Watering the trees for the forest: Drought alleviation in oaks and pines by ancestral ditches

Traditional ditches ("acequias" in Spanish) derive meltwater and infiltrate groundwater providing ecological services downstream in the semi-arid Sierra Nevada range (SE Spain). Therefore, they may act as a nature-based solution by alleviating drought stress in trees growing near ditches by enhancing growth and reducing their intrinsic water-use efficiency (iWUE). Such a mitigation role of acequias is critical given that some oak (Quercus pyrenaica) and pine (Pinus sylvestris) stands reach their xeric distribution limits in Europe. We compared tree-ring width data and wood δ13C, a proxy of iWUE, in oak and pine stands located near or far (control) from ditches with different infiltration capacity in two watersheds. We assessed how trees responded to climate data, drought stress, and vegetation greenness through correlations and resilience indices. Oak trees located near ditches grew more and responded less to precipitation, soil moisture, a drought index, and greenness than control trees. In pines, we did not find this pattern, and ditch trees grew more than control trees only during an extremely dry year (1995). Climate-growth correlations suggested a longer growing season in ditch pines. Growth of ditch oaks from the "Acequia Nueva" (AN), with high infiltration capacity, responded more to autumn soil moisture and showed the lowest δ13C. Growth was enhanced by cool-wet spring conditions in pines and also by warm-wet conditions in the prior winter in the case of oaks. Control trees showed lower resistance to drought. Control trees presented higher wood δ13C values except for old oaks from the "Acequia Grande" (AG) site which may show long-term acclimation. Traditional ditches alleviate drought stress in oak and pine stands subjected to regional xeric climate conditions.

Exploring interspecific hybridization dynamics in artificial forests of Pinus brutia and P. halepensis: Implications for sustainable afforestation

Interspecific hybridization increases genetic diversity, which is essential for coping with changing environments. Hybrid zones, occurring naturally in overlapping habitats of closely related species, can be artificially established during afforestation. The resulting interspecific hybridization may promote sustainability in artificial forests, particularly in regions facing degradation due to climate change. Currently, there is limited evidence of hybridization during regeneration of artificial forests. Here, we studied the frequency of Pinus brutia Ten. × P. halepensis Mill. hybridization in five planted forests in Israel in three stages of forest regeneration: seeds before dispersal, emerged seedlings and recruited seedlings at the end of the dry season. We found hybrids on P. brutia, but not on P. halepensis trees due to asynchronous cone production phenology. Using 94 single-nucleotide polymorphism (SNP) markers, we found hybrids at all stages, most of which were hybrids of advanced generations. The hybrid proportions increased from 4.7 ± 2.1 to 8.2 ± 1.4 and 21.6 ± 6.4 per cent, from seeds to emerged seedlings and to recruited seedlings stages, respectively. The increased hybrid ratio implies an advantage of hybrids over P. brutia during forest regeneration. To test this hypothesis, we measured seedling growth rate and morphological traits under controlled conditions and found that the hybrid seedlings exhibited selected traits of the two parental species, which likely contributed to the fitness and survival of the hybrids during the dry season. This study highlights the potential contribution of hybrids to sustainable-planted forests and contributes to the understanding of genetic changes that occur during the regeneration of artificial forests.

Drought legacies in mixed Mediterranean forests: Analysing the effects of structural overshoot, functional traits and site factors

Previous favorable climate conditions stimulate tree growth making some forests more vulnerable to hotter droughts. This so-called structural overshoot may contribute to forest dieback, but there is little evidence on its relative importance depending on site conditions and tree species because of limited field data. Here, we analyzed remote sensing (NDVI) and tree-ring width data to evaluate the impacts of the 2017 drought on canopy cover and growth in mixed Mediterranean forests (Fraxinus ornus, Quercus pubescens, Acer monspessulanum, Pinus pinaster) located in southern Italy. Legacy effects were assessed by calculating differences between observed and predicted basal area increment (BAI). Overall, the growth response of the study stands to the 2017 drought was contingent on site conditions and species characteristics. Most sites presented BAI and canopy cover reductions during the drought. Growth decline was followed by a quick recovery and positive legacy effects, particularly in the case of F. ornus. However, we found negative drought legacies in some species (e.g., Q. pubescens, A. monspessulanum) and sites. In those sites showing negative legacies, high growth rates prior to drought in response to previous wet winter-spring conditions may have predisposed trees to drought damage. Vice versa, the positive drought legacy found in some F. ornus site was linked to post-drought growth release due to Q. pubescens dieback and mortality. Therefore, we found evidences of structural drought overshoot, but it was restricted to specific sites and species. Our findings highlight the importance of considering site settings such as stand composition, pre-drought conditions and different tree species when studying structural overshoot. Droughts contribute to modify the composition and dynamics in mixed forests.

Variation in xylem vulnerability to cavitation shapes the photosynthetic legacy of drought

Increased drought conditions impact tree health, negatively disrupting plant water transport which, in turn, affects plant growth and survival. Persistent drought legacy effects have been documented in many diverse ecosystems, yet we still lack a mechanistic understanding of the physiological processes limiting tree recovery after drought. Tackling this question, we exposed saplings of a common Australian evergreen tree (Eucalyptus viminalis) to a cycle of drought and rewatering, seeking evidence for a link between the spread of xylem cavitation within the crown and the degree of photosynthetic recovery postdrought. Individual leaves experiencing >35% vein cavitation quickly died but this did not translate to a rapid overall canopy damage. Rather, whole canopies showed a gradual decline in mean postdrought gas exchange rates as water stress increased. This gradual loss of canopy function postdrought was due to a significant variation in cavitation vulnerability of leaves within canopies leading to diversity in the capacity of leaves within a single crown to recover function after drought. These results from the evergreen E. viminalis emphasise the importance of within-crown variation in xylem vulnerability as a central character regulating the dynamics of canopy death and the severity of drought legacy through time.

Growth of tree (Pinus sylvestris) and shrub (Amelanchier ovalis) species is constrained by drought with higher shrub sensitivity in dry sites

We lack understanding of how variable is radial growth of coexisting tree and shrub species, and how growth is constrained by drought depending on site aridity. Here, we compared the radial growth of two widespread and coexisting species, a winter deciduous shrub (Amelanchier ovalis Medik.) and an evergreen conifer tree (Pinus sylvestris L.). We sampled four sites in Northeastern Spain subjected to different aridity levels and used dendrochronological methods to quantify growth patterns and responses to climate variables. The growth of the two species varied between regions, being lower in the driest sites. The first-order autocorrelation (growth persistence) was higher in more mesic sites but without clear differences between species. Tree and shrub growth negatively responded to elevated summer temperatures and positively to spring-summer precipitation and wet conditions. However, negative growth responses of the shrub to drought were only observed in the two driest sites in contrast to widespread responses of the tree. Abrupt growth reductions were common in the drier sites, but resilience indices show that the two species rapidly recovered pre-drought growth levels. The lower growth synchrony of the shrub as compared to the tree can be due to the multistemmed architecture, fast growth and low stature of the shrub. Besides, the high dependency of the shrub growth on summer rainfall can explain why drought limitations were only apparent in the two driest sites. In any case, results point out to the dendrochronological potential of shrubs, which is particularly relevant giving its ability to inhabit woodlands and treeless regions under harsh climatic conditions. Nevertheless, further research is required to elucidate the capacity of shrub species to tolerate drought, as well as to understand how shrubs thrive in water- and cold-limited environments.

Drought-adapted leaves are produced even when more water is available in dry tropical forest

Species in dry environments may adjust their anatomical and physiological behaviors by adopting safer or more efficient strategies. Thus, species distributed across a water availability gradient may possess different phenotypes depending on the specific environmental conditions to which they are subjected. Leaf and vascular tissues are plastic and may vary strongly in response to environmental changes affecting an individual's survival and species distribution. To identify whether and how legumes leaves vary across a water availability gradient in a seasonally dry tropical forest, we quantified leaf construction costs and performed an anatomical study on the leaves of seven legume species. We evaluated seven species, which were divided into three categories of rainfall preference: wet species, which are more abundant in wetter areas; indifferent species, which are more abundant and occur indistinctly under both rainfall conditions; and dry species, which are more abundant in dryer areas. We observed two different patterns based on rainfall preference categories. Contrary to our expectations, wet and indifferent species changed traits in the sense of security when occupying lower rainfall areas, whereas dry species changed some traits when more water was available, such as increasing cuticle and spongy parenchyma thickness, or producing smaller and more numerous stomata. Trischidium molle, the most plastic and wet species, exhibited a similar strategy to the dry species. Our results corroborate the risks to vegetation under future climate change scenarios as stressed species and populations may not endure even more severe conditions.

The interplay of drought and hurricanes on tree recovery: insights from dynamic and weak functional responses

Identifying the functional traits that enable recovery after extreme events is necessary for assessing forest persistence and functioning. However, the variability of traits mediating responses to disturbances presents a significant limitation, as these relationships may be contingent on the type of disturbance and change over time. This study investigates the effects of traits on tree growth-for short and longer terms-in response to two vastly different extreme climatic events (droughts and hurricanes) in a Puerto Rican forest. I found that trees display a dynamic functional response to extreme climatic events. Leaf traits associated with efficient photosynthesis mediated faster tree growth after hurricanes, while trees with low wood density and high water use efficiency displayed faster growth after drought. In the longer term, over both drought and hurricanes, tree size was the only significant predictor of growth, with faster growth for smaller trees. However, despite finding significant trait-growth relationships, the predictive power of traits was overall low. As the frequency of extreme events increases due to climate change, understanding the dynamic relationships between traits and tree growth is necessary for identifying strategies for recovery.

Climate and Soil Microsite Conditions Determine Local Adaptation in Declining Silver Fir Forests

Ongoing climatic change is threatening the survival of drought-sensitive tree species, such as silver fir (Abies alba). Drought-induced dieback had been previously explored in this conifer, although the role played by tree-level genetic diversity and its relationship with growth patterns and soil microsite conditions remained elusive. We used double digest restriction-site-associated DNA sequencing (ddRADseq) to describe different genetic characteristics of five silver fir forests in the Spanish Pyrenees, including declining and non-declining trees. Single nucleotide polymorphisms (SNPs) were used to investigate the relationships between genetics, dieback, intraspecific trait variation (functional dendrophenotypic traits and leaf traits), local bioclimatic conditions, and rhizosphere soil properties. While there were no noticeable genetic differences between declining and non-declining trees, genome-environment associations with selection signatures were abundant, suggesting a strong influence of climate, soil physicochemical properties, and soil microbial diversity on local adaptation. These results provide novel insights into how genetics and diverse environmental factors are interrelated and highlight the need to incorporate genetic data into silver fir forest dieback studies to gain a better understanding of local adaptation.

Blue is the fashion in Mediterranean pines: New drought signals from tree-ring density in southern Europe

Long-term records of tree-ring width (TRW), latewood maximum density (MXD) and blue intensity (BI) measurements on conifers have been largely used to develop high-resolution temperature reconstructions in cool temperate forests. However, the potential of latewood blue intensity (LWBI), less commonly used earlywood blue intensity (EWBI), and delta (difference between EWBI and LWBI, dBI) blue intensity in Mediterranean tree species is still unexplored. Here we developed BI chronologies in moist-elevation limits of the most southwestern European distribution of Pinus nigra subsp. salzmanii Arnold. We tested whether BI variables derived from tree rings of black pine are better proxies than ring-width variables to reconstruct long-term changes in climatic factors and water availability. For this we applied correlations and regression analyses with daily and monthly climate data, a spatial and temporal drought index (Standardized Precipitation-Evapotranspiration Index-SPEI) and Vapour Pressure Deficit (VPD), as well as atmospheric circulation patterns: North Atlantic Oscillation (NAO), Southern Oscillation Index (SOI) and Western Mediterranean Oscillation (WeMO). We found a positive relation between black pine growth (RW) and temperature during the winter preceding the growing season. Among all variables LWBI and dBI were found to be more sensitive than TRW to SPEI at low-elevation site, with EWBI series containing an opposite climatic signal. LWBI and dBI were significantly related to June and September precipitation at high-elevation site. Winter VPD was related with higher EWI and LWI series, whereas dBI and EWBI were related with January SOI and February NAO. We confirm the potential of long-term dBI series to reconstruct climate in drought-prone regions. This novel study in combination with other wood anatomical measurements has wide implications for further use of BI to understand and reconstruct environmental changes in Mediterranean conifer forests.

Unexpected resilience in relict Abies pinsapo Boiss forests to dieback and mortality induced by climate change

Acute and early symptoms of forest dieback linked to climate warming and drought episodes have been reported for relict Abies pinsapo Boiss. fir forests from Southern Spain, particularly at their lower ecotone. Satellite, orthoimages, and field data were used to assess forest decline, tree mortality, and gap formation and recolonization in the lower half of the altitudinal range of A. pinsapo forests (850-1550 m) for the last 36 years (1985-2020). Field surveys were carried out in 2003 and in 2020 to characterize changes in stand canopy structure and mortality rates across the altitudinal range. Time series of the Normalized Difference Vegetation Index (NDVI) at the end of the dry season (derived from Landsat 5 and 7 imagery) were used for a Dynamic Factor Analysis to detect common trends across altitudinal bands and topographic solar incidence gradients (SI). Historical canopy cover changes were analyzed through aerial orthoimages classification. Here we show that extensive decline and mortality contrast to the almost steady alive basal area for 17 years, as well as the rising photosynthetic activity derived from NDVI since the mid-2000s and an increase in the forest canopy cover in the late years at mid and high altitudes. We hypothesized that these results suggest an unexpected resilience in A. pinsapo forests to climate change-induced dieback, that might be promoted by compensation mechanisms such as (i) recruitment of new A. pinsapo individuals; (ii) facilitative effects on such recruitment mediated by revegetation with other species; and (iii) a 'release effect' in which surviving trees can thrive with fewer resource competition. Future research is needed to understand these compensation mechanisms and their scope in future climate change scenarios.

Tree-ring evidence of ecological stress memory

Plants experiencing stress could develop the ability to reshape their response toward present stress based on past stress experience, called 'ecological stress memory' (ESM), which is important for plant acclimation to repeated stresses. Although ESM has been largely reported, it remains unclear whether ESM could improve tree resistance to recurrent stress in subsequent decades. Here, we explore it from a tree-ring network of 1491 trees from 50 long-living juniper forests on the Tibetan Plateau. Through comparing performances of tree radial growth in past sequential growth stresses, we found that trees could obtain ESM under antecedent stresses and elevate resistance to subsequent stress after several years or even decades. Such positive effects of ESM are associated with post-stress recovery. Trees with slow recovery trajectories after antecedent stress show significantly improved resistance to subsequent stress, while trees with extremely fast post-stress recovery showed decreased resistance to subsequent stress. These results imply that temporary depressive tree radial growth after antecedent stress might be a trigger of long storage of ESM. Incorporating positive effects of ESM and relationship between ESM activation and post-stress recovery into future Earth system models could advance our capacity to predict forest dynamics and forest ecosystem stabilization under future stress conditions.

Towards species-level forecasts of drought-induced tree mortality risk

Predicting species-level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections. We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South-Eastern Australia (mean annual precipitation range: 344-1424 mm yr^-1 ). We conducted three experiments: applying CABLE to the 2017-2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO₂ ). The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within-species' ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO₂ in ameliorating drought stress was consistent across species. Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision-making around the resilience of restoration plantings or net-zero emission strategies.

Tree growth response to drought partially explains regional-scale growth and mortality patterns in Iberian forests

Tree-ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes in forest dynamics. Here, we related tree-ring growth chronologies and stand-level forest changes in basal area from two independent data sets to test if tree-ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth, and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree-ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981-2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index, and their impacts on tree growth by quantifying tree-ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought-prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought-induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought-prone gymnosperm forests, we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth, and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought-induced growth reductions and their cumulative impacts have strong potential to be used as early-warning indicators of regional forest vulnerability.

The role of nutritional impairment in carbon-water balance of silver fir drought-induced dieback

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ¹⁸ O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ¹⁸ O than ND trees. The uncoupling between iWUE and δ¹⁸ O, together with the positive correlations between P and K leaf concentrations and δ¹⁸ O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

Species- and Age-Specific Growth Reactions to Extreme Droughts of the Keystone Tree Species across Forest-Steppe and Sub-Taiga Habitats of South Siberia

Over the coming decades, climate change can decrease forest productivity and stability in many semiarid regions. Tree-ring width (TRW) analysis allows estimation of tree sensitivity to droughts, including resistance (Rt) and resilience (Rc) indexes. It helps to find adaptive potential of individual trees and forest populations. On a forest stand scale, it is affected by habitat conditions and species’ ecophysiological characteristics, and on individual scale by tree genotype, age, and size. This study investigated TRW response to droughts in forest-steppe and sub-taiga of southern Siberia for keystone species Scots pine (Pinus sylvestris L.), Siberian larch (Larix sibirica Ledeb.), and silver birch (Betula pendula Roth.). Chronologies reacted positively to the Standardized Precipitation-Evapotranspiration Index (SPEI) of the previous July–September and current April–July. Depressed tree growth across region and droughts lasting over both intra-seasonal intervals were registered in 1965, 1974, and 1999. TRW-based Rt and Rc for these droughts did not reveal age- or size-related patterns. Higher growth stability indexes were observed for birch in sub-taiga and for conifers in forest-steppe. Larch at all sites had disadvantage against pine for 1965 and 1999 droughts aggravated by pest outbreaks, but adapted better to drought in 1974. Site aridity affected both tree growth stability and intensity of climatic response.

Declines in canopy greenness and tree growth are caused by combined climate extremes during drought-induced dieback

Several dieback episodes triggered by droughts are revealing the high vulnerability of Mediterranean forests, manifested as declines in growth, increased defoliation, and rising mortality rates. Understanding forest responses to such climate extreme events is of high priority for predicting their future vegetation dynamics. We examined how remotely sensed measures of vegetation activity (NDVI, Normalized Difference Vegetation Index) and radial growth (BAI, basal area increment) responded to climate extreme events. We considered tree (Pinus sylvestris, Quercus pubescens, Quercus frainetto) and shrub (Juniperus phoenicea) populations from Italy and Spain showing recent dieback phenomena. Two components of drought, namely elevated atmospheric demand (VPD, vapor pressure deficit) and low soil moisture were analyzed in nearby stands showing or not showing dieback symptoms. Dieback stands exhibited lower NDVI values than non-dieback stands. NDVI and BAI were positively related in all sites except for the dieback stand of Q. frainetto that was negatively related. Such NDVI-BAI linkages were related to specific time windows, which could be useful for identifying when climatic conditions have the greatest influence on vegetation. Growth decline occurred in response to increasing VPD, but responses differed among species. J. phoenicea was the most negatively impacted by higher VPD, whereas oaks responded to soil moisture. A high VPD was related to stronger growth reduction in dieback P. sylvestris trees regardless of soil moisture changes. We highlighted that coupling between proxies of forest productivity (NDVI, BAI) allows better understanding and forecasting of drought-induced dieback phenomena in forests and shrublands. Scaling up from tree to stand levels might be feasible when using the maximum growing season NDVI, which can be applied for retrospective modeling of the impact of drought stress on forest productivity and tree growth.

Short-Term Effects of Droughts and Cold Winters on the Growth of Scots Pine at Coastal Sand Dunes around the South Baltic Sea

Coastal dunes near the Baltic Sea are often stabilized by Scots pine forests and are characterized by a mild climate. These ecosystems are affected by water shortages and might be influenced by climate extremes. Considering future climate change, utilizing tree rings could help assess the role of climate extremes on coastal forest growth. We used superposed epoch analysis to study Scots pine responses to droughts and cold winters, with focus on frequency, timing, and duration. We measured ring widths (RW) and latewood blue intensity (LBI) on samples extracted from trees growing at dune ridge and bottom microsites at the south Baltic Sea. At the regional scale, we observed some similarities in tree responses to both extremes between RW and LBI within the same microsite type and region. At the local scale, RW and LBI were more frequently influenced by cold winters than droughts. RW and LBI from dune ridges were more frequently influenced by droughts than RW and LBI from dune bottoms. LBI from both microsites was more often influenced by droughts than RW. RW and LBI from both microsites were similarly often influenced by cold winters. At both scales, the response time of RW and LBI after droughts predominantly lagged by one year, while cold winters were recorded in the same year. The typical duration of growth reductions after both extremes was one year for both RW and LBI. Our study indicates that Scots pine from the Baltic Sea region is sensitive to climate extremes, especially cold winters.

A Tree Ring Proxy Evaluation of Declining Causes in Pinus sylvestris L. and Pinus nigra J.F. Arnold in Northeastern Romania

Drought-induced dieback has been extensively studied in various forests habitats. We used a retrospective tree ring width (TRW), basal area increment (BAI), oxygen isotope ratios in tree ring cellulose (δ18OTR) and carbon isotope ratios in tree ring cellulose (δ13CTR) to assess causes in declining Pinus sylvestris L. and Pinus nigra J.F. Arnold. The climate data analysis indicates a significant increased trend occurred after 1980 in minimum, mean and maximum temperature and a reduced amount of precipitation compared to the 1920–1980-time scale. According to the Palmer Drought Severity Index, we found two extreme drought years (1946 and 2000) and three years with severe drought (1990, 2003 and 2012). One-way ANOVA indicated no significant difference between P. nigra and P sylvestris tree ring width, basal area increment, but a considerable difference between δ13CTR and δ18OTR. Basal area increment evaluated the climate-growth relationship most accurately, comparing to δ18OTR and δ13CTR, which explained the influences of environmental factors in tree rings formation. The δ13CTR was mainly negatively correlated with high temperatures from April-August current growing seasons. The negative correlation between δ13CTR and NDVI indices (June, August) shows a decreased carbon uptake induced by drought from summer to early autumn. The low δ18OTR signal was associated with a complex of factors, including the strong influence of heavy precipitation occurring in the growing season and a weak reaction of declined trees to resources. Species-specific responses to drought in 1990, 2003 and 2012 indicated P. sylvestris as more sensitive to drought whit higher demand for water supply in the optimal compared with P. nigra. Weak and unstable correlations in time with increasing/decreasing values in drought periods were obtained more accurately using δ18OTR compared to δ13CTR. The species-specific resilience response to drought years showed a weak resilience and resistance in P. sylvestris occurred more evident after the 2012 event compared to less sensitive P. nigra trees. Decision-makers can use presented results to reinforce specific management plans capable of protecting and changing local compositions where is the case with species more resistant to drouth.

The Past Matters: Previous Management Strategies Modulate Current Growth and Drought Responses of Norway Spruce (Picea abies H. Karst.)

Due to high productivity and past management approaches, the forests of Central Europe are heavily dominated by conifers, even on sites where they do not occur naturally at all. One prominent example is Norway spruce (Picea abies H. Karst.), a species considered particularly vulnerable to severe droughts, especially outside of its ecological niche where it has been widely planted over the past centuries. In the face of global change, it is a major task for foresters to increase these forests’ ability to cope with the impacts of increasing climatic extremes. Therefore, gaining more knowledge about how different management strategies affect the drought responses of trees is crucial. However, we still know little about the influence of the individual treatment history of a tree on its growth. We used a dendroecological approach to address this issue and to assess how initial spacing, structural diversity, tree size, and density regulation approaches modulate annual growth, especially in drought years. We hypothesized that stand establishment and past silvicultural treatment codetermine tree growth and drought resilience. Our study took place at the combined spacing-thinning trial Fürstenfeldbruck 612 (FFB 612) in Southern Germany, since it delivered precise long-term data covering a broad range of treatments. Based on linear mixed effect models, we showed that the individual treatment history of a tree affects its annual growth and drought responses considerably. In more detail, we found that (i) high structural diversity in the vicinity of each tree favored growth and improved a tree’s performance under drought; (ii) larger trees were more climate-sensitive; (iii) previous high variations in thinning intensity, and consequently strong fluctuations in growth, affected current growth negatively and reduced recovery from droughts. Furthermore, we sought to understand the underlying mechanisms and to draw potential implications for refining silvicultural guidelines.

SilvAdapt.Net: A Site-Based Network of Adaptive Forest Management Related to Climate Change in Spain

Adaptive forest management (AFM) is an urgent need because of the uncertainty regarding how changes in the climate will affect the structure, composition and function of forests during the next decades. Current research initiatives for the long-term monitoring of impacts of silviculture are scattered and not integrated into research networks, with the consequent losses of opportunities and capacity for action. To increase the scientific and practical impacts of these experiences, it is necessary to establish logical frameworks that harmonize the information and help us to define the most appropriate treatments. In this context, a number of research groups in Spain have produced research achievements and know-how during the last decades that can allow for the improvement in AFM. These groups address the issue of AFM from different fields, such as ecophysiology, ecohydrology and forest ecology, thus resulting in valuable but dispersed expertise. The main objective of this work is to introduce a comprehensive strategy aimed to study the implementation of AFM in Spain. As a first step, a network of 34 experimental sites managed by 14 different research groups is proposed and justified. As a second step, the most important AFM impacts on Mediterranean pines, as one of the most extended natural and planted forest types in Spain, are presented. Finally, open questions dealing with key aspects when attempting to implement an AFM framework are discussed. This study is expected to contribute to better outlining the procedures and steps needed to implement regional frameworks for AFM.

Drought Drives Growth and Mortality Rates in Three Pine Species under Mediterranean Conditions

Drought constrains tree growth in regions with seasonal water deficit where growth decline can lead to tree death. This has been observed in regions such as the western Mediterranean Basin, which is a climate-warming hotspot. However, we lack information on intra- and inter-specific comparisons of growth rates and responses to water shortage in these hotspots, considering tree species with different drought tolerance. We sampled several sites located in north-eastern Spain showing dieback and high mortality rates of three pine species (Pinus sylvestris, Pinus pinaster, Pinus halepensis). We dated death years and reconstructed the basal area increment of coexisting living and recently dead trees using tree ring data. Then, we calculated bootstrapped Pearson correlations between a drought index and growth. Finally, we used linear mixed-effects models to determine differences in growth trends and the response to drought of living and dead trees. Mortality in P. sylvestris and P. pinaster peaked in response to the 2012 and 2017 droughts, respectively, and in sites located near the species’ xeric distribution limits. In P. halepensis, tree deaths occurred most years. Dead trees showed lower growth rates than living trees in five out of six sites. There was a strong growth drop after the 1980s when climate shifted towards warmer and drier conditions. Tree growth responded positively to wet climate conditions, particularly in the case of living trees. Accordingly, growth divergence between living and dead trees during dry periods reflected cumulative drought impacts on trees. If aridification continues, tree drought mortality would increase, particularly in xeric distribution limits of tree species.

Growth of 19 conifer species is highly sensitive to winter warming, spring frost and summer drought

BACKGROUND AND AIMS

Conifers are key components of many temperate and boreal forests and are important for forestry, but species differences in stem growth responses to climate are still poorly understood and may hinder effective management of these forests in a warmer and drier future.

METHODS

We studied 19 Northern Hemisphere conifer species planted in a 50-year-old common garden experiment in the Netherlands to (1) assess the effect of temporal dynamics in climate on stem growth, (2) test for a possible positive relationship between the growth potential and climatic growth sensitivity across species, and (3) evaluate the extent to which stem growth is controlled by phylogeny.

KEY RESULTS

Eighty-nine per cent of the species showed a significant reduction in stem growth to summer drought, 37 % responded negatively to spring frost and 32 % responded positively to higher winter temperatures. Species differed largely in their growth sensitivity to climatic variation and showed, for example, a four-fold difference in growth reduction to summer drought. Remarkably, we did not find a positive relationship between productivity and climatic sensitivity, but instead observed that some species combined a low growth sensitivity to summer drought with high growth potential. Both growth sensitivity to climate and growth potential were partly phylogenetically controlled.

CONCLUSIONS

A warmer and drier future climate is likely to reduce the productivity of most conifer species. We did not find a relationship between growth potential and growth sensitivity to climate; instead, some species combined high growth potential with low sensitivity to summer drought. This may help forest managers to select productive species that are able to cope with a warmer and drier future.

Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback

Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two Pinus pinaster stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.

Do Extreme Climate Events Cause the Degradation of Malus sieversii Forests in China?

Frequent extreme climate events have attracted considerable attention around the world. Malus sieversii in Xinjiang is the ancestor of cultivated apple, and it is mainly distributed in the Ili river valley at end of the Tianshan Mountains. Wild fruit forests have been degraded, but the cause remains unclear. In order to identify whether extreme climate events caused this degradation reanalysis data and atmospheric circulation indices were used to determine the trends and the reasons for extreme climate changes. Subsequently, we further investigated the effect of extreme climate events on wild fruit forest using characteristics of extreme climate indices and tree-ring chronology. We found increasing trends in both extreme precipitation and warm indices, and decreasing trends in cool indices. Extreme climate events were mainly associated with the Atlantic Multidecadal Oscillation (AMO). Analysis of data of wind and geopotential height field at 500 hPa showed that strengthening wind, increasing geopotential height, cyclone and anti-cyclone circulation drivers contributed to extreme climate events. In the non-degraded region, there were significant positive correlations between tree-ring chronology and both extreme precipitation and extreme warm indices (except for warm spell duration indicator). The other extreme indices (except for heavy rain days) had a large correlation range with tree-rings in a 4-8-year period. These results indicated that extreme precipitation and extreme warm indices intensified M. sieversii growth of the non-degraded region on multi-time scales. In contrast, the degraded region showed insignificant negative relationship between tree-ring chronology and both extreme precipitation and extreme warm indices [except for warm spell duration index (WSDI)], and significant negative correlations in a 4-8-year period were detected between tree-ring chronology and most of the extreme precipitation indices, including heavy rain days, very wet days, cold spell duration indicator, simple precipitation intensity index (SDII), and annual total precipitation. Under the long disturbance of inappropriate anthropic activities, extreme climate has caused the outbreak of pests and diseases resulting in the degeneration of wild fruit forest. Our study provides scientific guidance for the ecosystem conservation in wild fruit forest in China, and also across the region.

Xylem Phenology and Growth Response of European Beech, Silver Fir and Scots Pine along an Elevational Gradient during the Extreme Drought Year 2018

Highlights: European beech (Fagus sylvatica L.) and silver fir (Abies alba Mill.) displayed parabolic elevational trends of the cessation of xylem cell differentiation phases. Xylem phenology and growth rates of Scots pine (Pinus sylvestris L.) appeared to be less influenced by the 2018 drought, whereas beech reduced growth on the lowest elevation and fir seemed negatively affected in general. Background: The year 2018 was characterized by multiple drought periods and heat waves during the growing season. Our aim was to understand species-specific responses of xylem phenology and growth to drought and how this effect was modified along an elevational gradient. Materials and Methods: We sampled microcores and increment cores along an elevational gradient in the southwestern Black Forest (SW Germany) region and analyzed xylem phenology and growth response to drought. Results: Termination of cell enlargement and lignification occurred earliest in beech and latest in pine. Beech had the highest growth rates but shortest growth durations, fir achieved moderate rates and medium durations and pine had lowest growth rates despite long growth durations. In contrast to pine, onsets of cell differentiation phases of fir and beech did not show clear linear relationships with elevation. Cessation of cell production and lignification of beech and fir followed a parabolic elevational trend and occurred earliest on low elevations, whereas pine showed no changes with elevation. Tree-ring width, generally, depended 3–4 times more on the growth rate than on growth duration. Conclusions: The possibly drought-induced early cessation of cell differentiation and considerable growth reduction of beech appeared to be most severe on the lowest elevation. In comparison, growth reductions of fir were larger and seemed independent from elevation. We found evidence, that productivity might be severely affected at lower elevations, whereas at high elevations wood production might not equally benefit during global warming.

Disentangling Mechanisms of Drought-Induced Dieback in Pinus nigra Arn. from Growth and Wood Isotope Patterns

The increased frequency and intensity of warming-induced droughts have triggered dieback episodes affecting many forest types and tree species worldwide. Tree plantations are not exempt as they can be more vulnerable to drought than natural forests because of their lower structural and genetic diversity. Therefore, disentangling the physiological mechanisms leading to growth decline and tree mortality can provide tools to adapt forest management to climate change. In this study, we investigated a Pinus nigra Arn. plantation situated in northern Spain, in which some trees showed canopy dieback and radial-growth decline. We analyzed how radial growth and its responses to drought events differed between non-declining (ND) and declining (D) trees showing low and high canopy defoliation, respectively, in combination with carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings. The radial growth of P. nigra was constrained by water availability during the growing season and the previous autumn. The radial growth of D trees showed higher sensitivity to drought than ND trees. This fact is in accordance with the lower drought resilience and negative growth trends observed in D trees. Both tree classes differed in their growth from 2012 onwards, with D trees showing a reduced growth compared to ND trees. The positive δ13C-δ18O relationship together with the uncoupling between growth and intrinsic water-use efficiency suggest that D trees have less tight stomatal regulation than ND trees, which could involve a high risk of xylem embolism in the former class. Our results suggest that different water use strategies between coexisting ND and D trees were behind the differences in growth patterns and point to hydraulic failure as a possible mechanism triggering dieback and growth decline.

Resilience of Spanish forests to recent droughts and climate change

A widespread increase in forest cover is underway in northern Mediterranean forests because of land abandonment and decreased wood demand, but the resilience of these successional forests to climate change remains unresolved. Here we use 18-year time series of canopy greenness derived from satellite imagery (NDVI) to evaluate the impacts of climate change on Spain's forests. Specifically, we analyzed how NDVI was influenced by the climatic water balance (i.e. Standardized Precipitation-Evapotranspiration Index, SPEI), using monthly time-series extracted from 3,100 pixels of forest, categorized into ten forest types. The forests increased in leaf area index by 0.01 per year on average (from 1.7 in 2000 to 1.9 in 2017) but there was enormous variation among years related to climatic water balance. Forest types varied in response to drought events: those dominated by drought-avoiding species showed strong covariance between greenness and SPEI, while those dominated by drought-tolerant species showed weak covariance. Native forests usually recovered more than 80% of greenness within the 18 months and the remainder within 5 years, but plantations of Eucalyptus were less resilient. Management to increase the resilience of forests-a key goal of forestry in the Mediterranean region-appears to have had a positive effect: canopy greenness within protected forests was more resilient to drought than within non-protected forests. In conclusion, many of Spain's successional forests have been resilient to drought over the past 18 years, from the perspective of space. Future studies will need to combine remote sensing with field-based analyses of physiological tolerances and mortality processes to understand how Mediterranean forests will respond to the rapid climate change predicted for this region in the coming decades.

Linkages between Climate, Radial Growth and Defoliation in Abies pinsapo Forests from Southern Spain

Systematic forest networks of health monitoring have been established to follow changes in tree vigor and mortality. These networks often lack long-term growth data, but they could be complemented with tree ring data, since both defoliation and radial growth are proxies of changes in tree vigor. For instance, a severe water shortage should reduce growth and increase tree defoliation in drought-prone areas. However, the effects of climatic stress and drought on growth and defoliation could also depend on tree age. To address these issues, we compared growth and defoliation data with recent climate variability and drought severity in Abies pinsapo old and young trees sampled in Southern Spain, where a systematic health network (Andalucía Permanent Plot Network) was established. Our aims were: (i) to assess the growth sensitivity of old and young A. pinsapo trees and (ii) to test if relative changes in radial growth were related with recent defoliation, for instance, after severe droughts. We also computed the resilience indices to quantify how old and young trees recovered growth after recent droughts. Wet-cool conditions during the prior autumn and the current early summer improved the growth of old trees, whereas late-spring wet conditions enhanced the growth of young trees. Old trees were more sensitive to wet and sunny conditions in the early summer than young trees. Old and young trees were more responsive to the Standardized Precipitation-Evapotranspiration Index drought index of June–July and July–August calculated at short (one–three months) and mid (three–six months) time scales, respectively. Old trees presented a higher resistance to a severe drought in 1995 than young trees. A positive association was found between stand defoliation and relative growth. Combining monitoring and tree ring networks is useful for the detection of early warning signals of dieback in similar drought-prone forests.

Low Growth Sensitivity and Fast Replenishment of Non-structural Carbohydrates in a Long-Lived Endangered Conifer After Drought

There is an ongoing debate on whether a drought induced carbohydrate limitation (source limitation) or a direct effect of water shortage (sink limitation) limit growth under drought. In this study, we investigated the effects of the two driest summers recorded in southern Chile in the last seven decades, on the growth and non-structural carbohydrates (NSC) concentrations of the slow-growing conifer Fitzroya cupressoides. Specifically, we studied the seasonal variation of NSC in saplings and adults one and two years after the occurrence of a 2 year-summer drought at two sites of contrasting precipitation and productivity (mesic-productive vs. rainy-less productive). We also evaluated radial growth before, during and after the drought, and predicted that drought could have reduced growth. If drought caused C source limitation, we expected that NSCs will be lower during the first than the second year after drought. Conversely, similar NSC concentrations between years or higher NSC concentrations in the first year would be supportive of sink limitation. Also, due to the lower biomass of saplings compared with adults, we expected that saplings should experience stronger seasonal NSC remobilization than adults. We confirmed this last expectation. Moreover, we found no significant growth reduction during drought in the rainy site and a slightly significant growth reduction at the mesic site for both saplings and adults. Across organs and in both sites and age classes, NSC, starch, and sugar concentrations were generally higher in the first than in the second year following drought, while NSC seasonal remobilization was generally lower. Higher NSC concentrations along with lower seasonal NSC remobilization during the first post-drought year are supportive of sink limitation. However, as these results were found at both sites while growth decreased slightly and just at the mesic site, limited growth only is unlikely to have caused NSC accumulation. Rather, these results suggest that the post-drought dynamics of carbohydrate storage are partly decoupled from the growth dynamics, and that the rebuild of C reserves after drought may be a priority in this species.

Increased Drought Sensitivity Results in a Declining Tree Growth of Pinus latteri in Northeastern Thailand

Climate change may lead to alterations in tree growth and carbon cycling. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of tree species. However, the effects of climate change on pine forest dynamics in tropical region of Thailand remain poorly understood. This study develops three new tree ring-width chronologies of Pinus latteri (Tenasserim pine) in northern and northeastern Thailand and analyzes their climate-growth relationships and temporal stability. Ring-width chronologies of P. latteri at three sites showed significantly positive correlations with precipitation, relative humidity and self-calibrated Palmer Drought Severity Index (scPDSI) during the dry season (previous November to current April) and early rainy season (May–June). Conversely, significantly negative correlations were found between ring-width site chronologies and air temperatures (mean, maximum and minimum) from April to August. Therefore, our results revealed that radial growth of Tenasserim pines from northern and northeastern Thailand was mainly limited by moisture availability during the dry-to-wet transition season from April to June. Moving correlations revealed that Tenasserim pines in the lowland area of northeastern Thailand became more sensitive to moisture availability in recent 30 years (1985–2017) as compared with early period (1951–1984). Accompanying the shifted growth sensitivity to climate change, growth synchrony among trees was increasing and tree growth rates of Tenasserim pines have been declining during recent decades at two more moisture-limited sites in northeastern Thailand. Recent rapid warming and increasing drought during the transition season (April–June) together intensify climatic constrains on tree growth of Tenasserim pines in the lowland area of northeastern Thailand. Considering continued regional climate change, pine forests in tropical lowland areas may encounter intensified drought stresses, and thus, become more vulnerable to future climate change. Our results serve as an early indicator of potential effects of climate change on tropical pine species and raise concerns about sustainable managements of pine forests under a changing climate.

Physiological and Transcriptome Analyses Reveal Short-Term Responses and Formation of Memory Under Drought Stress in Rice

In some plants, exposure to stress can induce a memory response, which appears to play an important role in adaptation to recurrent stress environments. However, whether rice exhibits drought stress memory and the molecular mechanisms that might underlie this process have remained unclear. Here, we ensured that rice drought memory was established after cycles of mild drought and re-watering treatment, and studied gene expression by whole-transcriptome strand-specific RNA sequencing (ssRNA-seq). We detected 6,885 transcripts and 238 lncRNAs involved in the drought memory response, grouped into 16 distinct patterns. Notably, the identified genes of dosage memory generally did not respond to the initial drought treatment. Our results demonstrate that stress memory can be developed in rice under appropriate water deficient stress, and lncRNA, DNA methylation and endogenous phytohormones (especially abscisic acid) participate in rice short-term drought memory, possibly acting as memory factors to activate drought-related memory transcripts in pathways such as photosynthesis and proline biosynthesis, to respond to the subsequent stresses.

Drought Sensitiveness on Forest Growth in Peninsular Spain and the Balearic Islands

Drought is one of the key natural hazards impacting net primary production and tree growth in forest ecosystems. Nonetheless, tree species show different responses to drought events, which make it difficult to adopt fixed tools for monitoring drought impacts under contrasting environmental and climatic conditions. In this study, we assess the response of forest growth and a satellite proxy of the net primary production (NPP) to drought in peninsular Spain and the Balearic Islands, a region characterized by complex climatological, topographical, and environmental characteristics. Herein, we employed three different indicators based on in situ measurements and satellite image-derived vegetation information (i.e., tree-ring width, maximum annual greenness, and an indicator of NPP). We used seven different climate drought indices to assess drought impacts on the tree variables analyzed. The selected drought indices include four versions of the Palmer Drought Severity Index (PDSI, Palmer Hydrological Drought Index (PHDI), Z-index, and Palmer Modified Drought Index (PMDI)) and three multi-scalar indices (Standardized Precipitation Evapotranspiration Index (SPEI), Standardized Precipitation Index (SPI), and Standardized Precipitation Drought Index (SPDI)). Our results suggest that—irrespective of drought index and tree species—tree-ring width shows a stronger response to interannual variability of drought, compared to the greenness and the NPP. In comparison to other drought indices (e.g., PDSI), and our results demonstrate that multi-scalar drought indices (e.g., SPI, SPEI) are more advantageous in monitoring drought impacts on tree-ring growth, maximum greenness, and NPP. This finding suggests that multi-scalar indices are more appropriate for monitoring and modelling forest drought in peninsular Spain and the Balearic Islands.

Post-drought Resilience After Forest Die-Off: Shifts in Regeneration, Composition, Growth and Productivity

A better understanding on the consequences of drought on forests can be reached by paying special attention to their resilience capacity, i.e., the ability to return to a state similar to pre-drought conditions. Nevertheless, extreme droughts may surpass the threshold for the resilience capacity triggering die-off causing multiple changes at varying spatial and temporal scales and affecting diverse processes (tree growth and regeneration, ecosystem productivity). Combining several methodological tools allows reaching a comprehensive characterization of post-drought forest resilience. We evaluated the changes in the abundance, regeneration capacity (seedling abundance), and radial growth (annual tree rings) of the main tree species. We also assessed if drought-induced reductions in growth and regeneration of the dominant tree species scale-up to drops in vegetation productivity by using the Normalized Difference Vegetation Index (NDVI). We studied two conifer forests located in north-eastern Spain which displayed drought-induced die-off during the last decades: a Scots pine (Pinus sylvestris) forest under continental Mediterranean conditions and a Silver fir (Abies alba) forest under more temperate conditions. We found a strong negative impact of a recent severe drought (2012) on Scots pine growth, whereas the coexisting Juniperus thurifera showed positive trends in basal area increment (0.02 ± 0.003 cm² yr^-1). No Scots pine recruitment was observed in sites with intense die-off, but J. thurifera and Quercus ilex recruited. The 2012 drought event translated into a strong NDVI reduction (32% lower than the 1982-2014 average). In Silver fir we found a negative impact of the 2012 drought on short-term radial growth, whilst long-term growth of Silver fir and the coexisting Fagus sylvatica showed positive trends. Growth rates were higher in F. sylvatica (0.04 ± 0.003 cm² yr^-1) than in A. alba (0.02 ± 0.004 cm² yr^-1). These two species recruited beneath declining and non-declining Silver fir trees. The 2012 drought translated into a strong NDVI reduction which lasted until 2013. The results presented here suggest two different post-drought vegetation pathways. In the Scots pine forest, the higher growth and recruitment rates of J. thurifera correspond to a vegetation shift where Scots pine is being replaced by the drought-tolerant juniper. Conversely, in the Silver fir forest there is an increase of F. sylvatica growth and abundance but no local extinction of the Silver fir. Further research is required to monitor the evolution of these forests in the forthcoming years to illustrate the cumulative impacts of drought on successional dynamics.