No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Trends in climatically-driven extreme growth reductions of Picea abies and Pinus sylvestris in Central Europe
Abstract
Extreme tree growth reductions represent events of abrupt forest productivity decline and carbon sequestration reduction. An increase in their magnitude can represent an early warning signal of impending tree mortality. Yet the long-term trends in extreme growth reductions remain largely unknown. We analysed trends in the proportion of trees exhibiting extreme growth reductions in two Central-European conifer species - Pinus sylvestris (PISY) and Picea abies (PCAB) - between 1901 and 2018. We employed a novel approach for extreme growth reduction quantification by relating their size to their mean recurrence interval. Twenty-eight sites throughout Czechia and Slovakia with 1120 ring width series representing high and low-elevation forests were inspected for extreme growth reductions with recurrence intervals of 15 and 50 years along with their link to climatic drivers. Our results show the greatest growth reductions at low-elevation PCAB sites, indicating high vulnerability of PCAB to drought. The proportions of trees exhibiting extreme growth reductions increased over time at low-elevation PCAB, decreased recently following an abrupt increase in the 1970-80s at high-elevation PCAB, and showed nonsignificant trends in high and low-elevation PISY. Climatic drivers of extreme growth reductions, however, shifted over time for all site categories as the proportion of low temperature-induced extreme growth reductions declined since the 1990s, whereas events caused by drought consistently increased in frequency during the same period. We observed higher growth volatility at the lower range of distribution compared to the upper range margin of PISY and PCAB. This will undoubtedly considerably impact tree growth and vitality as temperatures and incidence of drought in Central Europe are expected to further increase with ongoing climate change.
... Low summer water availability combined with high temperatures strongly reduces growth for both of these species, suggesting that they may be unable to cope with prolonged and frequent dry conditions in Europe (Lévesque et al., 2014;Zang et al., 2012). In fact, there is clear evidence that extreme growth reductions (GRs) due to drought in Europe increased after 1990 for both species (Treml et al., 2021). Norway spruce is regarded as more vulnerable to drought than other coexisting species Vitasse et al., 2019;Zang et al., 2014), with significant wood damage and economic loss (Rosner et al., 2018). ...
... Norway spruce is regarded as more vulnerable to drought than other coexisting species Vitasse et al., 2019;Zang et al., 2014), with significant wood damage and economic loss (Rosner et al., 2018). Accordingly, Treml et al. (2021) showed that Norway spruce is more vulnerable to drought than Scots pine at low altitudes in Central Europe. ...
... Scots pine was the more drought tolerant species, with higher resistance to drought and shorter recovery times, compared to Norway spruce. Dry summers have been shown to strongly reduce photosynthesis, stomatal conductance, carbohydrate transfer and growth for Norway spruce (Lévesque et al., 2014;Treml et al., 2021). Later termination of shoot elongation and later radial growth above-ground and below-ground make Norway spruce more sensitive to summer water stress than Scots pine (Zang et al., 2012). ...
Climate change is increasing the severity and frequency of droughts around the globe, leading to tree mortality that reduces production and provision of other ecosystem services. Recent studies show that growth of mixed stands may be more resilient to drought than pure stands. The two most economically important and widely distributed tree species in Europe are Norway spruce ( Picea abies (L.) Karst) and Scots pine ( Pinus sylvestris L.), but little is known about their susceptibility to drought when coexist.
This paper analyses the resilience (resistance, recovery rate and recovery time) at individual‐tree level using a network of tree‐ring collections from 22 sites along a climatic gradient from central Europe to Scandinavia. We aimed to identify differences in growth following drought between the two species and between mixed and pure stands, and how environmental variables (climate, topography and site location) and tree characteristics influence them.
We found that both the timing and duration of drought drive the different responses between species and compositions. Norway spruce showed higher vulnerability to summer drought, with both lower resistance and a longer recovery time than Scots pine. Mixtures provided higher drought resistance for both species compared to pure stands, but the benefit decreases with the duration of the drought. Especially climate sensitive and old trees in climatically marginal sites were more affected by drought stress.
Synthesis . Promoting Scots pine and mixed forests is a promising strategy for adapting European forests to climate change. However, if future droughts become longer, the advantage of mixed stands could disappear which would be especially negative for Norway spruce.
... It has been increasingly recognized that tipping points, corresponding to abrupt and irreversible state shifts, occur in diverse ecosystems including those found in cold biomes (Henttonen et al., 2017;Wu et al., 2019;Song et al., 2020;Zhang et al., 2020;Treml et al., 2022). The likelihood of state transitions in natural ecosystems increases as the environment conditions gradually approach critical thresholds. ...
The loss of resilience and prolonged recovery times after extreme climate events can be used as early warning signals of impending tipping points or abrupt irreversible changes. However, evidence of such critical slowing down in growth series is still lacking in terrestrial ecosystems under harsh environmental conditions such as alpine shrublands. Alpine shrublines, at which shrublands shift into grasslands under climate thresholds, constitute unique settings to look for critical slowing down phenomena. On the northeastern Tibetan Plateau, we sampled Salix oritrepha Schneid. (willow shrubs) in three elevational sites along each of two transects (QL, Qilian; HSX, Huashixia) running from alpine treeline position to shrubline. We then quantified the radial growth resilience and recovery indices of alpine willow shrubs to cold spells along elevational gradients by using dendrochronology. Willow growth was primarily constrained by low winter temperatures in QL and low summer temperatures in HSX. Cold spell events were detected in 2008 and 2011 in HSX and QL sites, respectively. Because temperature decreases with elevation, both growth resilience and recovery indices differed within each transect, showing a decreasing trend with elevation. Growth in 78% and 56% willow shrubs recovered back to the level before cold spell within two years in QL and HSX sites, respectively. Moreover, the average growth recovery period was longer at the shrubline (ca. 2-4 years) than near the treeline position (ca. 1-2 years). Critical slowing down theory could thus be applied to assess growth resilience towards the alpine shrubline, a thermal threshold for shrub growth. Our results and framework highlight the priority to directly measure the resilience metrics and also offer a promising perspective that the critical slowing down phenomenon may be universal in various ecosystems.
... Note, that charts might have different y-axis scales to aid visual comparison between clusters and species. A chart with results for the retrospective calibration mode is shown in Figure S4. the growth of P. sylvestris often responds positively to temperature in late winter and early spring (Harvey et al., 2020), but it may also be significantly correlated with summer temperatures and/or soil moisture both at dry and cold sites (Bose et al., 2020;Pärn, 2009;Treml et al., 2022). ...
Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non‐stationarity, i.e., a decoupling of tree growth responses to climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non‐stationarity. To systematically assess potential drivers of non‐stationarity, we compiled tree‐ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non‐linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate‐growth models were tested in independent verification periods to quantify their non‐stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non‐stationarity varied between species, site climatic conditions and models. Chronologies of Pinus sylvestris showed stronger non‐stationarity compared to Picea abies stands with a high degree of stationarity. In general, sites with mixed climatic signals were most affected by non‐stationarity compared to sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non‐stationarity compared to monthly‐resolved non‐linear models. We conclude that non‐stationarity in climate‐growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non‐stationarity, we recommend that temporal non‐stationarity rather than stationarity should be considered as the baseline model of climate‐growth response for temperate forests.
... There have been evidences of climatically driven reduction in survival of spruce regeneration as well as adult trees in different parts of its range(Brandl et al. (2020);Schurman et al. (2019);Pretzsch et al. (2014)). This effect is stronger in lower altitudes and latitudes, such as in central Europe, owing to more frequent hot and dry conditions, suggesting a reduction of range in its southern edges(Treml et al. (2022);Feurdean et al. (2011);Bradshaw and Lindbladh (2005); Kullman and Engelmark ...
Global environmental changes are affecting tree population demography with potentially significant impacts on forest biodiversity and wood industry. Forest regeneration processes include seed production, growth and survival of saplings to the recruitment sizes at which trees are considered in forest inventories. Changes in regeneration dynamics directly affect forest composition and structure and can jeopardize the sustainability of forest management. This is especially the case in mountain forests where environmental gradients are strong and where forests are often uneven-aged, i.e. combining trees of all ages in a single stand. Regeneration processes are difficult to monitor. Large data sets often give only fixed pictures of sapling densities with little information on demographic processes. In this thesis, we quantified the effects of different biotic and abiotic factors on regeneration dynamics of Picea abies (spruce), Abies alba (fir) and Fagus sylvatica (beech) in the French Alps and Jura mountains. We also predicted changes in tree recruitment fluxes in these forests, for potential climate change situations. We recorded sapling height increment and density of spruce, fir and beech in 152 plots across the French Alps and Jura mountains. We then analysed how biotic and abiotic factors known to affect regeneration, namely altitude, slope, aspect, light availability, soil characteristics, ungulate browsing, temperature, precipitation and evapotranspiration, affected sapling density and growth using non-linear mixed models. We showed that temperature has a positive non-linear effect on sapling height growth and water resource availability has a positive effect on sapling density. Terminal shoot browsing, which prevents sapling height growth, is especially frequent on fir. In a second analysis, we built a more comprehensive model of regeneration dynamics, representing explicitly the process of new seedling production, sapling growth, browsing and survival, and finally their recruitment into adult trees. We predicted parameters for these processes in combination, using Approximate Bayesian Computation (ABC), based on the field data collected earlier. The results imply that more frequent and intense heat and drought events could negatively influence sapling growth and survival of the three species, with probable reduction of forest renewal fluxes. An increase of ungulate populations leading to increased browsing could be especially detrimental to fir and possibly also to beech saplings. We also predicted the potential tree recruitment fluxes for different IPCC climate projection scenarios for the year 2100, and showed that a reduction in tree recruitments is highly likely. This study shows that the ABC method can be efficiently used to estimate regeneration dynamic processes, based on sapling density, height increment and browsing data. It highlights the vulnerability of future forest regeneration to water availability and ungulate presence, urging researchers and forest managers alike to anticipate future potential important changes in mountain forest dynamics.
... In the statistical models, we included climatic variables for seasonal windows within the growing season that directly control cambial activity, incl. mean June-August SPEI-4 (defined as mean SPEI-4 of June, July and August) and June-July temperature (for correlation coefficients between tree-ring chronologies and climatic variables, see Treml et al., 2022). While the former climatic variable typically drives tree growth in lowlands, the latter determines tree growth at cold sites (Ponocná et al., 2016, Kolář et al., 2017, Bosela et al., 2021. ...
Climate controls forest biomass production through direct effects on cambial activity and indirectly through interactions with CO2, air pollution, and nutrient availability. The atmospheric concentration of CO2, sulfur and nitrogen deposition can also exert a significant indirect control on wood formation since these factors influence the stomatal regulation of transpiration and carbon uptake, that is, intrinsic water use efficiency (iWUE). Here we provide 120-year long tree-ring time series of iWUE, stem growth, climatic and combined sulfur and nitrogen (SN) deposition trends for two common tree species, Pinus sylvestris (PISY) and Picea abies (PCAB), at their lower and upper distribution margins in Central Europe. The main goals were to explain iWUE trends using theoretical scenarios including climatic and SN deposition data, and to assess the contribution of climate and iWUE to the observed growth trends. Our results showed that after a notable increase in iWUE between the 1950s and 1980s, this positive trend subsequently slowed down. The substantial rise of iWUE since the 1950s resulted from a combination of an accelerated increase in atmospheric CO2 concentrations (Ca) and a stable level of leaf intercellular CO2 (Ci). The offset of observed iWUE values above the trajectory of a constant Ci/Ca scenario was explained by trends in SN deposition (all sites) together with the variation of drought conditions (low-elevation sites only). Increasing iWUE over the 20th and 21st centuries improved tree growth at low-elevation drought-sensitive sites. In contrast, at high-elevation PCAB sites, growth was mainly stimulated by recent warming. We propose that SN pollution should be considered in order to explain the steep increase in iWUE of conifers in the 20th century throughout Central Europe and other regions with a significant SN deposition history.
IntroductionUnder 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.ResultsTrees 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.DiscussionPrecipitation 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.
Against the backdrop of global change, the intensity, duration, and frequency of droughts are projected to increase and threaten forest ecosystems worldwide. Tree responses to drought are complex and likely to vary among species, drought characteristics, and site conditions. Here, we examined the drought response patterns of three major temperate tree species, s. fir (Abies alba), E. beech (Fagus sylvatica), and N. spruce (Picea abies), along an ecological gradient in the South – Central – East part of Germany that included a total of 37 sites with varying climatic and soil conditions. We relied on annual tree-ring data to assess the influence of different drought characteristics and (micro-) site conditions on components of tree resilience and to detect associated temporal changes. Our study revealed that nutrient regime, drought frequency, and hydraulic conditions in the previous and subsequent years were the main determinants of drought responses, with pronounced differences among species. Specifically, we found that (a) higher drought frequency was associated with higher resistance and resilience for N. spruce and E. beech; (b) more favorable climatic conditions in the two preceding and following years increased drought resilience and determined recovery potential of E. beech after extreme drought; (c) a site's nutrient regime, rather than micro-site differences in water availability, determined drought responses, with trees growing on sites with a balanced nutrient regime having a higher capacity to withstand extreme drought stress; (d) E. beech and N. spruce experienced a long-term decline in resilience. Our results indicate that trees under extreme drought stress benefit from a balanced nutrient supply and highlight the relevance of water availability immediately after droughts. Observed long-term trends confirm that N. spruce is suffering from persistent climatic changes, while s. fir is coping better. These findings might be especially relevant for monitoring, scenario analyses, and forest ecosystem management.
Long-lived tree species optimize resource allocation processes, and there are more than likely trade-offs between the growth and response to environmental stressful conditions. The study aims to investigate the growth of healthy and damaged Norway spruce trees using dendrochronological analysis. We have selected a natural high mountain spruce population and established 27 sampling plots in Poľana Mts. We measured 270 trees, from which we visually evaluated 133 individuals. Moreover, 51 trees were sampled for increment cores. Growth response of spruce trees to specific combination of winter, spring and summer temperatures and precipitations within a particular (extreme) year 1974 was analysed. Single-year extreme weather conditions (extremely warm winter, dry early spring and cold early summer) induced long-term growth divergence of neighbouring spruce trees of comparable age and size and differentiated between healthy and declining (highly defoliated) trees. In high mountain temperate forests, declining trees' long-term growth response reach about 40 years. This period accounted for a decrease in size of relativized increments by 30.2% in declining trees compared to visually healthy individuals. Trees with a low ability to recover from climatic stress compensate the response to environmental conditions by lower growth rates. The single-tree selection silvicultural system should be preferred in high mountain conditions, where frequent small-scale, low-intensity disturbances drive stand dynamics. Trees showing low growth resilience to revealed combination of weather conditions should be preferentially removed from stand to reduce its vulnerability, especially within the transformation of even-aged spruce monocultures using selective cuttings.
Norway spruce is an ecologically and commercially important species, which currently suffers from climate change and is expected to decline because of heat and drought stress. A manipulative experiment with drought-stressed and well-watered (control) seedlings was established, comprising five populations of Norway spruce coming from a small territory but distributed along a steep elevational transect from 550 to 1280 m.a.s.l. Plants were phenotyped at physiological traits (content of phytohormones and monoterpenes, chlorophyll a fluorescence, gas exchange and hyperspectral indices) and genotyped using ddRAD sequencing. After filtering, 34 127 polymorphisms (SNPs) were retained and used for analysis. Association tests revealed 42 SNP-variable pairs located across 29 scaffolds exhibiting significant associations, among them 20 were found for drought-stressed seedlings and 22 for control (watered) plants. However, there was no consistency between drought-stressed and control plants, no SNP-trait pair was common for both treatments. The study demonstrated that the identified genotype–phenotype associations can only be interpreted in the context of environmental setup preceding the assessment of phenotypic traits.
Significant alterations of cambial activity might be expected due to climate warming, leading to growing season extension and higher growth rates especially in cold-limited forests. However, assessment of climate-change-driven trends in intra-annual wood formation suffers from the lack of direct observations with a timespan exceeding a few years. We used the Vaganov-Shashkin process-based model to: (i) simulate daily resolved numbers of cambial and differentiating cells; and (ii) develop chronologies of the onset and termination of specific phases of cambial phenology during 1961-2017. We also determined the dominant climatic factor limiting cambial activity for each day. To asses intra-annual model validity, we used 8 years of direct xylogenesis monitoring from the treeline region of the Krkonoše Mts. (Czechia). The model exhibits high validity in case of spring phenological phases and a seasonal dynamics of tracheid production, but its precision declines for estimates of autumn phenological phases and growing season duration. The simulations reveal an increasing trend in the number of tracheids produced by cambium each year by 0.42 cells/year. Spring phenological phases (onset of cambial cell growth and tracheid enlargement) show significant shifts toward earlier occurrence in the year (for 0.28-0.34 days/year). In addition, there is a significant increase in simulated growth rates during entire growing season associated with the intra-annual redistribution of the dominant climatic controls over cambial activity. Results suggest that higher growth rates at treeline are driven by (i) temperature-stimulated intensification of spring cambial kinetics, and (ii) decoupling of summer growth rates from the limiting effect of low summer temperature due to higher frequency of climatically optimal days. Our results highlight that the cambial kinetics stimulation by increasing spring and summer temperatures and shifting spring phenology determine the recent growth trends of treeline ecosystems. Redistribution of individual climatic factors controlling cambial activity during the growing season questions the temporal stability of climatic signal of cold forest chronologies under ongoing climate change.
The increase in frequency and intensity of droughts due to climate change might threaten forests under stress levels causing dieback and mortality episodes. Thus, deciphering how tree species from within a region respond to drought along environmental gradients should help us to understand forest vulnerability to climate change. To enlighten contrasting drought responses of dominant tree species, we reconstructed vegetation activity using Normalized Difference Vegetation Index (NDVI) and radial growth using tree-ring width series. We studied six tree species, three angiosperms (Fagus sylvatica, Quercus humilis, and Quercus ilex) and three gymnosperms (Pinus sylvestris, Pinus nigra, and Pinus halepensis), inhabiting a Mediterranean region in north-eastern Spain. We investigated if reduced growth resilience and increased growth synchrony after successive droughts (1986, 1989, 2005, and 2012): (i) were related to cumulative drought stress and (ii) preceded forest dieback in dry sites as compared to wet sites. In 2016, dieback affected Q. ilex and P. sylvestris stands in dry sites showing lower growth rates and NDVI. No dieback symptoms were observed in other species from dry (P. nigra, P. halepensis) or wet (F. sylvatica, Q. humilis, P. sylvestris) sites. Hot and dry summer conditions constrained growth and reduced NDVI. During 2005, a severe drought affected all species, but growth drops were more marked in dry places. All species were able to recover after extreme droughts, albeit angiosperms displayed lower than expected values of growth after the 2012 drought. Growth synchrony was higher in dry sites than in wet sites, and the differences were higher after the 2005 drought. This study reveals that the sensitivity of tree species to drought in species inhabiting the same region is species dependent, and it is contingent on local conditions with higher effects in dry sites than in wet sites. We describe how a cumulative impact of successive droughts increases growth synchrony and triggers the occurrence of dieback events in Mediterranean forests.
The combination of drought and heat affects forest ecosystems by deteriorating the health of trees, which can lead to large‐scale die‐offs with consequences on biodiversity, the carbon cycle, and wood production. It is thus crucial to understand how drought events affect tree health and which factors determine forest susceptibility and resilience. We analyze the response of Central European forests to the 2018 summer drought with 10 × 10 m satellite observations. By associating time‐series statistics of the Normalized Difference Vegetation Index (NDVI) with visually classified observations of early wilting, we show that the drought led to early leaf‐shedding across 21,500 ± 2,800 km2, in particular in central and eastern Germany and in the Czech Republic. High temperatures and low precipitation, especially in August, mostly explained these large‐scale patterns, with small‐ to medium‐sized trees, steep slopes, and shallow soils being important regional risk factors. Early wilting revealed a lasting impact on forest productivity, with affected trees showing reduced greenness in the following spring. Our approach reliably detects early wilting at the resolution of large individual crowns and links it to key environmental drivers. It provides a sound basis to monitor and forecast early‐wilting responses that may follow the droughts of the coming decades.
Climate change-driven increases in drought frequency and severity could compromise forest ecosystems and the terrestrial carbon sink1–3. While the impacts of single droughts on forests have been widely studied4–6, understanding whether forests acclimate to or become more vulnerable to sequential droughts remains largely unknown and is crucial for predicting future forest health. We combine cross-biome datasets of tree growth, tree mortality and ecosystem water content to quantify the effects of multiple droughts at a range of scales from individual trees to the globe from 1900 to 2018. We find that subsequent droughts generally have a more deleterious impact than initial droughts, but this effect differs enormously by clade and ecosystem, with gymnosperms and conifer-dominated ecosystems more often exhibiting increased vulnerability to multiple droughts. The differential impacts of multiple droughts across clades and biomes indicate that drought frequency changes may have fundamentally different ecological and carbon-cycle consequences across ecosystems.
Key message
Effect of drought during 2017 and 2018 resulted in radial stem increment reduction to 78% and 61%, respectively, of the levels occurring in normal year 2016 in Central Europe.
Abstract
Norway spruce (Picea abies (L.) Karst.) is currently the most threatened commercial tree species in Central Europe. This is due to increased drought stress from advancing climate change as well as the species’ distribution outside its natural range. Tree water status and water movement through a tree are key parameters influencing tree growth and vitality. This study is focused on the growth and stress reaction of spruce to climatic conditions, analysing stem diameter variation along an elevation gradient (381–995 m a.s.l.) in the Czech Republic. Tree water deficit based on the zero-growth concept (TWD), calculated from high-frequency dendrometer records and the temporal dynamics of radial growth, was studied for 3 years (2016–2018). Two of these 3 years were affected by severe drought during the growing season. Contrary to our expectations, the observed TWD showed no clear linear decline with rising elevation. The most severe tree desiccation was observed in experimental sites at middle elevations of about 600 m a.s.l. Here, we show that both the timing and level of tree water deficit had an impact on annual stem radial increment (SRIannual). Severe drought had a substantial negative impact on SRIannual of Norway spruce in both 2017 and 2018. Drought conditions in 2017 and 2018 resulted in reduction of SRIannual relative to measurements for the wetter year in 2016 to 78% and 61%, respectively. We report the evidence that the current climatic conditions in the Central European region are not suitable for growing Norway spruce at lower and middle elevations and that forest management needs to react immediately to this situation.
Global climate change is expected to further raise the frequency and severity of extreme events, such as droughts. The effects of extreme droughts on trees are difficult to disentangle given the inherent complexity of drought events (frequency, severity, duration, and timing during the growing season). Besides, drought effects might be modulated by trees’ phenotypic variability, which is, in turn, affected by long‐term local selective pressures and management legacies. Here, we investigated the magnitude and the temporal changes of tree‐level resilience (i.e., resistance, recovery, and resilience) to extreme droughts. Moreover, we assessed the tree‐, site‐, and drought‐related factors and their interactions driving the tree‐level resilience to extreme droughts. We used a tree‐ring network of the widely distributed Scots pine (Pinus sylvestris) along a 2800 km latitudinal gradient from southern Spain to northern Germany. We found that the resilience to extreme drought decreased in mid‐elevation and low productivity sites from 1980‐1999 to 2000‐2011 likely due to more frequent and severe droughts in the later period. Our study showed that the impact of drought on tree‐level resilience was not dependent on its latitudinal location, but rather on the type of sites trees were growing at and on their growth performances (i.e., magnitude and variability of growth) during the pre‐drought period. We found significant interactive effects between drought duration and tree growth prior to drought, suggesting that Scots pine trees with higher magnitude and variability of growth in the long term are more vulnerable to long and severe droughts. Moreover, our results indicate that Scots pine trees that experienced more frequent droughts over the long‐term were less resistant to extreme droughts. We therefore conclude that the physiological resilience to extreme droughts might be constrained by their growth prior to drought, and that more frequent and longer drought periods may overstrain their potential for acclimation.
In 2018, Central Europe experienced one of the most severe and long-lasting summer drought and heat wave ever recorded. Before 2018, the 2003 millennial drought was often invoked as the example of a “hotter drought”, and was classified as the most severe event in Europe for the last 500 years. First insights now confirm that the 2018 drought event was climatically more extreme and had a greater impact on forest ecosystems of Austria, Germany and Switzerland than the 2003 drought. Across this region, mean growing season air temperature from April to October was more than 3.3°C above the long-term average, and 1.2°C warmer than in 2003. Here, we present a first impact assessment of the severe 2018 summer drought and heatwave on Central European forests. In response to the 2018 event, most ecologically and economically important tree species in temperate forests of Austria, Germany and Switzerland showed severe signs of drought stress. These symptoms included exceptional low foliar water potentials crossing the threshold for xylem hydraulic failure in many species and observations of widespread leaf discoloration and premature leaf shedding. As a result of the extreme drought stress, the 2018 event caused unprecedented drought-induced tree mortality in many species throughout the region. Moreover, unexpectedly strong drought-legacy effects were detected in 2019. This implies that the physiological recovery of trees was impaired after the 2018 drought event, leaving them highly vulnerable to secondary drought impacts such as insect or fungal pathogen attacks. As a consequence, mortality of trees triggered by the 2018 events is likely to continue for several years. Our assessment indicates that many common temperate European forest tree species are more vulnerable to extreme summer drought and heat waves than previously thought. As drought and heat events are likely to occur more frequently with the progression of climate change, temperate European forests might approach the point for a substantial ecological and economic transition. Our assessment also highlights the urgent need for a pan-European ground-based monitoring network suited to track individual tree mortality, supported by remote sensing products with high spatial and temporal resolution to track, analyse and forecast these transitions.
Key message
Under severe drought, growth of Norway spruce suffered much more than European beech. Norway spruce benefited from growing in the environment of beech, and both species acclimated slightly to 5 years of experimentally extended drought.
Abstract
Recent studies show that the detrimental effects of drought on stand growth are mitigated when the stand contains mixed tree species. We analysed the growth responses of Norway spruce and European beech to episodic and experimentally extended drought in intra- and inter-specific mature stands. We used annual diameter growth records dating back to 1998 to determine the impact of the natural episodic drought in 2003 and 2015. To analyse extended drought, spruce and beech trees were exposed to extreme drought under automatic throughfall exclusion roofs from 2014 to 2018. The growth of spruce in an inter-specific environment with beech was 20–50% less affected by natural episodic drought compared with an intra-specific constellation. When beech grew in an inter-specific environment, it was by 23% more affected by drought compared to intra-specific conditions, but seemed to recover faster. The induced drought from 2014 to 2018 resulted in a strong growth reduction in the first year particularly for spruce, followed by a slight acclimation to the dry conditions. Beech acclimated and recovered faster than spruce across all growing conditions, while spruce only acclimatized faster in the environment of beech. Both species showed a higher mortality under induced drought compared with the controls; for spruce, the mortality rate was fivefold higher compared to the long-term mortality. The long-term moderate-growth stabilization and the growth increase after the 5-year exposure to drought suggest a gradual acclimation to drought by beech. The resistance and acclimation to drought of spruce when growing in mixture should be considered when designing resource efficient and productive mixed conifer-broadleaved stands for future climates.
Using measurements from high resolution monitoring of radial tree-growth we present new data of the growth reactions of four widespread broadleaved tree-species to the combined European drought years 2018 and 2019. We can show that, in contrast to field crops, trees could make better use of the winter soil moisture storage in 2018 which buffered them from severe drought stress and growth depressions in this year. Nevertheless, legacy effects of the 2018 drought accompanied by sustained low soil moisture conditions (missing recharge in winter) and again higher than average temperatures and low precipitation in spring/summer 2019 have resulted in severe growth reductions for all studied tree-species in this year. This highlights the pivotal role of soil water recharge in winter. Although short term resistance to hot summers can be high if sufficient winter precipitations buffers forest stands from drought damage, legacy effects will strongly impact tree growth in subsequent years if the drought persists. The two years 2018 and 2019 are extreme with regard to historical instrumental data but, according to regional climate models, resemble rather normal conditions of the climate in the second half of the 21st century. Therefore the observed strongly reduced growth rates can provide an outlook on future forest growth potential in northern Central Europe and beyond.
Significance
This study demonstrates how different lines of evidence from tree rings in widely spread growth locations can combine to fix an approximately dated tree-ring record from the East Mediterranean Bronze–Iron Age to an exact calendar-dated range. This tree-ring record is of high importance for regional chronology and spans the time period in which the major volcanic eruption of Thera (Santorini) occurred. Exact dating of this eruption is important because it provides a prominent marker horizon through which ancient timelines of the East Mediterranean, Egypt, and the Levant can be synchronized. Chemical analysis of the dated tree-ring sequence identifies a chemical change in their growth environment around 1560 BC, which while requiring further substantiation, may be evidence of the Thera eruption.
Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality.
The response of forest productivity to climate extremes strongly depends on ambient environmental and site conditions. To better understand these relationships at a regional scale, we used nearly 800 observation years from 271 permanent long‐term forest monitoring plots across Switzerland, obtained between 1980 and 2017. We assimilated these data into the 3‐PG forest ecosystem model using Bayesian inference, reducing the bias of model predictions from 14% to 5% for forest stem carbon stocks and from 45% to 9% for stem carbon stock changes. We then estimated the productivity of forests dominated by Picea abies and Fagus sylvatica for the period of 1960‐2018, and tested for productivity shifts in response to climate along elevational gradient and in extreme years. Simulated net primary productivity (NPP) decreased with elevation (2.86 ± 0.006 Mg C ha‐1 y‐1 km‐1 for P. abies and 0.93 ± 0.010 Mg C ha‐1 y‐1 km‐1 for F. sylvatica). During warm‐dry extremes, simulated NPP for both species increased at higher and decreased at lower elevations, with reductions in NPP of more than 25% for up to 21% of the potential species distribution range in Switzerland. Reduced plant water availability had a stronger effect on NPP than temperature during warm‐dry extremes. Importantly, cold‐dry extremes had negative impacts on regional forest NPP comparable to warm‐dry extremes. Overall, our calibrated model suggests that the response of forest productivity to climate extremes is more complex than simple shift towards higher elevation. Such robust estimates of NPP are key for increasing our understanding of forests ecosystems carbon dynamics under climate extremes.
Forest mortality is accelerating due to climate change and the largest trees may be at the greatest risk, threatening critical ecological, economic, and social benefits. Here, we combine high-resolution airborne LiDAR and optical data to track tree-level mortality rates for ~2 million trees in California over 8 years, showing that tree height is the strongest predictor of mortality during extreme drought. Large trees die at twice the rate of small trees and environmental gradients of temperature, water, and competition control the intensity of the height-mortality relationship. These findings suggest that future persistent drought may cause widespread mortality of the largest trees on Earth.
Drought has promoted large‐scale, insect‐induced tree mortality in recent years, with severe consequences for ecosystem function, atmospheric processes, sustainable resources and global biogeochemical cycles. However, the physiological linkages among drought, tree defences, and insect outbreaks are still uncertain, hindering our ability to accurately predict tree mortality under on‐going climate change. Here we propose an interdisciplinary research agenda for addressing these crucial knowledge gaps. Our framework includes field manipulations, laboratory experiments, and modelling of insect and vegetation dynamics, and focuses on how drought affects interactions between conifer trees and bark beetles. We build upon existing theory and examine several key assumptions: 1) there is a trade‐off in tree carbon investment between primary and secondary metabolites (e.g. growth vs. defence); 2) secondary metabolites are one of the main component of tree defence against bark beetles and associated microbes; and 3) implementing conifer‐bark beetle interactions in current models improves predictions of forest disturbance in a changing climate. Our framework provides guidance for addressing a major shortcoming in current implementations of large‐scale vegetation models, the under‐representation of insect‐induced tree mortality.
Extreme climate events (ECEs) such as severe droughts, heat waves and late spring frosts are rare but exert a paramount role in shaping tree species distributions. The frequency of such ECEs is expected to increase with climate warming, threatening the sustainability of temperate forests. Here, we analyzed 2844 tree‐ring width series of five dominant European tree species from 104 Swiss sites ranging from 400 to 2200 m a.s.l. for the period 1930–2016. We found that (i) the broadleaved oak and beech are sensitive to late frosts that strongly reduce current year growth; however, tree growth is highly resilient and fully recovers within two years; (ii) radial growth of the conifers larch and spruce is strongly and enduringly reduced by spring droughts—these species are the least resistant and resilient to droughts; (iii) oak, silver fir, and to a lower extent beech, show higher resistance and resilience to spring droughts and seem therefore better adapted to the future climate. Our results allow a robust comparison of the tree growth responses to drought and spring frost across large climatic gradients and provide striking evidence that the growth of some of the most abundant and economically important European tree species will be increasingly limited by climate warming. These results could serve for supporting species selection to maintain the sustainability of forest ecosystem services under the expected increase in ECEs.
Climate-induced tree mortality became a global phenomenon during the last century and it is expected to increase in many regions in the future along with a further increase in the frequency of drought and heat events. However, tree mortality at the ecosystem level remains challenging to quantify since long-term, tree- individual, reliable observations are scarce. Here, we present a unique data set of monitoring records from 126 forest stands across Switzerland, which include five major European tree species (Norway spruce, Scots pine, silver fir, European beech, and sessile and common oak) and cover a time span of over one century (1898-2013), with inventory periods of 5 to 10 years. The long-term average annual mortality rate of the investigated forest stands was 1.5%. In general, species-specific annual mortality rates did not consistently increase over the last decades, except for Scots pine forests at lower altitudes, which exhibited a clear increase of mortality since the 1960s. Temporal trends of tree mortality varied also depending on diameter at breast height (DBH), with large trees generally experiencing an increase in mortality, while mortality of small trees tended to decrease. Normalized mortality rates were remarkably similar between species and a modest, but consistent and steady increasing trend was apparent throughout the study period. Mixed effects models revealed that gradually changing stand parameters (stand basal area and stand age) had the strongest impact on mortality rates, modulated by climate, which had increasing importance during the last decades. Hereby, recent climatic changes had highly variable effects on tree mortality rates, depending on the species in combination with abiotic and biotic stand and site conditions. This suggests that forest species composition and species ranges may change under future climate conditions. Our data set highlight the complexity of forest dynamical processes such as long-term, gradual changes of forest structure, demography and species composition, which together with climate determine mortality rates.
The ability to accurately predict ecosystem drought response and recovery is necessary to produce reliable forecasts of land carbon uptake and future climate. Using a suite of models from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP), we assessed modeled net primary productivity (NPP) response to, and recovery from, drought events against a benchmark derived from tree ring observations between 1948 and 2008 across forested regions of the US and Europe. We find short lag times (0–6 months) between climate anomalies and modeled NPP response. Although models accurately simulate the direction of drought legacy effects (i.e. NPP decreases), projected effects are approximately four times shorter and four times weaker than observations suggest. This discrepancy between observed and simulated vegetation recovery from drought reveals a potential critical model deficiency. Since productivity is a crucial component of the land carbon balance, models that underestimate drought recovery time could overestimate predictions of future land carbon sink strength and, consequently, underestimate forecasts of atmospheric CO2.
Tree growth and reproduction are subject to trade-offs in resource allocation. At the same time, they are both influenced by climate. In this study, we combined long records of reproductive effort at the individual- (29 years), population- (41 years) and regional (up to 53 years) scale, and tree ring chronologies, to investigate the effects of climate and reproductive allocation on radial growth in an Alpine Norway spruce forest.
Seed and cone production was highly variable between years (mean individual CV = 1.39, population CV = 1.19), but showed high reproductive synchrony between individuals (mean inter-tree correlation = 0.72). No long-term trend in reproductive effort was detected over four decades of observations. At the stand scale, cone production was dominated by a small number of individuals (“super-producers”), who remained dominant over three decades.
Individual tree growth responded positively to summer temperature, but the response to cone production varied between individual trees. Consequently, we found some evidence that mast years were associated with a divergence in growth between high and low cone producing individuals, and a decline in within-population growth synchrony.
At the population level we found limited evidence of a relationship between growth and reproduction. Radial growth was lower than average in some mast years, but not in others. This was partly explained by summer temperature during the year of growth, with growth reductions restricted to mast years that coincided with colder than average summers. Regional mast records and tree ring chronologies provided some support to indicate that our results were consistent in other spruce stands, although the effect of mast years on growth appeared to vary between sites.
Tree ring variation at the individual and population level, and between-tree growth synchrony are influenced by masting, and consequently dendrochronologists should consider both the occurrence of masting and the individual differences in reproductive effort when interpreting tree ring datasets. Our results also indicate that tree ring chronologies contain information to facilitate reconstruction of mast events, which will help address outstanding questions regarding the future response of masting to climate change.
Energy and water limitations of tree growth remain insufficiently understood at large spatiotemporal scales, hindering model representation of interannual or longer-term ecosystem processes. By assessing and statistically scaling the climatic drivers from 2710 tree-ring sites, we identified the boreal and temperate land areas where tree growth during 1930–1960 CE responded positively to temperature (20.8 ± 3.7 Mio km ² ; 25.9 ± 4.6%), precipitation (77.5 ± 3.3 Mio km ² ; 96.4 ± 4.1%), and other parameters. The spatial manifestation of this climate response is determined by latitudinal and altitudinal temperature gradients, indicating that warming leads to geographic shifts in growth limitations. We observed a significant ( P < 0.001) decrease in temperature response at cold-dry sites between 1930–1960 and 1960–1990 CE, and the total temperature-limited area shrunk by −8.7 ± 0.6 Mio km ² . Simultaneously, trees became more limited by atmospheric water demand almost worldwide. These changes occurred under mild warming, and we expect that continued climate change will trigger a major redistribution in growth responses to climate.
Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.
Populations of many species are genetically adapted to local historical climate conditions. Yet most forecasts of species’ distributions under climate change have ignored local adaptation (LA), which may paint a false picture of how species will respond across their geographical ranges. We review recent studies that have incorporated intraspecific variation, a potential proxy for LA, into distribution forecasts, assess their strengths and weaknesses, and make recommendations for how to improve forecasts in the face of LA. The three methods used so far (species distribution models, response functions, and mechanistic models) reflect a trade‐off between data availability and the ability to rigorously demonstrate LA to climate. We identify key considerations for incorporating LA into distribution forecasts that are currently missing from many published studies, including testing the spatial scale and pattern of LA, the confounding effects of LA to non‐climatic or biotic drivers, and the need to incorporate empirically‐based dispersal or gene flow processes. We suggest approaches to better evaluate these aspects of LA and their effects on species‐level forecasts. In particular, we highlight demographic and dynamic evolutionary models as promising approaches to better integrate LA into forecasts, and emphasize the importance of independent model validation. Finally, we urge closer examination of how LA will alter the responses of central vs. marginal populations to allow stronger generalizations about changes in distribution and abundance in the face of LA.
This article is protected by copyright. All rights reserved.
Predicted increases in temperature and aridity across the boreal forest region have the potential to alter timber supply and carbon sequestration. Given the widely-observed variation in species sensitivity to climate, there is an urgent need to develop species-specific predictive models that can account for local conditions. Here, we matched the growth of 270,000 trees across a 761,100 km 2 region with detailed site-level data to quantify the growth responses of the seven most common boreal tree species in Eastern Canada to changes in climate. Accounting for spatially-explicit species-specific responses, we find that while 2 °C of warming may increase overall forest productivity by 13 ± 3% (mean ± SE) in the absence of disturbance, additional warming could reverse this trend and lead to substantial declines exacerbated by reductions in water availability. Our results confirm the transitory nature of warming-induced growth benefits in the boreal forest and highlight the vulnerability of the ecosystem to excess warming and drying.
Droughts, which are characterized by multiple dimensions including frequency, duration, severity and onset timing, can impact tree stem radial growth profoundly. Different dimensions of drought influence tree stem radial growth independently or jointly, which makes the development of accurate predictions a formidable challenge. Measurement‐based tree‐ring data have obvious advantages for studying the drought responses of trees. Here, we explored the use of abundant tree‐ring records for quantifying regional response patterns to key dimensions of drought. Specifically, we designed a series of regional‐scaled “natural experiments”, based on 357 tree‐ring chronologies from Southwest USA and location‐matched monthly water balance anomalies, to reveal how tree‐ring responds to each dimension of drought. Our results showed that tree‐ring was affected significantly more by the water balance condition in the current hydrological year than that in the prior hydrological year. Within the current hydrological year, increased drought frequency (number of dry months) and duration (maximum number of consecutive dry months) resulted in “cumulative effects” which amplified the impacts of drought on trees and reduced the drought resistance of trees. Drought events that occurred in the pre‐growing seasons strongly affected subsequent tree stem radial growth. Both the onset timing and severity of drought increased “legacy effects” on tree stem radial growth, which reduced the drought resilience of trees. These results indicated that the drought impact on trees is a dynamic process: even when the total water deficits are the same, differences among the drought processes could lead to considerably different responses from trees. This study thus provides a conceptual framework and probabilistic patterns of tree‐ring growth response to multiple dimensions of drought regimes, which in turn may have a wide range of implications for predictions, uncertainty assessment and forest management.
Early 21st-century droughts in Europe have been broadly regarded as exceptionally severe, substantially affecting a wide range of socio-economic sectors. These extreme events were linked mainly to increases in temperature and record-breaking heatwaves that have been influencing Europe since 2000, in combination with a lack of precipitation during the summer months. Drought propagated through all respective compartments of the hydrological cycle, involving low runoff and prolonged soil moisture deficits. What if these recent droughts are not as extreme as previously thought? Using reconstructed droughts over the last 250 years, we show that although the 2003 and 2015 droughts may be regarded as the most extreme droughts driven by precipitation deficits during the vegetation period, their spatial extent and severity at a long-term European scale are less uncommon. This conclusion is evident in our concurrent investigation of three major drought types – meteorological (precipitation), agricultural (soil moisture) and hydrological (grid-scale runoff) droughts. Additionally, unprecedented drying trends for soil moisture and corresponding increases in the frequency of agricultural droughts are also observed, reflecting the recurring periods of high temperatures. Since intense and extended meteorological droughts may reemerge in the future, our study highlights concerns regarding the impacts of such extreme events when combined with persistent decrease in European soil moisture.
Understanding which variables affect forest resilience to extreme drought is key to predict future dynamics under ongoing climate change. In this study, we analyzed how tree resistance, recovery and resilience to drought have changed along three consecutive droughts and how they were affected by species, tree size, plot basal area (as a proxy for competition) and climate. We focused on the three most abundant pine species in the northeast Iberian Peninsula: Pinus halepensis, P. nigra and P. sylvestris during the three most extreme droughts recorded in the period 1951–2010 (occurred in 1986, 1994, and 2005–2006). We cored trees from permanent sample plots and used dendrochronological techniques to estimate resistance (ability to maintain growth level during drought), recovery (growth increase after drought) and resilience (capacity to recover pre-drought growth levels) in terms of tree stem basal area increment. Mixed-effects models were used to determine which tree- and plot-level variables were the main determinants of resistance, recovery and resilience, and to test for differences among the studied droughts. Larger trees were significantly less resistant and resilient. Plot basal area effects were only observed for resilience, with a negative impact only during the last drought. Resistance, recovery and resilience differed across the studied drought events, so that the studied populations became less resistant, less resilient and recovered worse during the last two droughts. This pattern suggests an increased vulnerability to drought after successive drought episodes.
Climate change is expected to exacerbate the frequency of drought-induced tree mortality world-wide. To better predict the associated change of species composition and forest dynamics on various scales and develop adequate adaptation strategies, more information on the mechanisms driving the often observed patchiness of tree die-back is needed. Although forest-edge effects may play an important role within the given context, only few corresponding studies exist. Here, we investigate the regional die-back of Scots pine in Franconia, Germany, after a hot and dry summer in 2015, thereby emphasizing possible differences in mortality between forest edge and interior. By means of dendroecological investigations and close-range remote sensing, we assess long-term growth performance and current tree vitality along five different forest-edge distance gradients. Our results clearly indicate a differing growth performance between edge and interior trees, associated with a higher vulnerability to drought, increased mortality rates, and lower tree vitality at the forest edge. Prior long-lasting growth decline of dead trees compared to live trees suggests depletion of carbon reserves in course of a long-term drought persisting since the 1990ies to be the cause of regional Scots pine die-back. These findings highlight the forest edge as a potential focal point of forest management adaptation strategies in the context of drought-induced mortality.
Key message
Conifer radial growth reductions may be related to unusual snow conditions or a mismatch between frost hardiness level and minimum temperature, but not typically to low winter temperature extremes.
Abstract
The aim of this study was to examine if temperature conditions potentially causing frost damage have an effect on radial growth in Norway spruce and Scots pine in Finland. We hypothesized that frost damage occurs and reduces radial growth after (1) extreme cold winter temperatures, (2) frost hardiness levels insufficient to minimum temperatures, and (3) the lack of insulating snow cover during freezing temperatures, resulting in increased frost and decreased temperatures in soil. Meteorological records were used to define variables describing the conditions of each hypothesis and a dynamic frost hardiness model was used to find events of insufficient frost hardiness levels. As frost damage is likely to occur only under exceptional conditions, we used generalized extreme value distributions to describe the frost variables. Our results did not show strong connections between radial growth and the frost damage events. However, significant growth reductions were found at some Norway spruce sites after events insufficient frost hardiness levels, and alternatively, after winters with high frost sum of snowless days. Scots pine did not show significant growth reductions associated with any of the studied variables. Thus, radial growth in Norway spruce may be more sensitive to future changes in winter conditions. Our results demonstrate that considering only temperature is unlikely to be sufficient in studying winter temperature effects on tree growth. Instead, understanding the effects of changing temperature and snow conditions in relation to tree physiology and phenology is needed.
Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long-term alternatives for susceptible yet economically important tree species such as Norway spruce (Picea abies) are required if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought-tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed-species stands along an altitudinal gradient (400-1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought-prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.
Key messageRecent growth changes (1980–2007) in Western European forests strongly vary across tree species, and range from +42% in mountain contexts to −17% in Mediterranean contexts. These changes reveal recent climate warming footprint and are structured by species' temperature (−) and precipitation (+) growing conditions. ContextUnprecedented climate warming impacts forests extensively, questioning the respective roles of climatic habitats and tree species in forest growth responses. National forest inventories ensure a repeated and spatially systematic monitoring of forests and form a unique contributing data source. AimsA primary aim of this paper was to estimate recent growth changes in eight major European tree species, in natural contexts ranging from mountain to Mediterranean. A second aim was to explore their association with species’ climatic habitat and contemporary climate change. Methods
Using >315,000 tree increments measured in >25,000 NFI plots, temporal changes in stand basal area increment (BAI) were modelled. Indicators of climate normals and of recent climatic change were correlated to species BAI changes. ResultsBAI changes spanned from −17 to +42% over 1980–2007 across species. BAI strongly increased for mountain species, showed moderate/no increase for generalist and temperate lowland species and declined for Mediterranean species. BAI changes were greater in colder/wetter contexts than in warmer/drier ones where declines were observed. This suggested a role for climate warming, further found more intense in colder contexts and strongly correlated with species BAI changes. Conclusion
The predominant role of climate warming and species climatic habitat in recent growth changes is highlighted in Western Europe. Concern is raised for Mediterranean species, showing growth decreases in a warmer climate with stable precipitation.
Decline or health deterioration of Norway spruce (Picea abies (L.) Karst.) dominated forest stands has recently been observed mainly in sub-mountainous parts of Central Europe. Forest inventory of 208 randomly distributed circular plots including field observations of spruce tree health and rot symptoms by honey fungus (Armillaria ostoyae) was used for assessing intensity of spruce forest health decline in a managed forest area of 12.7 th. ha located in Beskids Mts., NE Czech Republic. First, principal component analysis was used to separate inventory variables related to environmental stress (reduced apical increment, dry tree top and stem resin exudation due to A. ostoyae infestation) into PC1, and health deterioration symptoms associated with mechanical damage (peeling, crown breaks) into PC2. The first two principal components explained 59% of the total variability in health decline symptoms. Spatial variability of both principal components was explained using spatial lag regression model identified from a set of environmental variables including sulfur and nitrogen deposition, elevation, solar radiation, age of the forest stands and geological properties (geochemical reactivity index). Environmental stress (PC1) was associated with low elevations (sub-optimal for spruce), high level of nitrogen and sulfur deposition (their interaction), low geochemical reactivity and also stand age. On the other hand, mechanical damage (PC2) significantly increased with elevation and stand age. As the forest decline in Beskids Mts. is related to A. ostoyae spreading from local infestation hot spots, both principal components had a significant spatial autocorrelation, partly distorting the signal of environmental conditions. The results indicate that the disturbed forest soils by long-term acid deposition and subsequent nutrient degradation and more pronounced drought stress at low elevations are the most important drivers of the recent spruce health decline in Beskids Mts.
Key message
Differences in temporal dynamics of radial growth and use of stem water reserves in co-occurring saplings and mature conifers are caused by soil water availability and canopy structure.
Abstract
High-resolution time series of stem radius variations (SRVs) record fluctuations in tree water status and temporal dynamics of radial growth. The focus of this study was to evaluate the influence of tree size (i.e., saplings vs. mature trees) and soil water availability on SRVs. Dendrometers were installed on Pinus sylvestris at an open xeric site and on Picea abies at a dry-mesic site, and the SRVs of co-occurring saplings and mature trees were analyzed during two consecutive years. The results revealed that irrespective of tree size, radial growth in P. sylvestris occurred in April–May, whereas the main growing period of P. abies was April–June (saplings) and May–June (mature trees). Linear relationships between growth-detrended SRVs (SSRVs) of mature trees vs. saplings and climate-SSRV relationships revealed greater use of water reserves by mature P. abies compared with saplings. This suggests that the strikingly depressed growth of saplings compared with mature P. abies was caused by source limitation, i.e., restricted photosynthesis beneath the dense canopy. In contrast, a tree size effect on the annual increment, SSRV, and climate–SSRV relationships was less obvious in P. sylvestris, indicating comparable water status in mature trees and saplings under an open canopy. The results of this study provided evidence that water availability and a canopy atmosphere can explain differences in temporal dynamics of radial growth and use of stem water reserves among mature trees and saplings.
Many forest ecosystems have experienced recent declines in productivity; however, in some alpine regions, tree growth and forest expansion are increasing at marked rates. Dendrochronological analyses at the upper limit of alpine forests in the Tibetan Plateau show a steady increase in tree growth since the early 1900s, which intensified during the 1930s and 1960s, and have reached unprecedented levels since 1760. This recent growth acceleration was observed in small/young and large/old trees and coincided with the establishment of trees outside the forest range, reflecting a connection between the physiological performance of dominant species and shifts in forest distribution. Measurements of stable isotopes (carbon, oxygen, and nitrogen) in tree rings indicate that tree growth has been stimulated by the synergistic effect of rising atmospheric CO2 and a warming-induced increase in water and nutrient availability from thawing permafrost. These findings illustrate the importance of considering soil-plant-atmosphere interactions to understand current and anticipate future changes in productivity and distribution of forest ecosystems.
Forests cover approximately one third of Central Europe. Based on a century of research tradition in phytosociology, potential vegetation mapping and palynology, oak (Quercus sp.) and European beech (Fagus sylvatica) are considered to be the natural dominants at low and middle altitudes, respectively. By contrast, currently many coniferous forests (especially of Picea abies) can be found especially at mid-altitudes, but these are thought to have resulted from forestry plantations in the past 200 years. Both nature conservation and forestry policy seek to promote broadleaved trees at the expense of conifers. However, several older and more recent papers pointed out discrepancies between conservation guidelines (included in Natura 2000) and historical/palaeoecological data with regard to the distribution of conifers. In this interdisciplinary paper, our aim was to bring new evidence into the debate on the conservation implications of coniferous tree species at mid-altitudes in Central Europe. We created a pollen-based vegetation and land-cover model for a highland area of 11,300 km(2) in the Czech Republic and assessed tree species composition in the forests before the onset of modern forestry based on 18(th) -century archival sources. The landscape model and pre-forestry archival evidence unequivocally demonstrated the dominance of coniferous trees in the study region throughout the entire Holocene. Broadleaved trees were present in a much smaller area than envisaged by current ideas of natural vegetation. Rather than casting doubt on the principles of Central European nature conservation in general, our results highlighted the necessity of detailed regional investigations as well as the importance of past data in challenging established notions on the natural distribution of tree species. This article is protected by copyright. All rights reserved.
As climate change drives increased drought in many forested regions, mechanistic understanding of the factors conferring drought tolerance in trees is increasingly important. The dendrochronological record provides a window through which we can understand how tree size and traits shape growth responses to droughts.
We analyzed tree‐ring records for twelve species in a broadleaf deciduous forest in Virginia (USA) to test hypotheses for how tree height, microenvironment characteristics, and species’ traits shaped drought responses across the three strongest regional droughts over a 60‐year period.
Drought tolerance (resistance, recovery, and resilience) decreased with tree height, which was strongly correlated with exposure to higher solar radiation and evaporative demand. The potentially greater rooting volume of larger trees did not confer a resistance advantage, but marginally increased recovery and resilience, in sites with low topographic wetness index. Drought tolerance was greater among species whose leaves lost turgor (wilted) at more negative water potentials and experienced less shrinkage upon desiccation.
The tree‐ring record reveals that tree height and leaf drought tolerance traits influenced growth responses during and after significant droughts in the meteorological record. As climate change‐induced droughts intensify, tall trees with drought‐sensitive leaves will be most vulnerable to immediate and longer‐term growth reductions.
Covering large parts of Europe, Norway spruce (Picea abies L Karst.) plays an important role in the adaptation strategy of forest services to future climate change. Although dendroecology can provide valuable information on the past relationships between tree growth and climate, most previous studies were biased towards species-specific distribution limits, where old individuals grow slowly under extreme conditions. In the present study, we investigated the growth variability and climate sensitivity of 2851 Norway spruce trees along longitudinal (E 12–26°), latitudinal (N 45–51°), and elevation (118–1591 m a.s.l.) gradients in central-eastern Europe. We reveal that summer weather significantly affects the radial growth of spruce trees, but the effects strongly vary along biogeographical gradients. Extreme summer heatwaves in 2000 and 2003 reduced the growth rates by 10–35%, most pronounced in the southern Carpathians. In contrast to the population in the Czech Republic, climate warming induced a synchronous decline in the growth rates across biogeographical gradients in the Carpathian arc. By demonstrating the increased vulnerability of Norway spruce under warmer climate conditions, we recommended that the forest services and conservation managers replace or admix monocultures of this species with more drought-resilient mixtures including fir, beech and other broadleaved species.
Evaporation from open water surfaces is often estimated based on the pan evaporation (Epan), which is an essential measure for estimating atmospheric evaporative demand. Within the Central European region, Epan appears to be slightly underestimated in the case of the hydrological balance of water bodies. In the context of the recent multi-year period of drought, significant losses of surface water deposits were observed in countries of Central Europe. In spite of the ‘evaporation paradox’ phenomenon, Epan is not generally decreasing as expected by many studies from past decades. Recorded observations from the Czech Republic show an increase in Epan, which is associated with an increase in global radiation and vapor pressure deficit. The vast majority of meteorological stations show a strong or very strong increase in Epan during April, June, July and August. During the 1971–2018 period, the annual mean Epan has been increasing by an average of 2.97 mm yr⁻¹. For the period 2001–2018, the mean Epan was 18% higher (519 mm) than the 1971–2000 average (440 mm). Our simulations of future scenarios, using regional climate models, predicted a growth in Epan of up to 27–54%. Such an increase in evaporation would cause serious consequences for surface water availability and agricultural production during the periods of drought in the Czech Republic, as the drought period 2014-2018 has clearly demonstrated.
Earth's hydroclimatic variability is increasing, with changes in the frequency of extreme events that may negatively affect forest ecosystems. We examined possible consequences of changing precipitation variability using tree rings in the conterminous United States. While many growth records showed either little evidence of precipitation limitation or linear relationships to precipitation, growth of some species (particularly those in semi-arid regions) responded asymmetrically to precipitation such that tree growth reductions during dry years were greater than, and not compensated by, increases during wet years. The U.S. Southwest, in particular, showed a large increase in precipitation variability, coupled with asymmetric responses of growth to precipitation. Simulations suggested roughly a twofold increase in the probability of large negative growth anomalies across the Southwest resulting solely from 20th century increases in variability of cool-season precipitation. Models project continued increases in precipitation variability, portending future growth reductions across semiarid forests of the western United States.
Climatic constraints on tree growth mediate an important link between terrestrial and atmospheric carbon pools. Tree rings provide valuable information on climate‐driven growth patterns, but existing data tend to be biased towards older trees on climatically extreme sites. Understanding climate change responses of biogeographic regions requires data that integrate spatial variability in growing conditions and forest structure. We analyzed both temporal (c. 1901‐2010) and spatial variation in radial growth patterns in 9 876 trees from fragments of primary Picea abies forests spanning the latitudinal and altitudinal extent of the Carpathian arc. Growth was positively correlated with summer temperatures and spring moisture availability throughout the entire region. However, important seasonal variation in climate responses occurred along geospatial gradients. At northern sites, winter precipitation and October temperatures of the year preceding ring formation were positively correlated with ring width. In contrast, trees at the southern extent of the Carpathians responded negatively to warm and dry conditions in autumn of the year preceding ring formation. An assessment of regional synchronization in radial growth variability showed temporal fluctuations throughout the 20th century linked to the onset of moisture limitation in southern landscapes. Since the beginning of the study period, differences between high and low elevations in the temperature sensitivity of tree growth generally declined, while moisture sensitivity increased at lower elevations. Growth trend analyses demonstrated changes in absolute tree growth rates linked to climatic change, with basal area increments in northern landscapes and lower altitudes responding positively to recent warming. Tree growth has predominantly increased with rising temperatures in the Carpathians, accompanied by early indicators that portions of the mountain range are transitioning from temperature to moisture limitation. Continued warming will alleviate large‐scale temperature constraints on tree growth, giving increasing weight to local drivers that are more challenging to predict. This article is protected by copyright. All rights reserved.
In the last years, large-scale mass forest withering and dieback have been reported for Scots pine (Pinus sylvestris)across eastern Europe, particularly in Romania. In these regions, the climate models forecast an increase in intensity and frequency of extreme climate events such as drought. Taking into account these aspects, the exact identification of the influences of drought on the loss of radial growth and vitality in Scots pine stands becomes mandatory. To achieve this aim, we developed the first country-wide Scots pine dendrochronological network in Romania consisting of 34 chronologies of basal area increment (BAI), and including 1401 individual tree-ring width series. Romanian Scots pine forests were severely impacted by the 2000 and 2012 droughts. The high temperatures and low precipitation from April to August were the main climatic causes of radial-growth reduction and large-scale withering in some areas. By mapping post-drought growth resilience, we identified locations where resilience was low and could identify foci of future forest dieback and high tree mortality. The projected appearance of similar prolonged and severe droughts in the future will lead to the damage or local extinction of some Scots pine forests in Romania, regardless of their age, composition or spatial location. The elaboration of adaptive forest management strategies to the impact of climate changes, specifically designed for the Scots pine stands, is not possible without knowing and understanding these aspects.
The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for assessing the science related to climate change. It provides regular assessments of the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation. This IPCC Special Report is a comprehensive assessment of our understanding of global warming of 1.5°C, future climate change, potential impacts and associated risks, emission pathways, and system transitions consistent with 1.5°C global warming, and strengthening the global response to climate change in the context of sustainable development and efforts to eradicate poverty. It serves policymakers, decision makers, stakeholders and all interested parties with unbiased, up-to-date, policy-relevant information. This title is also available as Open Access on Cambridge Core.
To study growth responses of trees to extreme events, analyses of so-called ‘pointer years’ are often performed. Thereby, the term pointer year refers to years in which a majority of trees shows extraordinary growth responses, like very narrow (or wide) ring widths. A wide variety of methods has been used to analyze pointer years, hampering comparisons between studies. The latter illustrates that there is a strong need to harmonize pointer year detection methods. This review contributes to that by describing and comparing the main methods (and variants thereof) found in the literature for the last two decades, both in a qualitative and quantitative way. We discuss methodological considerations and provide recommendations how to proceed with pointer year detection in future. Given that the individual methods for pointer year detection were found to highlight different aspects of extraordinary growth, the one method cannot completely substitute the other. Hence, we suggest to use multiple methods in a harmonized way to get the deepest insight into the nature of pointer years.
Interaction of disturbance agents may cause cascading effects in forests. The three most important disturbance agents of Norway spruce (Picea abies) in northern Europe are Heterobasidion root rot, wind and the European spruce bark beetle (Ips typographus). In this study, we present a mechanistic individual agent based model to simulate the dynamics of the bark beetle and integrate it in the simulation framework WINDROT to further study the interactive dynamics of root rot, wind and bark beetles. We carried out model performance analysis concluding that the model is sensitive to the parameters of optimal bark thickness for reproduction. Stand level interaction between wind and bark beetle disturbances was also evaluated against field data. The stand level simulations show the interaction between the disturbance agents; the root rot increases wind disturbance and bark beetles benefit from wind fallen trees. No direct interaction was found in the simulation study between the root rot and bark beetles. Further experimental research and large scale simulation studies are needed to better understand the underlying mechanisms in the interaction between the disturbance agents.
The similarity over long distances of dendroecological pointer years (with extreme ring-widths) were studied at both regional and country scales in order to investigate the geographical extension of climate influences on tree-rings. Two common species, Norway spruce (Picea abies Karst.) and white fir (Abies alba Mill.) were compared. The regional study was carried out on 33 populations located in four alpine valleys along a climatic gradient of summer aridity (Tarentaise, Maurienne, and Briançonnais, in France, and Susa valley in Italy). For most of species and regions, several negative ring-width pointer years with abrupt growth reductions such as 1976, 1922, 1986, and 1944 were common (listed in order of decreasing importance). However, spruce growth was more reduced in 1948 than that of fir. At the country scale, some of the strongest positive (e.g., 1932, 1964, 1969) and negative (e.g., 1956, 1962, 1976, 1986) pointer years extended over the whole of France, whereas the geographic variability was explainable by the autoecology of species. At both studied scales, evident climatic interpretations such as severe winter frosts, unusual summer droughts, or excessive wet and cold springs can explain most of the negative pointer years. Conversely, most positive growth responses are caused by a local combination of favorable climatic factors rather than simple extreme events, and therefore are less efficient for wood dating.
Tree growth response to recent environmental changes is of key interest for forest ecology. This study addressed the following questions with respect to Norway spruce (Picea abies, L. Karst.) in Central Europe: Has tree growth accelerated during the last five decades? What are the main environmental drivers of the observed tree radial stem growth and how much variability can be explained by them? Using a nationwide dendrochronological sampling of Norway spruce in the Czech Republic (1246 trees, 266 plots), novel regional tree-ring width chronologies for 40(±10)- and 60(±10)-year old trees were assembled, averaged across three elevation zones (break points at 500 and 700m). Correspondingly averaged drivers, including temperature, precipitation, nitrogen (N) deposition and ambient CO2 concentration, were used in a general linear model (GLM) to analyze the contribution of these in explaining tree ring width variability for the period from 1961 to 2013. Spruce tree radial stem growth responded strongly to the changing environment in Central Europe during the period, with a mean tree ring width increase of 24 and 32% for the 40- and 60-year old trees, respectively. The indicative General Linear Model analysis identified CO2, precipitation during the vegetation season, spring air temperature (March-May) and N-deposition as the significant covariates of growth, with the latter including interactions with elevation zones. The regression models explained 57% and 55% of the variability in the two tree ring width chronologies, respectively. Growth response to N-deposition showed the highest variability along the elevation gradient with growth stimulation/limitation at sites below/above 700m. A strong sensitivity of stem growth to CO2 was also indicated, suggesting that the effect of rising ambient CO2 concentration (direct or indirect by increased water use efficiency) should be considered in analyses of long-term growth together with climatic factors and N-deposition.
Significance
Climate extremes are major drivers of long-term forest growth trends, but we still lack appropriate knowledge to anticipate their effects. Here, we apply a conceptual framework to assess the vulnerability of Circum-Mediterranean Abies refugia in response to climate warming, droughts, and heat waves. Using a tree-ring network and a process-based model, we assess the future vulnerability of Mediterranean Abies forests. Models anticipate abrupt growth reductions for the late 21st century when climatic conditions will be analogous to the most severe dry/heat spells causing forest die-off in the past decades. However, growth would increase in moist refugia. Circum-Mediterranean fir forests currently subjected to warm and dry conditions will be the most vulnerable according to the climate model predictions for the late 21st century.
Circumboreal forest ecosystems are exposed to a larger magnitude of warming in comparison with the global average, as a result of warming-induced environmental changes. However, it is not clear how tree growth in these ecosystems responds to these changes. In the present study, we investigated the sensitivity of forest productivity to climate change using ring width indices (RWI) from a tree-ring width dataset accessed from the International Tree-Ring Data Bank and gridded climate datasets from the Climate Research Unit. A negative relationship of RWI with summer temperature and recent reductions of RWI were typically observed in continental dry regions, such as inner Alaska and Canada, southern Europe, and the southern part of eastern Siberia. We then developed a multiple regression model with regional meteorological parameters to predict RWI, and then applied to these models to predict how tree growth will respond to twenty-first-century climate change (RCP8.5 scenario). The projections showed a spatial variation and future continuous reduction in tree growth in those continental dry regions. The spatial variation, however, could not be reproduced by a dynamic global vegetation model (DGVM). The DGVM projected a generally positive trend in future tree growth all over the circumboreal region. These results indicate that DGVMs may overestimate future wood net primary productivity (NPP) in continental dry regions such as these; this seems to be common feature of current DGVMs. DGVMs should be able to express the negative effect of warming on tree growth, so that they simulate the observed recent reduction in tree growth in continental dry regions. This article is protected by copyright. All rights reserved.
Norway spruce has experienced unprecedented forest declines in recent decades, leading to exten-sive salvage logging. Currently, because of the conversion of conifer forests into more natural mixedforests in Central Europe, spruce has begun to be replaced, mainly by European beech. The frequentlydiscussed changing climate has a crucial effect on the vitality of both species. To improve our under-standing of spruce and beech responses to climate change, including more frequent temperature anddrought extremes, we investigated the impact of temporal climate variability on the growth of thesespecies along an elevation gradient. In total, 79 spruce and 90 beech trees were used to build species-specific tree-ring width chronologies representing the altitudinal range in which both species grow (450,650, 800, and 950 m asl) under the conditions of the Czech Republic. The climate–growth relationshipindicates strong dependence of spruce and beech tree-ring growth on spring temperature (Mar–May)at all sites and summer (Jun–Aug) water availability at lower altitudes. Significant temporal shifts inthe climate–growth relationships of both species indicate an increasing negative effect of summer tem-perature and positive effect of water availability in summer. The increasing drought and temperaturesensitivity of both species suggest a significant impact of the predicted climate change on such forestecosystems. Discussion emphasizes the current importance of adaptive forest management strategies.
An increase in drought frequency, duration and severity is expected for the Central European region as a direct consequence of climate change. This will have profound effects on a number of key sectors (e.g. agriculture, forestry, energy production and tourism) and also affect water resources, biodiversity and the landscape as a whole. However, global circulation models significantly differ in their projections for Central Europe with respect to the magnitude and timing of these changes. Therefore, analysis of changes in drought characteristics during the last 54 yr in relation to prevailing climate trends might significantly enhance our understanding of present and future drought risks. This study is based on a set of drought indices, including the Standardized Precipitation Index (SPI), the Palmer Drought Severity Index (PDSI), the Palmer Zindex (Z-index) and the Standardized Precipitation-Evapotranspiration Index (SPEI), in their most advanced formulations. The time series of the drought indices were calculated for 411 climatological stations across Austria (excluding the Alps), the Czech Republic and Slovakia. Up to 45% of the evaluated stations (depending on the index) became significantly drier during the 1961-2014 period except for areas in the west and north of the studied region. In addition to identifying the regions with the most pronounced drying trends, a drying trend consistency across the station network of 3 independent national weather services was shown. The main driver behind this development was an increase in the evaporative demand of the atmosphere, driven by higher temperatures and global radiation with limited changes in precipitation totals. The observed drying trends were most pronounced during the April-September period and in lower elevations. Conversely, the majority of stations above 1000 m exhibited a significant wetting trend for both the summer and winter (October-March) half-years.