which the interaction between water molecules subject to negative pressure has been elaborated and tested by computer simulation. The hypothesis explains typical reactions of affected trees (e.g. formation of “stork nests”, “anxiety” sprouts, reduction of leaf and needle sizes, shedding of needles) as adaptations to decreased tensile strength of water in tree xylem water conduits. In support of the proposed mechanism a comparative study of tree diseases has been made in which rupture of sap flow (cavitation) is caused by fungi (e.g. Dutch elm disease). It is shown that symptoms occur comparable to those trees damaged by air pollution.The presented hypothesis predicts that the forest decline is basically proportional to the product of dry deposition of pollutants (particulate matter, ozone, hydrogen peroxide and acidic gas) and the water stress experienced by trees. A comparative study of forest decline in Europe and Japan, as well as an evaluation of preliminary results of large scale experimental research on forest decline in Germany suggests that the observed patterns can qualitatively be explained. Possible mechanisms for pollution induced cavitation and embolism in xylem water conduits are discussed and necessary experimental research for verification of the hypothesis outlined.
. The mechanism of water-stress-induced xylem embolism was studied in three species of conifers: Abies balsamea (L.) Mill., Picca rubens Sarg, and Juniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air embolism. Abics balsamea and Picca rubens began to embolize at tensions between 2 and 3 MPa and were completely non-conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to embolize at 4 and still retaining approximately 10% conductivity at 10 MPa. As with a previous study of the vessel-bearing Accr saccharum Marsh., a brief perfusion of branch segments with an oxalic acid and calcium solution (10 and 0.1 mol m−3. respectively) increased the vulnerability of the xylem to embolism; this was especially pronounced in Abies balsamea. In order to test whether embolism was caused by aspiration of air into functional tracheids from neighbouring embolized, ones (the ‘air-seeding’hypothesis), hydrated branch segments were injected with air at various pressures and measured for embolism. Results supported the air-seeding hypothesis because the relationship between injection pressure and embolism for both native and oxalic-calcium-treated segments was essentially the same as for embolism induced by xylem tension. Structural and experimental evidence suggested the air seeding occurred through inter-tracheid pit membranes when the thickened torus region of the membrane became displaced from its normal sealing position over the pit aperture. Thus, the embolism-inducing tension may be a function of pit membrane flexibility. This tension is of ecological significance because it reflects to some extent the range of xylem tensions to which a species is adapted.
A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.
The extent to which stomatal conductance (gs) was capable of responding to reduced hydraulic conductance (k) and preventing cavitation-induced xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. from 1 to 18d after shoot injection we measured midday transpiration rate (E), gs, and xylem pressure (Yp-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2-3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and gs. The k measurement was expressed per unit leaf area (k-1, leaf-specific conductance). Leaves measured within 2d of shoot injection showed reduced gs and E relative to non-injected controls, and both parameters were strongly correlated with k-1. At this time, there was no difference in leaf Yp-xylem between injected shoots and controls, and leaf Yp-xylem was not significantly different from the highest cavitation-induced pressure (Yp-cav) in the branch xylem (-.43 +- 0.029 MPa, n = 8). Leaves measured 7-18d after shoots were injected exhibited a partial return of g-s and E values to the control range. This was associated with a decrease in leaf Yp-xylem below Yp-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.