Dr. Kerstin Treydte

Ongoing climate change is having profound impacts on the growth and distribution of trees worldwide. However, there remain substantial gaps in our understanding of how environmental factors influence tree-growth responses to climate at larger scales, which is critical for identifying regions susceptible to the impacts of climate change. In this study, we aimed to reveal the main environmental factors that determine the spatial heterogeneity of tree-growth sensitivity to temperature, precipitation, and drought across temperate forests in Northeast Asia (30–45° N, 124–146° E). Utilising an extensive tree-ring network of 101 chronologies of 22 tree species, across 79 sites, we found that local climate, and especially climate water deficit, plays a dominant role in shaping the spatial heterogeneity of tree-growth sensitivity, while geospatial variables were less important. Our analysis revealed a pervasive pattern of increased tree susceptibility to drought across Northeast Asian forests. Specifically, at sites experiencing high climate water deficit, tree growth was consistently reduced across both broadleaved and coniferous species under conditions of low precipitation, elevated temperatures, and increased dryness during the growing season. Our findings suggest that ongoing climate warming may further negatively affect tree-growth performance, especially at drier sites, across Northeast Asian forests.

Given growing concerns about global climate change, it is critical to understand both historical and current shifts in the hydroclimate, particularly in regions critically entwined with global circulation. The Tibetan Plateau, the Earth’s largest and highest plateau, is a nexus for global atmospheric processes, significantly influencing East Asian hydroclimate dynamics through the synergy of the Asian Monsoon and the Westerlies. Yet, understanding historical and recent hydroclimate fluctuations and their wide-ranging ecological and societal consequences remains challenging due to short instrumental observations and partly ambiguous proxy reconstructions. Here, we present a precisely-dated 3476-year precipitation reconstruction derived from tree-ring δ¹⁸O data on the Tibetan Plateau, representing one of the few multi-millennia-long annually-resolved terrestrial δ¹⁸O records to date. Our findings reveal that the 20^th century drought extremes are severe within the past three millennia, and likely linked to the weakening of both the Asian Monsoon and Westerlies due to anthropogenic aerosol emissions. Additionally, our analyses identified three distinct stages (110 BC–AD 280, AD 330–770 and AD 950–1300) characterized by shifts toward arid hydroclimate conditions, corresponding to significant social unrest and dynasty collapses, which underscores the potential societal impacts of severe hydroclimatic shifts.

The world's longest tree-ring chronology comprises thousands of oak and pine series from Germany and continuously covers the Holocene back to 12,325 cal BP. A lack of relict wood from the Younger Dryas cold reversal ca. 12,900-11,700 cal BP, however, challenges the extension of this absolutely dated ring width record further back in time. Here, we combine 646 high-resolution stable oxygen isotope and 795 radiocarbon measurements from subfossil pines that grew during the Younger Dryas at three different sites near Zurich, Switzerland, to extend the record. Coherency of the oxygen isotope variations secures internal crossdating, and radiocarbon wiggle-matching places the final 425-year-long ring-width chronology between 12,716 and 12,292 cal BP with an uncertainty of ±8 years. Our study describes an important step towards annual dating precision further into the Late Glacial period.

In ecohydrology, water isotopologues are used to assess potential sources of root water uptake by comparing xylem water signatures with source water signatures. Such comparisons are affected by the variability and uncertainty of the isotope signatures of plant water and water sources. The tree-scale and stand-scale variabilities of the isotope signatures in stem xylem water are often unknown but are important for sampling design and uncertainty estimation in assessing the sources of tree water uptake. Here, we quantified tree-scale and stand-scale variabilities of xylem water isotope signatures in beech, oak and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub-daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotope signatures at different heights along the stem suggests that water residence times within trees need to be considered, along with their effects on the isotope signatures in different compartments (stem, branches, leaves). Further, concerning the hydrogen signatures, we found height- and species-specific offsets (SW-excess δ²H). Stem xylem water's tree-scale variability was similar in magnitude to its stand-scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem water from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and facilitate more accurate source water attribution.

Oxygen (δ¹⁸O) and hydrogen (δ²H) stable isotope ratios are tightly coupled in precipitation and, albeit damped, in leaf water, but are often decoupled in tree-ring cellulose. The environmental and physiological conditions in which this decoupling occurs are not yet well understood. We investigated the relationships between δ¹⁸O and δ²H and tree-ring width (TRW), tree crown volume, tree age and climate in silver fir and Douglas-fir and found substantial differences between δ¹⁸O and δ²H. Overall, δ¹⁸O–δ²H correlations were weak to absent but became significantly negative under high summer vapour pressure deficit (VPD). δ¹⁸O and δ²H had positive and negative nonlinear relationships with TRW, respectively, with clear relationships at the site and tree levels for silver fir and, to a lesser extent, for Douglas-fir. Age trends for silver fir were weakly negative in δ¹⁸O but positive in δ²H. Tree crown volume and δ¹⁸O or δ²H had no significant relationships. Most strikingly, δ¹⁸O strongly depended on spring climate (precipitation and VPD), whereas δ²H depended on summer climate (temperature and VPD) for both species. Our study shows that the δ¹⁸O–δ²H decoupling in tree-ring cellulose in two temperate conifer species could be highlighted by their contrasting relationships to climate and tree intrinsic variables (TRW, age).

Disentangling the factors influencing the climate sensitivity of trees is crucial to understanding the susceptibility of forests to climate change. Reducing tree-to-tree competition and mixing tree species are two strategies often promoted to reduce the drought sensitivity of trees, but it is unclear how effective these measures are in different ecosystems. Here, we studied the growth and physiological responses to climate and severe droughts of silver fir and Douglas-fir growing in pure and mixed conditions at three sites in Switzerland. We used tree-ring width data and carbon (δ¹³C), oxygen (δ¹⁸O) and hydrogen (δ²H) stable isotope ratios from tree-ring cellulose to gain novel information on water relations and the physiology of trees in response to drought and how tree species mixture and competition modulate these responses. We found significant differences in isotope ratios between trees growing in pure and mixed conditions for the two species, although these differences varied between sites, e.g. trees growing in mixed conditions had higher δ¹³C values and tree-ring width than trees growing in pure conditions for two of the sites. For both species, differences between trees in pure and mixed conditions regarding their sensitivity to temperature, precipitation, climatic water balance and vapor pressure deficit were minor. Furthermore, trees growing in pure and mixed conditions showed similar responses of tree-ring width and isotope ratios to the past severe droughts of 2003, 2015 and 2018. Competition had only a significantly negative effect on δ¹³C of silver fir, which may suggest a decrease in photosynthesis due to higher competition for light and nutrients. Our study highlights that tree species mixture may have only moderate effects on the radial growth and physiological responses of silver fir and Douglas-fir to climatic conditions and that site condition effects may dominate over mixture effects.

Understanding how trees prioritize carbon gain at the cost of drought vulnerability under severe drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of severe drought in 29 mature white spruce (Picea glauca [Moench] Voss) families grown in a common garden trial. During the drought episode, the majority of families displayed decreased growth and exhibited either sustained or increased intrinsic water-use efficiency (iWUE), which was largely influenced by reduced stomatal conductance as revealed by the dual carbon‑oxygen isotope approach. Different water-use strategies were detected within white spruce populations in response to drought conditions. Our results revealed intraspecific variation in the prevailing physiological mechanisms underlying drought response within and among populations of Picea glauca. The presence of different genetic groups reflecting diverse water-use strategies within this largely-distributed conifer is likely to lessen the negative effects of drought and decrease the overall forest ecosystems' sensitivity to it.

Cellulose of tree rings is often assumed to be predominantly formed by direct assimilation of CO₂ by photosynthesis and consequently can be used to reconstruct past atmospheric ¹⁴C concentrations at annual resolution. Yet little is known about the extent and the age of stored carbon from previous years used in addition to the direct assimilation in tree rings. Here, we studied ¹⁴C in earlywood and latewood cellulose of four different species (oak, pine, larch and spruce), which are commonly used for radiocarbon calibration and dating. These trees were still growing during the radiocarbon bomb peak period (1958-1972). We compared cellulose ¹⁴C measured in tree-ring subdivisions with the atmospheric ¹⁴C corresponding to the time of ring formation. We observed that cellulose ¹⁴C carried up to about 50% of the atmospheric ¹⁴C signal from the previous 1-2 years only in the earlywood of oak, whereas in conifers it was up to 20% in the earlywood and in the case of spruce also in the latewood. The bias in using the full ring of trees growing in a temperate oceanic climate to estimate atmospheric ¹⁴C concentration might be minimal considering that earlywood has a low mass contribution and that the variability in atmospheric ¹⁴C over a few years is usually less than 3‰.

Increases in temperatures and atmospheric CO₂ concentration influence the growth performance of trees worldwide. The direction and intensity of tree growth and physiological responses to changing climate do, however, vary according to environmental conditions. Here we present complex, long-term, tree-physiological responses to unprecedented temperature increase in East Asia. For this purpose, we studied radial growth and isotopic (δ¹³C and δ¹⁸O) variations using tree-ring data for the past 100 yr of dominant Quercus mongolica trees from the cool-temperate forests from Hallasan, South Korea. Overall, we found that tree stem basal area increment, intercellular CO₂ concentration and intrinsic water-use efficiency significantly increased over the last century. We observed, however, short-term variability in the trends of these variables among four periods identified by change point analysis. In comparison, δ¹⁸O did not show significant changes over time, suggesting no major hydrological changes in this precipitation-rich area. The strength and direction of growth–climate relationships also varied during the past 100 yr. Basal area increment (BAI) did not show significant relationships with the climate over the 1924–1949 and 1975–1999 periods. However, over 1950–1974, BAI was negatively affected by both temperature and precipitation, while after 2000, a temperature stimulus was observed. Finally, over the past two decades, the increase in Q. mongolica tree growth accelerated and was associated with high spring–summer temperatures and atmospheric CO₂ concentrations and decreasing intrinsic water-use efficiency, δ¹⁸O and vapour pressure deficit, suggesting that the photosynthetic rate continued increasing under no water limitations. Our results indicate that the performance of dominant trees of one of the most widely distributed species in East Asia has benefited from recent global changes, mainly over the past two decades. Such findings are essential for projections of forest dynamics and carbon sequestration under climate change.

The effects of rising atmospheric CO₂ concentrations (C_a) with climate warming on intrinsic water-use efficiency and radial growth in boreal forests are still poorly understood. We measured tree-ring cellulose δ¹³C, δ¹⁸O, and tree-ring width in Larix dahurica (larch) and Betula platyphylla (white birch), and analyzed their relationships with climate variables in a boreal permafrost region of northeast China over past 68 years covering a pre-warming period (1951–1984; base period) and a warm period (1985–2018; warm period). We found that white birch but not larch significantly increased their radial growth over the warm period. The increased intrinsic water-use efficiency in both species was mainly driven by elevated C_a but not climate warming. White birch but not larch showed significantly positive correlations between tree-ring δ¹³C, δ¹⁸O and summer maximum temperature as well as vapor pressure deficit in the warm period, suggesting a strong stomatal response in the broad-leaved birch to temperature changes. The climate warming-induced radial growth enhancement in white birch is primarily associated with a conservative water-use strategy. In contrast, larch exhibits a profligate water-use strategy. It implies an advantage for white birch over larch in the warming permafrost regions.

The vapor pressure deficit reflects the difference between how much moisture the atmosphere could and actually does hold, a factor that fundamentally affects evapotranspiration, ecosystem functioning, and vegetation carbon uptake. Its spatial variability and long-term trends under natural versus human-influenced climate are poorly known despite being essential for predicting future effects on natural ecosystems and human societies such as crop yield, wildfires, and health. Here we combine regionally distinct reconstructions of pre-industrial summer vapor pressure deficit variability from Europe’s largest oxygen-isotope network of tree-ring cellulose with observational records and Earth system model simulations with and without human forcing included. We demonstrate that an intensification of atmospheric drying during the recent decades across different European target regions is unprecedented in a pre-industrial context and that it is attributed to human influence with more than 98% probability. The magnitude of this trend is largest in Western and Central Europe, the Alps and Pyrenees region, and the smallest in southern Fennoscandia. In view of the extreme drought and compound events of the recent years, further atmospheric drying poses an enhanced risk to vegetation, specifically in the densely populated areas of the European temperate lowlands.

Climate change poses a major threat to global forest ecosystems. In particular, rising temperatures and prolonged drought spells have led to increased rates of forest decline and dieback in recent decades. Under this framework, forest edges are particularly prone to drought-induced decline since they are characterized by warmer and drier micro-climatic conditions amplifying impacts of drought on tree growth and survival. Previous research indicated that forest-edge Scots pine trees have a higher growth sensitivity to water availability compared to the forest interior with consequent reduction of canopy greenness (remotely sensed NDVI) and higher mortality rates. Yet, the underlying physiological mechanisms remain largely unknown. Here, we address this knowledge gap by comparing stable carbon isotope signatures and wood anatomical traits in annual rings of trees growing at the forest edge vs. the forest interior and between trees that either survived or died during the extreme drought of 2015. Our analyses suggest that the exposure to drought of forest-edge Scots pine likely results in a reduction of stomatal conductance, as reflected by a higher δ¹³C of stem wood, thinner cell walls, and lower mean ring density. Moreover, we found dead trees to feature larger mean hydraulic lumen diameters and a lower cell-wall reinforcement, indicating a higher risk to suffer from cavitation. In conclusion, the typically drier micro-climatic conditions at the forest edge seem to have triggered a larger reduction of stomatal conductance of Scots pine trees, resulting in a lower carbon availability and significantly altered wood anatomical properties under an increasingly drier climate.

The Qinling Mountains (Qinling Mts.) are characterized as a distinct transition of bioclimatic zones along the north-south geographic boundary in China. Although ongoing global warming may affect the growth of most tree species across elevation gradients, the effect of modern warming on the stable hydrogen isotopes of tree-ring wood is still unknown. In this study, we developed early- and late-wood δ²H chronologies from 1900 to 2018 CE of lignin methoxy groups (δ²H_LM) at 700-m intervals from 1700 to 3100 m a.s.l. along the elevation gradient in Mount Taibai (Mt. Taibai), the peak of the Qinling Mts. The δ²H_LM values of the earlywood were relatively enriched in ²H compared to the latewood, and an elevation-dependent depleting trend along the altitudinal gradient was observed. Comparing the δ²H_LM values with modelled δ²H values of precipitation (δ²H_Pre) of the sample sites, similar isotopic patterns can be obtained. The reconstructions of δ²H_Pre values confirm the strong coherence between the isotopic composition of the source water and the methoxy groups in the growing season. The consistency of δ²H_LM chronologies between the early- and late-wood at the higher sites was stronger than that at the lower sites. The additional lagged effect and climate response revealed significant seasonal and altitudinal differences. At the timberline site, δ²H_LM values of earlywood were mainly related to the January and February temperatures, whereas δ²H_LM values of latewood correlated most strongly with temperature from July to September, and both values were controlled by the amount of total precipitation. In addition, the combined new δ²H_LM chronologies, based on the lag-1 autocorrelation and the weighted average of the current year earlywood and previous latewood, could be used to merge the corresponding temperature signals from the middle- and low-elevation sites. Altitudinal correlations of elevation offset between the annual δ²H_LM chronologies and interpolations of precipitation and temperature suggested that the δ²H_LM values at higher elevations better represent regional climatic changes. Therefore, topographic effects such as elevation differences should be considered in large-scale applications of δ²H_LM values in future studies.

• The oxygen isotope composition (δ¹⁸O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways.
• We assessed the relative contribution of seasonal needle and xylem water δ¹⁸O variations to the intra-annual tree-ring cellulose δ¹⁸O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ¹⁸O measurements of soil water, needle water, and twig xylem water with intra-annual δ¹⁸O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling.
• Intra-annual cellulose δ¹⁸O values resembled source water δ¹⁸O mean levels better than needle water δ¹⁸O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (p_ex). Maximum p_ex values were achieved in August and imprinted on sections at 50–75% of the ring. High p_ex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ¹⁸O variability, we estimated a limited Péclet effect at both sites.
• Due to a variable p_ex, source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ¹⁸O variations, potentially masking signals coming from needle-level processes.

The effects of tree age-related intrinsic water-use efficiency (iWUE) under climate change such as warming and rising atmospheric CO₂ concentrations (C_a) on tree growth are still poorly understood. We measured tree-ring δ¹³C, δ¹⁸O, tree-ring width, and their relationships with climate variables, calculated iWUE for relatively young (90 ± 5 yr) and mature (230 ± 40 yr) Larix dahurica trees in a boreal permafrost region in northeastern China. The period of tree-ring measurements covering 70 years includes a pre-warming period (1951-1984; base period) and a warm period (1985-2018; warm period). The growth rate of both mature and young trees did not change with time over the base period, while the mature trees but not the young ones significantly increased the growth rate with time over the warm period. The iWUE of mature trees consistently increased from 1951 to 2018, whereas that of the young trees increased only with time during the warm period from 1985 to 2018. The tree-ring δ¹³C and δ¹⁸O were not correlated for both young and mature trees over both base and warm periods, except for a significantly positive correlation between tree-ring δ¹³C and δ¹⁸O for mature trees during the warm period. We found that mature trees were more sensitive to C_a than young trees. Our results suggest that mature trees have a more ‘conservative’ (low stomatal conductance (g_s), constant assimilation rate (A)) water use strategy than young trees (constant g_s, high A) in the permafrost regions in a warming world.

Stable isotope ratio analysis of tree rings has been widely and successfully applied in recent decades for climatic and environmental reconstructions. These studies were mostly conducted at an annual resolution, considering one measurement per tree ring, often focusing on latewood. However, much more information could be retrieved with high-resolution intra-annual isotope studies, based on the fact that the wood cells and the corresponding organic matter are continuously laid down during the growing season. Such studies are still relatively rare, but have a unique potential for reconstructing seasonal climate variations or short-term changes in physiological plant properties, like water-use efficiency. The reason for this research gap is mostly technical, as on the one hand sub-annual, manual splitting of rings is very tedious, while on the other hand automated laser ablation for high-resolution analyses is not yet well established and available. Here, we give an update on the current status of laser ablation research for analysis of the carbon isotope ratio (δ¹³C) of wood, describe an easy-to-use laser ablation system, its operation and discuss practical issues related to tree core preparation, including cellulose extraction. The results show that routine analysis with up to 100 laser shot-derived δ¹³C-values daily and good precision and accuracy (ca. 0.1‰) comparable to conventional combustion in an elemental analyzer are possible. Measurements on resin-extracted wood is recommended as most efficient, but laser ablation is also possible on cellulose extracted wood pieces. Considering the straightforward sample preparation, the technique is therefore ripe for wide-spread application. With this work, we hope to stimulate future progress in the promising field of high-resolution environmental reconstruction using laser ablation.

The European Dendroecological Fieldweek (EDF) is a one-week course that takes place every year at varying locations in Europe according to the principle "Bring tree-ring research to the people". The EDF welcomes early-career to advanced researchers, but also forest service and other federal agency employees and private people interested in tree-ring sciences from all over the world. It encompasses a large spectrum of dendrochronological field, laboratory and data analysis methods and scientific fields including climatology, ecology, physiology, geomorphology and archaeology. Multiple scales of observations from the individual cell to the ecosystem level and from seasonal to multi-centennial periods are covered. Work on mini research projects in topic groups alternates with keynote lectures and individual participants’ presentations.
As one of the first in-person tree-ring meetings since the start of the COVID-19 pandemic the 31st EDF was held in summer 2021 in Val Müstair, Switzerland. Topics included i) Tree age and climate sensitivity of a relict, old-growth Scots pine stand, ii) Blue intensity-based climate sensitivity of Norway spruce growth, iii) Tree rings as indicators of grey larch budmoth outbreaks, iv) Growth of larch trees along an abandoned irrigation channel, v) Wood anatomical characteristics of two alpine creeping shrub species, and vi) Historical dating of a stable and a residential house. Alongside with their educational value these projects allowed novel insight into the age structure and growth dynamics of the sub-alpine forests and beyond in the valley and provided valuable outcome to the local stakeholders such as the Nature Park Biosfera Val Müstair, the local forest service and the public of Val Müstair.
Under hindered conditions due to the pandemic, the 31st EDF still demonstrated its strength as an international educational and interdisciplinary scientific field and lab course, combining teaching with the application of cutting-edge technologies.

Recent experiments have underlined the potential of δ²H in tree-ring cellulose as a physiological indicator of shifts in autotrophic versus heterotrophic processes (i.e., the use of fresh versus stored non-structural carbohydrates). However, the impact of these processes has not yet been quantified under natural conditions. Defoliator outbreaks disrupt tree functioning and carbon assimilation, stimulating remobilization, therefore providing a unique opportunity to improve our understanding of changes in δ²H. By exploring a 700-year tree-ring isotope chronology from Switzerland, we assessed the impact of 79 larch budmoth (LBM, Zeiraphera griseana) outbreaks on the growth of its host tree species, Larix decidua. LBM outbreaks significantly altered the tree-ring isotopic signature, creating a ²H-enrichment and an ¹⁸O- and ¹³C-depletion. Changes in tree physiological functioning in outbreak years are shown by the decoupling of δ²H and δ¹⁸O (O–H relationship), in contrast to the positive correlation in non-outbreak years. Across the centuries, the O–H relationship in outbreak years was not significantly affected by temperature, indicating that non-climatic physiological processes dominate over climate in determining δ²H. We conclude that the combination of these isotopic parameters can serve as a metric for assessing changes in physiological mechanisms over time.

Climate warming has profoundly altered the status of permafrost and has caused extensive permafrost degradation in the Northern Hemisphere. However, long-term observations investigating the hydrological dynamics of permafrost and its ecological effects on plant growth are lacking. Previous studies have reported tree-ring stable hydrogen isotope ratios of lignin methoxy groups (δ²H_LM) as an archive of hydrological signals. This study sampled tree-ring cores from a Larix gmelinii forest in Nanwenghe Forest Park, Northeastern China, and separately measured the tree-ring δ²H_LM for earlywood and latewood from 1900 to 2020. Earlywood and latewood δ²H_LM values, as well as the difference between them, showed no significant long-term trend from 1900 to 1987; however, they both exhibited significant increasing trends since 1988 at rates of 2.6 ‰ and 4.9 ‰ per decade, respectively. This variance changes the magnitude of the difference between the two chronologies and can be explained by the shift in source water δ²H values during tree growth. Based on a structural equation model analysis, when the influence of permafrost melting weakened due to permafrost degradation, the growing season temperature was better recorded in latewood δ²H_LM through the effects of precipitation δ²H from July to September. Based on the environmental response of tree-ring δ²H_LM in the permafrost region, permafrost degradation influences the source water δ²H values of trees, thereby affecting the expression of temperature signals in tree-ring δ²H_LM. The novel results in this study provide a new perspective on permafrost degradation based on the dynamic responses of tree-ring δ²H_LM to source water δ²H during permafrost degradation.

• The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.
• We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-hour resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.
• Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting.
• These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.