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Climate change challenges the adaptive capacity of several forest tree species in the face of increasing drought and rising temperatures. Therefore, understanding the mechanistic connections between genetic diversity and drought resilience is highly valuable for conserving drought-sensitive forests. Nonetheless, the post-drought recovery in trees from a transcriptomic perspective has not yet been studied by comparing contrasting phenotypes. Here, experimental drought treatments, gas-exchange dynamics and transcriptomic analysis (RNA-seq) were performed in the relict and drought-sensitive fir Abies pinsapo Boiss. to identify gene expression differences over immediate (24 h) and extended drought (20 days). Post-drought responses were investigated to define resilient and sensitive phenotypes. Single nucleotide polymorphisms (SNPs) were also studied to characterize the genomic basis of A. pinsapo drought resilience. Weighted gene co-expression network analysis showed an activation of stomatal closing and an inhibition of plant growth-related genes during the immediate drought, consistent with an isohydric dynamic. During the extended drought, transcription factors, as well as cellular damage and homeostasis protection-related genes prevailed. Resilient individuals activate photosynthesis-related genes and inhibit aerial growth-related genes, suggesting a shifting shoot/root biomass allocation to improve water uptake and whole-plant carbon balance. About, 152 fixed SNPs were found between resilient and sensitive seedlings, which were mostly located in RNA-activity-related genes, including epigenetic regulation. Contrasting gene expression and SNPs were found between different post-drought resilience phenotypes for the first time in a forest tree, suggesting a transcriptomic and genomic basis for drought resilience. The obtained drought-related transcriptomic profile and drought-resilience candidate genes may guide conservation programs for this threatened tree species.

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.

21 páginas, figuras, 4 tablas, 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., This study was financially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) via competitive grant RTI2018-096884-B-C31 (FORMAL project), RTI2018-096884-B-C33 (LESENS project) and by the Government of Aragón via competitive grant LMP242_18. AG acknowledges financial support from the Spanish Ministry of Science and the AEI through the Ramón y Cajal grant (RyC2020- 030647-I). JIQ acknowledges the financial support from Spanish Ministry of Science through the project (PID2019-107382RB-I00). JMI acknowledges financial support from the Project “CLU-2019-05—IRNASA/CSIC Unit of Excellence”, funded by the Junta de Castilla y León and cofinanced by the European Union (ERDF “Europe drives our growth”). RSS and JCL acknowledge the financial support from the Andalusian government (P20_00813 VUERCLIM), (IE19_074 UPO DendroOlavide) and (UPO-1263216 VULBOS), and from the Spanish Ministry of Science trough the project (EQC2018-005303-P)

Spanish black pine showed greater resilience and resistance, but generally lower recovery to drought events in managed than in unmanaged forest stands under Mediterranean humid climate. Drought negative effects on forest ecosystems are projected to increase under global warming all over the world. In this context, forest management can be an effective option for reducing drought impacts and increasing tree growth stability to extreme drought events. Here, we aim to evaluate black pine (Pinus nigra subsp. salzmannii) growth response to climatic variability and drought events in managed and unmanaged stands under similar Mediterranean climatic conditions. Drought events were identified using long-term climatic data, and basal area increments were calculated for 100–120-year old trees cored in managed and unmanaged plots. Results showed that tree size, temperature, and the interaction between management treatment and water availability significantly influenced tree growth. Basal area increment was reduced in response to the 1983, 1991, 1994‒1995, 1999–2000 and 2005 drought events. Trees in managed plots showed lower growth reductions in response to drought than those located in unmanaged plots, probably experiencing higher competition for soil water, whereas the reverse happened under wet climate conditions. Black pines showed greater resilience and resistance, but generally lower recovery to drought events in managed than in unmanaged stands. Our results suggest that forest management enhances drought tolerance in black pine stands, which may help to ameliorate the negative impacts of global warming across Mediterranean forest ecosystems.

Significance Forests are experiencing growing risks of drought-induced mortality in a warming world. Yet, ecosystem dynamics following drought mortality remain unknown, representing a major limitation to our understanding of the ecological consequences of climate change. We provide an emerging picture of postdrought ecological trajectories based on field indicators of forest dynamics. Replacement patterns following mortality indicate limited short-term persistence of predrought dominant tree species, highlighting the potential for major ecosystem reorganization in the coming decades. The great variability of the observed dynamics within and among species reinforces the primary influence of drought characteristics and ecosystem legacies, modulated by land use, management, and past disturbances, on ongoing drought-related species turnover and their potential implications for future forest biodiversity and ecosystem services., Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

Current climate change constitutes a challenge for the survival of several drought-sensitive forests. The study of the genetic basis of adaptation offers a suitable way to understand how tree species may respond to future climatic conditions, as well as to design suitable conservation and management strategies. Here, we focus on selected genetic signatures of the drought-sensitive relict fir, Abies pinsapo Boiss. Field sampling of 156 individuals was performed in two elevation ecotones, characterized by widespread A. pinsapo decline and mortality. The DNA from dead trees was investigated and compared to living individuals, accounting for different ages and elevations. We studied the genes gated outwardly-rectifying K+ (GORK) channel and Plasma membrane Intrinsic Protein (PIP1) aquaporin, previously related to drought response in plant model species, to test whether drought was the main abiotic factor driving the decline of A. pinsapo forests. A combination of linear regression and factor models were used to test these selection signatures, as well as a fixation index (Fst), used here to analyze the genetic structure. The results were consistent among these approaches, supporting a statistically significant association of the GORK gene with survival in one of the A. pinsapo populations. These results provide preliminary evidence for the potential role of the GORK gene in the resilience to drought of A. pinsapo.

The magnitude of drought impact in forest ecosystems depends on which group of trees are more severely affected, greater mortality of smaller trees can modulate the trajectories of succession, while the mortality of larger trees can disproportionately offset the ecosystem’s carbon balance. Several studies have documented a greater vulnerability of large trees to extreme droughts while some other studies reported a greater growth reduction in smaller trees during droughts. We tested these hypotheses by comparing tree basal area increment (BAI), drought resistance (i.e., magnitude of growth decline during drought), and resilience (i.e., magnitude of growth recovery after drought) across five different age-classes in black pine (Pinus nigra Arn. Ssp salzmannii) forests in Spain. Our results showed that the BAI patterns, drought resistance, and resilience were strongly influenced by tree age-classes. In addition, the effect of climatic water balance (precipitation minus potential evapotranspiration) on BAI significantly varied among age-classes. The effect of water balance on BAI was lower for younger age-classes (1–39 years of age) compared to older age-classes. We observed a greater growth reduction (i.e., lower resistance) in older trees (&gt, 40 years of age) during droughts compared to younger trees (&lt, 40 years of age). However, all trees, irrespective of their ages, were able to recover the growth rates after the drought. In general, younger trees showed a greater capacity in recovering the growth rate (i.e., more resilient) than older trees. We detected no significant effects of stand basal area and stand density on BAI, drought resistance, and resilience. Overall, our results indicated that growth of older trees was more negatively affected during drought. Therefore, these older/larger trees can be selected for commercial thinning, or can be released from competition, which can minimize the potential impacts of future droughts in black pine forests in Spain.

Current knowledge of climate change effects on forest ecology and species conservation should be linked to understanding of the past-time. Abies pinsapo forests constitute a model of an endangered ecosystem, highly vulnerable to ongoing warming, whose populations have been declining for centuries, while the drivers of this local depletion trend remain poorly understood. We hypothesized that long-term disturbances, both human- and natural-induced, have shaped A. pinsapo forests, contributing to these decline processes. Until today, studies using fossil pollen record to identify past climate impacts and land-use changes on A. pinsapo populations have not been done. Here, we investigate forests’ dynamics since the late Holocene (1180 cal. AD to present) in Southern Iberian Peninsula from a fossil pollen record by comparing the results obtained with climate fluctuations and land-uses changes. The pollen sequence shows a phase of stability during the Islamic Period (~1180–1400 cal. AD; ‘Medieval Climate Anomaly’), followed by increasing degradation at Christian Period concurrent with ‘Little Ice Age’ (LIA) (ca. 1487–1530 cal. AD). The Modern Period (1530–1800 cal. AD; LIA) is linked to intensive forest management, related to the naval industry. Afterwards, a progressive reduction is recorded during the Contemporary Age period (‘Industrial Period’) until ‘Recent Warming’. In short, historical severe forest management coupled with increasing aridity since LIA appear to influence A. pinsapo forest current species composition and poor structural diversity. These disturbances might be limiting the resilience of A. pinsapo forests under a climate change scenario. A selected forest management could promote a more complex forest structure.

Forests are being impacted by climate and land-use changes which have altered their productivity and growth. Understanding how tree growth responds to climate in natural and planted stands may provide valuable information to prepare management in sight of climate change. Plantations are expected to show higher sensitivity to climate and lower post-drought resilience than natural stands, due to their lower compositional and structural diversity. We reconstructed and compared the radial growth of six conifers with contrasting ecological and climatic niches (Abies pinsapo, Cedrus atlantica, Pinus sylvestris, Pinus nigra, Pinus pinea, Pinus pinaster) in natural and planted stands subjected to seasonal drought in 40 sites. We quantified the relationships between individual growth variability and climate variables (temperature, precipitation and the SPEI drought index), as well as post-drought resilience. Elevated precipitation during the previous autumn-winter and current spring to early summer enhanced growth in both natural and planted stands of all species. Temperature effects on growth were less consistent: only plantations of A. pinsapo, C. atlantica, P. nigra, P. pinea, P. sylvetris and a natural stand of P. nigra showed negative impacts of summer temperature on growth. Drought reduced growth of all species in both plantations and natural stands, with variations in the temporal scale of the response. Drought constrained growth more severely in natural stands than in plantations of C. atlantica, P. pinaster and P. nigra, whereas the inverse pattern was found for A. pinsapo. Resilience to drought varied between species: natural stands of A. pinsapo, C. atlantica and P. pinaster recovered faster than plantations, while P. pinea plantations recovered faster than natural stands. Overall, plantations did not consistently show a higher sensitivity to climate and a lower capacity to recover after drought. Therefore, plantations are potential tools for mitigating climate warming.

Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree-ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature–growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature–growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature–growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming. climate warming, forest limit, growth model, mountain ecosystems, tree rings acceptedVersion

Current climate change in the Mediterranean basin is associated to increasing frequency and intensity of droughts. This climate dryness entails a serious impact on drought-sensitive forests, several of them considered as hot spots of biodiversity. Adaptive management, as experimental thinning for stand structural diversity enhancement, may increase tree-level resources availability. However, the long-term stand-level effectiveness of this approach at sustaining forest ecosystem functioning remains uncertain. Here, we attempt to place experimental thinning in a climate change adaptation context, using as experimental system the drought-sensitive fir Abies pinsapo. We conducted a long-term study (2004–2019), focused on tree basal area increment (BAI) and quantified to what extent thinning treatments improved drought resistance, recovery and resilience to drought in the remaining trees. The results support a post-thinning (after 2004) BAI increase in thinned stands (+104% if 30% thinning of basal area was applied; +141% for 60% thinning) compared to controls (+21%). Short-term growth resilience, estimated for an extreme drought occurred after thinning (2005), yielded higher resilience for thinned stands, throughout improved resistance, but similar recovery than control. However, extreme droughts observed thereafter (for instance in 2012) do not support a long-term increase in resilience. Growth resilience, estimated for wetter-than-average years showed similar short-term enhanced resilience in thinned stand, while these differences dispelled afterward.

Droughts chronically alter resource availability in forest ecosystems. The increased frequency and severity of such extreme climate events challenge the acclimation potential of tree species especially across the drought-prone Mediterranean region. Pinus nigra is a widely distributed tree species in the Mediterranean region and considered vulnerable to extreme droughts. We used a 1000 km latitudinal gradient from northern Morocco to north-eastern Spain incorporating four regions (Edge-South, Core-South, Core-North and Edge-North) and including different P. nigra provenances. We aim to identify the climate and forest structure related drivers that influence tree radial growth (BAI, basal area increment). We developed statistical models for BAI by incorporating the potential effects of climate and forest structure (diameter and age distributions). Then, we forecasted the future growth of P. nigra forests during the 21st century considering the emission scenario A2 with an expected increase of +2.7 °C at the end of this century. Our results showed large variability across P. nigra populations in terms of environmental conditions, forest structure, and growth. The northernmost P. nigra populations, subjected to wetter and cooler conditions were those presenting the lowest BAI (4.9 cm2), whereas the southernmost P. nigra populations subjected to drier and warmer conditions presented the highest BAI values (11.5 cm2). Pinus nigra growth was enhanced by high spring precipitation, but this positive effect was probably modulated by forest structure. Temperature explained a higher proportion of the BAI variance than precipitation, with warmer summer conditions decreasing growth. Growth projections forecasted a decline in BAI (from 9.6 to 7.0 cm2) across all ecological regions starting around the mid-21st century but being lower in Edge-North populations compared to the other populations. Our study provides quantitative knowledge related to how P. nigra populations have been growing across four distinctive ecological regions. We also provide a forecasting tool that incorporates both climate and stand structure related information to project dynamics in tree populations.

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 2,800 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 predrought 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., We examined tree growth resilience of Scots pine along a 2,800 km latitudinal gradient from southern Spain to north‐eastern Germany using 615 adult trees from 30 different sites. We found that the resilience of Scots pine to extreme drought decreased in mid‐elevation and low productivity sites from 1980–1999 to 2000–2011 due to more frequent and severe droughts in the later period. We 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 during the pre‐drought period.

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., Resilience to drought is crucial for tree survival under climate change. Here, DeSoto et al. show that trees that died during drought were less resilient to previous dry events compared to surviving conspecifics, but the resilience strategies differ between angiosperms and gymnosperms.

12 Pags.- 34Figs.- 3 Tabls., The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species remain unclear. We used an extensive network of tree‐ring width (RWI, ring‐width index) records of 16 tree species from 567 forests, and high‐resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982‒2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended‐only RWI and NDVI series (i.e. after removing long‐term growth trends) and pre‐whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity. We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first‐year drought legacies were negative and different from random in 9% and 5% of the RWI and NDVI series respectively. The number of significant second‐ and third‐year legacies was substantially lower. Differences between RWI and NDVI legacies indicate that canopy greenness and radial growth responses to drought are decoupled. We found variations in legacies between tree species with gymnosperms presenting larger first‐year drought legacies than angiosperms, which were exposed to less severe droughts. Greater growth variability can explain the presence of first‐year RWI legacies in gymnosperms from dry sites despite that the relationship between growth variability and legacies was complex. Synthesis. Accounting for species and site responses to drought provides a better understanding of the magnitude and duration of drought legacies on forest growth and productivity. Despite the widespread occurrence of growth reductions in the years during and after drought occurrence, significant legacies were not very common, mostly lasted one year, and were more widespread in gymnosperms. These are relevant factors to be considered in the future when studying the consequences of drought on forest productivity and tree growth., This study was financially supported by: Xunta de Galicia, Grant/Award Number: PGIDIT06PXIB502262PR, GRC GI‐1809; INIA, Grant/Award Number: RTA2006‐00117; CANOPEE, 2014‐2020‐FEDER funds; and Spanish Science Ministry: RTI2018‐096884‐B‐C31, RTI2018‐096884‐B‐C33, AGL2017‐83828‐C2‐2R and ENV4‐CT97‐0641 projects. GSB was supported by a ‘Juan de la Cierva Formación’ grant from MINECO (FJCI 2016‐30121). This research was supported by the BERC 2018‐2021 program, and by the Spanish Ministry of Science, Innovation and Universities through the BC3 María de Maeztu excellence accreditation (MDM‐2017‐0714) and the IBERYCA (CGL2017‐84723‐P) project. R.S.S. was supported by VULBOS project (UPO‐1263216, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014‐2020), and A.H. by PinCaR project (UHU‐1266324, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014‐2020). K.N. was supported by the Ministry of Education, Science and Sports (Republic of Slovenia, Raziskovalci‐2.1‐UL‐BF‐ C3330‐19‐952011).

21 Pags.- 5 Figs.- 1 Appendix. - Suppl. Data., This study links tree-ring growth and gross primary production for a variety of forest types under different environmental conditions across Spain. NOAA-AVHRR satellite imagery data were combined with dendrochronological records and climate data at a fine spatial resolution (1.21 km2) to analyze the interannual variability of tree-ring growth and vegetation activity for different forest biomes from 1981 to 2015. Specifically, we assessed the links between tree-ring width indices (TRWi), the Normalized Difference Vegetation Index (NDVI) and a variety of environmental conditions, represented by climatic variables (air temperature, precipitation, evapotranspiration and water balance) and elevation. The impact of these variables on tree growth was assessed by means of the Predictive Discriminant Analysis (PDA). Results reveal a general positive and significant relationship between inter-annual variability of the NDVI at a high spatial resolution (1.21 km2) and tree-ring growth. Maximum correlations between NDVI and tree-ring growth were recorded when cumulative NDVI values were considered, in some cases covering long time periods (6–10 months), suggesting that tree growth is mainly related to Gross Primary Production (GPP) at annual scale. The relationship between tree-ring growth and inter-annual variability of the NDVI, however, strongly varies between forest types and environmental conditions., This work was supported by the research projects CGL2014-52135-C03-01, CGL2015-69186-C2-1-R, CGL2015-69985-R, CGL2013-48843-C2-1-R, AGL2014-53822-C2-1-R, (BFU2010-21451), PCIN-2015-220, CGL2016-81706-REDT and CGL2017-82216-R financed by the Spanish Commission of Science and Technology and FEDER, 1560/2015: Herramientas de monitorización de la vegetación mediante modelización ecohidrológica en parques continentales financed by the Red de Parques Nacionales, IMDROFLOOD financed by the Water Works 2014 co-funded call of the European Commission, CROSSDRO financed by the AXIS (Assessment of Cross(X) – sectorial climate Impacts and pathways for Sustainable transformation) JPI-Climate co-funded call of the European Commission and INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), ANR (FR) with co-funding by the European Union (Grant 690462). Financed by the Spanish Commission of Science and Technology and FEDER, and INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), ANR (FR) with co-funding by the European Union (Grant 690462). Natalia Martin-Hernandez was supported by a doctoral grant by the Aragon Regional Government, Marina Peña-Gallardo was granted by the Spanish Ministry of Economy and Competitiveness and Miquel Tomas-Burguera was supported by a doctoral grant by the Spanish Ministry of Education, Culture and Sport. Raúl Sánchez-Salguero, Antonio Gazol and Gabriel Sangüesa-Barreda were supported by Postdoctoral grants (IJCI-2015-25845, MINECO-FPDI 2013-16600 and FJCI 2016-30121, respectively; FEDER funds). This work also benefited from funding from Xunta de Galicia (PGIDIT06PXIB502262PR, GRC GI-1809, ROCLIGAL-10MDS291009PR), INIA (RTA2006-00117), and Interreg V-A POCTEFA (CANOPEE, 2014-2020-FEDER funds) projects.

Plants in Mediterranean mountains are particularly vulnerable to climatic change. In these environments, low temperature is combined with water shortage during summer, and as a result, the positive effect of global warming theoretically expanding the growing season length may be counterbalanced by rising drought stress. These circumstances may be exacerbated in the rear edge of species distribution, where warmer conditions occur. Here, we examined the climate-growth relationships of Juniperus sabina, a major prostrate shrub above the treeline in Mediterranean mountains, to investigate climate sensitivity and long-term signals stability in four rear-edge populations from southern Spain. We demonstrate that, over recent decades, local climatic conditions have modulated the response of J. sabina secondary growth to the ongoing climate change. We observed a negative effect of winter-spring moisture on secondary growth that suggests a limitation for earlier growth activity at higher elevation, potentially hindering the ability of J. sabina to compensate forthcoming increases in summer drought. At the driest site, we also detected a positive effect of October precipitation, suggesting a second growth pulse by early autumn. Our results provide an example of how local climatic conditions may limit plastic responses of secondary growth to climatic variability. The implications of growth limitation in J. sabina exceed the species-scale level, since these prostrate shrubs play a critical role as nurse plants and local biodiversity foci in Mediterranean mountains.

Intraspecific variability in morphological and ecophysiological leaf traits might be theorized to be present in declining populations, since they seem to be exposed to stress and plasticity could be advantageous. Here we focused on declining Persian oaks (Quercus brantii Lindl. var. persica (Jaub and Spach) Zohary) in the Zagros Mountains of western Iran, representing the most important tree species of this region. We selected trees with contrasting crown dieback, from healthy to severely defoliated, to investigate the relationships between canopy dieback and leaf morphology, water content and pigments. We also measured esterase and peroxidase, as enzymatic antioxidants and indicators of contrasting genotypes. Trees showing moderate to severe defoliation showed higher leaf mass area (LMA), reduced relative water content (RWC), and lower stomatal density (SD). Increasing LMA indicates a more sclerophyllic structure, according to drier conditions. We did not find significant differences in leaf pigments (chlorophyll a and b, and carotenoids) among crown dieback classes, suggesting that Persian oak trees are able to maintain accurate photochemical efficiency, while reduced RWC and SD suggest hydraulic limitations. Our results do not provide a consistent pattern as regards enzymatic antioxidant defense in Persian oak. Morphological leaf traits would be important drivers of future adaptive evolution in Persian oak, leading to smaller and thicker leaves, which have fitness benefits in dry environments. Nonetheless, drought responses may be critically affecting carbon uptake, as photosynthetic compounds are less effectively used in leaves with higher sclerophylly.

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.

AIM: We investigate the effects of the environmental and geographical processes driving growth resilience and recovery in response to drought in Mediterranean Pinus pinaster forests. We explicitly consider how intraspecific variability modulates growth resilience to drought. LOCATION: Western Mediterranean basin. METHODS: We analysed tree rings from a large network of 48 forests (836 trees) encompassing wide ecological and climatic gradients, including six provenances. To characterize the major constraints of P. pinaster growth under extremely dry conditions, we simulated growth responses to temperature and soil moisture using a process‐based growth model coupled with the quantification of climate–growth relationships. Then, we related growth–resilience indices to provenance and site variables considering different drought events. RESULTS: Pinus pinaster displayed strong variation in growth resilience across its distributional range, but common patterns were found within each provenance. Post‐drought resilience increased with elevation and drier conditions but decreased with spring precipitation. Trees from dry sites were less resistant to drought but recovered faster than trees from wet sites. MAIN CONCLUSIONS: Resilience strategies differed among tree provenances: wet forests showed higher growth resistance to drought, while dry forests presented faster growth recovery, suggesting different impacts of climate warming on forest productivity. We detected geographically structured resilience patterns corresponding to different provenances, confirming high intraspecific variability in response to drought. This information should be included in species distribution models to simulate forest responses to climate warming and forecasted aridification.

Climate extremes are expected to increase, which will affect oak forest ecosystems in Central European areas. Intensively managed forests, such as sessile oak stands, may alter their structure and function under a warming scenario. Here we analyse and project the climate-growth relationships of sessile oak ( Quercus petraea ) from high forests, originated from seed, and coppice forests, originated from vegetative reproduction in the Czech Republic. Dendrochronological data and linear mixed-effects models show similar effects of precipitation for April–May (positive) for both seedling- and coppice-origin trees. Previous autumn and current June temperature show significant negative correlations to growth in coppice forests. Nonetheless, trees from high forest stands showed increasing drought sensitivity and tended to show a stronger response to the previous autumn’s temperature during the 20th century. The positive effect of warmer autumns, only found on high stands, might be related to the extended growing season, suggesting improved adaptive capacity to cope with impending warmer conditions. In contrast, coppice trees might be able to buffer soil water shortage during spring and summer by higher root/shoot ratio. The obtained models were used to estimate the impact of 21st century-emission scenarios on tree basal area increments and stand basal area dynamics under different stand structures. Our results support that growth responses to climate warming are age and/or structure-dependent in sessile oak, particularly with regards to coppiced stands. Converting coppice forests to high stand structures, as well as thinning high stands, would enhance sessile oak forest adaptive capacity to cope with warming in Central Europe.

Understanding forest responses to the current climate change requires to investigate the effects of competition, buffering or enhancing process of forests decline. Here we attempt to place intra-specific competition in a climate change context, using as experimental system the drought-sensitive fir Abies pinsapo. We conducted a decade-long (2004–2015) investigation at four permanent plots (two thinned plots and two controls), where all trees were identified, tagged and mapped. Experimental thinning was performed attempting to enhance stand structure diversity by (1) reducing tree competition, (2) increasing the equitability of size classes (i.e. diversity) and (3) randomizing tree spatial pattern. We focus on tree basal area increment and sap flow density, as regards microclimate and competition, to quantify the extent to that thinning alleviates drought in the remaining trees. The results support that lower competition fosters short- to long-term tree-level physiological responses, specifically, improved growth and water uptake in the remaining trees. Density reduction provides a promising strategy for minimizing climate change effects on drought-sensitive tree species by improving resources availability to the remaining trees. Nevertheless, our results also support a higher air temperature and soil water depletion according to density reduction, suggesting that lower competition might also increase soil and understory evapotranspiration. Adaptive management, as the experimental thinning reported here, may increase tree-level resources availability, but the long-term stand-level effectiveness of this approach at sustaining forest ecosystem functioning remains uncertain.

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought-prone areas, tree populations located at the driest and southernmost distribution limits (rear-edges) should be particularly threatened. Here we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris, and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear-edges of the continuous distributions of these tree species. We used tree-ring width data from a network of 110 forests in combination with the process-based Vaganov-Shashkin Lite growth model and climate-growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO2 emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear-edge. By contrast, growth of high-elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of –10.7% and –16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear-edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear-edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions. This article is protected by copyright. All rights reserved.

The effects of climate change on forest growth are not homogeneous across tree species distribution ranges because of inter-population variability and spatial heterogeneity. Although latitudinal and thermal gradients in growth patterns have been widely investigated, changes in these patterns along longitudinal gradients due to the different timing and severity of regional droughts are less studied. Here, we investigated these responses in Mediterranean Black pine (Pinus nigra Arn.). We built a tree-ring width dataset comprising 77 forests (1202 trees) across the Mediterranean Basin. The biogeographical patterns in growth patterns and the relationships between growth and mean temperature, precipitation, drought and atmospheric circulations patterns (NAO -North Atlantic Oscillation-, SOI -Southern Oscillation Index- and MOI -Mediterranean Oscillation index-) were analyzed. Then, we evaluated the spatial and temporal growth synchrony between and within east and west populations. We found different growth and climate patterns in west vs. east Black pine populations, although in both regions growth was driven by similar temperature and precipitation variables. MOI significantly influenced tree growth, whilst NAO and SOI showed weaker effects. Growth of east and west Black pine populations desynchronized after the 1970s when several and uncoupled regional droughts occurred across the Mediterranean Basin. We detected a climate shift from the 1970s to the 1980s affecting growth patterns, changing growth-climate relationships, and reducing forest growth from west to east Black pine forests. Afterwards, climate and growth of east and west populations became increasingly more divergent. Our findings imply that integral bioclimatic and biogeographical analyses across the species distribution area must be considered to adequately assess the impact of climate change on tree growth under warming and more arid conditions.

Wood constitutes the unique source of DNA in dead trees, but extraction of adequate quality DNA from dry wood is usually challenging. However, many different molecular studies require the use of such DNA. We have standardized and validated a modified CTAB protocol to isolate DNA from dry wood from Abies pinsapo and Cedrus atlantica species. Due to the degradation and very little DNA that is normally present in the wood from dead trees we have developed a PCR based test to certify the quality of the extracted samples. In the present study, we have proved too the effectiveness of this methodology to isolate DNA from conifer dry wood samples of sufficient quality to perform further molecular genetic experiments.

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.

Alpine treeline ecotones are considered early-warning monitors of the effects of climate change on terrestrial ecosystems, but it is still unclear how accurately treeline dynamics may track the expected temperature rises. Site-specific abiotic constraints, such as topography and demographic trends may make treelines less responsive to environmental fluctuations. A better understanding on how local processes modulate treelines’ response to warming is thus required. We developed a model of treeline dynamics based on individual data of growth, mortality and reproduction. Specifically, we modeled growth patterns, mortality rates and reproductive size thresholds as a function of temperature and stand structure to evaluate the influence of climate- and stand-related processes on treeline dynamics. In this study, we analyze the dynamics of four Pyrenean mountain pine treeline sites with contrasting stand structures, and subjected to differing rates of climate warming. Our models indicate that Pyrenean treelines could reach basal areas and reproductive potentials similar to those currently observed in high-elevation subalpine forest by the mid twenty-first century. The fastest paces of treeline densification are forecasted by the late twenty-first century and are associated with higher warming rates. We found a common densification response of Pyrenean treelines to climate warming, but contrasting paces arise due to current size structures. Treelines characterized by a multistratified stand structure and subjected to lower mean annual temperatures were the most responsive to climate warming. In monostratified stands, tree growth was less sensitive to temperature than in multistratified stands and trees reached their reproductive size threshold later. Therefore, our simulations highlight that stand structure is paramount in modulating treeline responsiveness to ongoing climate warming. Synthesis. Treeline densification over the twenty-first century is likely to occur at different rates contingent on current stand structure and its effects on individual-level tree growth responses to warming. Accurate projections of future treeline dynamics must thus incorporate site-specific factors other than climate, specifically those related to stand structure and its influence on tree growth.

Aim Stressful environments, like those at the limits of species ranges, are closely associated with the selective establishment of particular traits, yet the mechanisms behind this relationship are poorly understood. Our main aim was to investigate spatial and temporal phenotypic plasticity related to environmental conditions and biogeographical origin. Location A latitudinal gradient along Iberian Peninsula and Morocco, western Mediterranean Basin. Methods We investigate the intra-specific variability in the Mediterranean oak Quercus ilex subsp. ballota, covering most of its latitudinal range. Acorns were collected in the years 2009, 2011, 2012 and 2013 from natural populations and were grown under controlled conditions. We measured acorn mass and germination dynamics, as well as seedling morphology. Results Field data showed significant differences in stand structure among populations. The southernmost population presented the smallest trees, followed by the northernmost population. Core-range trees registered higher values for leaf-nitrogen concentration. Leaf size showed latitudinal variation, with the lowest values at the northern range limit. In addition, the germination pattern also significantly differed among populations, showing a latitudinal pattern of variation (i.e. the southern populations germinated earlier). Fresh acorn mass showed significant within- and among-population differences, displaying a range margin pattern (i.e. similarities between the edge populations). Furthermore, seedling total weight and biomass allocation also followed the same pattern. The models explaining the most part of the trait variance were related mainly to climatic variables (reflecting temporal variability), while the random effect of the models (population) showed a range margin pattern with lower values in edge populations compared with core populations. Main conclusions Similar phenotypic traits among individuals native to the periphery of the distribution range were found despite the temporal variability, suggesting adaptation to stressful environments at the range edges.