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The soil microbiome plays an important role in forest functioning. However, the impact of drought-induced dieback and tree death on soil microbial biomass, community structure, and functional composition is unknown. We also lack understanding on how soil microbiota varies seasonally in such declining stands. We used Phospholipid Fatty Acids (PLFA) analysis to quantify soil microbial biomass and study its seasonal changes in three Mediterranean forests showing dieback and dominated by three pine species (Pinus halepensis Mill, Pinus pinaster Ait. and Pinus sylvestris L.). We also measured microclimatic parameters and soil physical and chemical parameters under trees with different vigor, assessed as canopy defoliation degree, and related them to seasonal changes in the soil microbial community. We found marked differences in soil microbial community structure, total biomass, and the relative abundance of major functional groups among forests. First, soil microbial biomass peaked either in the dry summer (P. halepensis) or in autumn (P. pinaster and P. sylvestris). Accordingly, the relative abundance of most functional groups, excluding Arbuscular mycorrhizal fungi, displayed substantial variation between seasons. In addition, the relative abundance of fungi and Gram-positive bacteria exhibited an opposite pattern compared to actinomycetes and Gram-negative bacteria. Second, soil physical and chemical parameters had a significant impact on within-site PLFA variation, although their influence was less important than that of seasonal variation. Third, differences between defoliated and healthy trees were minor and restricted to averaged ratios between different PLFA markers. Overall, the structure, biomass and relative abundance of major functional groups of the soil microbiome vary considerably among stand types and seasons in forests showing ongoing dieback and high mortality. However, while the seasonal dynamics show predictable patterns, which should be accounted for in future studies, the within-site variation is highly variable and mainly depends on soil physical and chemical parameters., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Local differentiation at distribution limits may influence species' adaptive capacity to environmental changes. However, drivers, such gene flow and local selection, are still poorly understood. We focus on the role played by range limits in mountain forests to test the hypothesis that relict tree populations are subjected to genetic differentiation and local adaptation. Two alpine treelines of mountain pine (Pinus uncinata Ram. ex DC) were investigated in the Spanish Pyrenees. Further, an isolated relict population forming the species' southernmost distribution limit in north-eastern Spain was also investigated. Using genotyping by sequencing, a genetic matrix conformed by single nucleotide polymorphisms (SNPs) was obtained. This matrix was used to perform genotype-environment and genotype-phenotype associations, as well as to model risk of non-adaptedness. Increasing climate seasonality appears as an essential element in the interpretation of SNPs subjected to selective pressures. Genetic differentiations were overall weak. The differences in leaf mass area and radial growth rate, as well as the identification of several SNPs subjected to selective pressures, exceeded neutral predictions of differentiation among populations. Despite genetic drift might prevail in the isolated population, the Fst values (0.060 and 0.066) showed a moderate genetic drift and Nm values (3.939 and 3.555) indicate the presence of gene flow between the relict population and both treelines. Nonetheless, the SNPs subjected to selection pressures provide evidences of possible selection in treeline ecotones. Persistence in range boundaries seems to involve several selective pressures in species' traits, which were significantly related to enhanced drought seasonality at the limit of P. uncinata distribution range. We conclude that gene flow is unlikely to constrain adaptation in the P. uncinata rear edge, although this species shows vulnerability to future climate change scenarios involving warmer and drier conditions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Climatic warming is assumed to expand the geographic range of insect pests whose distribution is mainly constrained by low temperatures. This is the case of the pine processionary moth (Thaumetopoea pityocampa), which is one of the main conifer defoliators in the Mediterranean Basin. Warmer winters may lead to a northward/upward expansion of this insect, as short-term studies have shown. However, no long-term data, i.e. spanning at least one century, has been used to examine these projections. We test the hypotheses that climatic warming (i) has caused an upward shift of the pine processionary moth, and (ii) has increased the frequency of severe defoliations. We used dendrochronological methods to reconstruct defoliations over the period 1900-2006 in 14 sites spanning a wide altitudinal range (1070-1675 m) in Teruel, eastern Spain. We built local ring-width chronologies for four co-occurring pine species with different degree of susceptibility against the moth defoliations, from highly suitable or palatable species (Pinus nigra) to moderately (Pinus sylvestris, Pinus halepensis) or rarely defoliated species (Pinus pinaster). We validated the tree-ring reconstructions of outbreaks using a field record of stand defoliations spanning the period 1971-2006. Outbreaks in the most affected P. nigra stands corresponded to abrupt one- to two-year growth reductions (70-90% growth loss). Reconstructed outbreaks occurred on average every 9-14 years. The growth memory of outbreaks was weaker but lasted longer (1-6 years) than that due to droughts (1-3 years). Neither an upward expansion nor an increase in outbreak frequency was observed. Severe PPM defoliations did not increase as climate warmed, rather they were positively related to the winter North Atlantic Oscillation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Crown Copyright © 2022. Published by Elsevier B.V. All rights reserved.)

Climate change can lead to the simultaneous occurrence of extreme droughts and heat waves increasing the frequency of compound events with unknown impacts on forests. Here we use two independent datasets, a compiled database of tree drought mortality events and the ICP-Forest level I plots, to study the impacts of the simultaneous occurrence of hot summers, with elevated vapour pressure deficit (VPD), and dry years on forest defoliation and mortality across Europe. We focused on tree drought mortality and background mortality rates, and we studied their co-occurrence with compound events of hot summers and dry years. In total, 143 out of 310 mortality events across Europe, i.e. 46% of cases, corresponded with rare compound events characterized by hot summers and dry years. Over the past decades, summer temperature increased in most sites and severe droughts resulted in compound events not observed before the 1980s. From the ICP-Forest plots we identified 291 (1718 trees) and 61 plots (128 trees) where severe defoliation and mortality, respectively, were caused by drought. The analyses of these events showed that 34% and 27% of the defoliation and mortality cases corresponded with rare compound climate events, respectively. Background mortality rates across Europe in the period 1993-2013 presented higher values in regions where summer temperature and VPD more steeply rose, where drought frequency increased. The steady increase in summer temperatures and VPD in Southern and Eastern Europe may favor the occurrence of compound events of hot summers and dry conditions. Giving that both, local and intense tree drought mortality events and background forest mortality rates, are linked to such compound events we can expect an increase in forest drought mortality in these European regions over the next decades., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Old-growth mountain forests represent an ideal setting for studying long-term impacts of climate change. We studied the few remnants of old-growth forests located within the Pollino massif (southern Italy) to evaluate how the growth of conspecific young and old trees responded to climate change. We investigated two conifer species (Abies alba and Pinus leucodermis) and two hardwood species (Fagus sylvatica and Quercus cerris). We sampled one stand per species along an altitudinal gradient, ranging from a drought-limited low-elevation hardwood forest to a cold-limited subalpine pine forest. We used a dendrochronological approach to characterize the long-term growth dynamics of old (age > 120 years) versus young (age < 120 years) trees. Younger trees grew faster than their older conspecifics during their juvenile stage, regardless of species. Linear mixed effect models were used to quantify recent growth trends (1950-2015) and responses to climate for old and young trees. Climate sensitivity, expressed as radial growth responses to climate during the last three decades, partially differed between species because high spring temperatures enhanced conifer growth, whereas F. sylvatica growth was negatively affected by warmer spring conditions. Furthermore, tree growth was negatively impacted by summer drought in all species. Climate sensitivity differed between young and old trees, with younger trees tending to be more sensitive in P. leucodermis and A. alba, whereas older F. sylvatica trees were more sensitive. In low-elevation Q. cerris stands, limitation of growth due to drought was not related to tree age, suggesting symmetric water competition. We found evidence for a fast-growth trend in young individuals compared with that in their older conspecifics. Notably, old trees tended to have relatively stable growth rates, showing remarkable resistance to climate warming. These responses to climate change should be recognized when forecasting the future dynamics of old-growth forests for their sustainable management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have influenced the research reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Climate warming is expected to enhance forest growth in cold-limited biomes while triggering reductions in drought-limited biomes. However, as temperature raises, it is unclear how temperature- and drought-growth couplings shift across elevation gradients in different biomes. We still lack comprehensive analyses on how altitude modulates the influence of temperature and drought on tree growth during the second half of the 20th century when climate warming accelerated. We compared the worldwide responses of tree growth (RWI, ring-width indices) to two of its major climatic constraints, growing-season minimum temperatures and drought (SPEI index), across biomes and elevation gradients during two periods with different warming rates (1960-1980 vs. 1980-2000). We found a decrease in the correlations of minimum temperatures with growth, but a strengthening of drought-growth relationships. However, these patterns varied across biomes because correlations between growth and temperature decreased in temperate forests and woodland shrubland, while correlations between growth and SPEI increased in boreal forests and decreased in temperate forests. Differences in growth responsiveness to climate between the two periods were more marked for mid-latitude forests situated between 1200 and 1600 m. The slopes of the relationships between growth-temperature correlations and elevation decreased in late spring and midsummer. The slopes of the relationships between growth-drought correlations and elevation increased in temperate forests and woodland shrubland suggesting that drought impacts are "climbing" in these biomes. Temperature controls on forest growth are relaxing as the climate warms, while drought is becoming a more significant constraint for tree growth, particularly for mid-elevation forests and in drought-prone woodland and shrubland. The strengthening of drought-growth coupling should be considered in vegetation models to reduce the uncertainty on forest climate mitigation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Drought-induced forest dieback is causing reductions in productivity, increasing tree mortality and impairing terrestrial carbon uptake worldwide. However, the role played by long-term nutrient imbalances during drought-induced dieback is still unknown. To improve our knowledge on the relationships between dieback and nutrient imbalances, we analysed wood anatomical traits (tree-ring width and wood density), soil properties and long-term chemical information in tree-ring wood (1900-2010) by non-destructive Micro X-ray fluorescence (μXRF) and destructive (ICP-OES) techniques. We studied two major European conifers with ongoing drought-induced dieback in mesic (Abies alba, silver fir) and xeric (Pinus sylvestris, Scots pine) sites. In each site we compared coexisting declining (D) and non-declining (ND) trees. We used dendrochronology and generalized additive and linear mixed models to analyse trends in tree-ring nutrients and their relationships with wood traits. The D trees presented lower growth and higher minimum wood density than ND trees, corresponding to a smaller lumen area of earlywood tracheids and thus a lower theoretical hydraulic conductivity. These differences in growth and wood-anatomy were more marked in silver fir than in Scots pine. Moreover, most of the chemical elements showed higher concentrations in D than in ND trees during the last two-five decades (e.g., Mn, K and Mg), while Ca and Na increased in the sapwood of ND trees. The Mn concentrations, and related ratios (Ca:Mn, Mn:Al and P:Mn) showed the highest differences between D and ND trees for both tree species. These findings suggest that a reduced hydraulic conductivity, consistent with hydraulic impairment, is affecting the use of P in D trees, making them more prone to drought-induced damage. The retrospective quantifications of Mn ratios may be used as early-warning signals of impending dieback., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Forests play an important role in regulating the global carbon cycle. Yet, how abiotic (i.e. soil nutrients) and biotic (i.e. tree diversity, stand structure and initial biomass) factors simultaneously contribute to aboveground biomass (coarse woody) productivity, and how the relative importance of these factors changes over succession remain poorly studied. Coarse woody productivity (CWP) was estimated as the annual aboveground biomass gain of stems using 10-year census data in old growth and secondary forests (25-ha and 4.8-ha, respectively) in northeast China. Boosted regression tree (BRT) model was used to evaluate the relative contribution of multiple metrics of tree diversity (taxonomic, functional and phylogenetic diversity and trait composition as well as stand structure attributes), stand initial biomass and soil nutrients on productivity in the studied forests. Our results showed that community-weighted mean of leaf phosphorus content, initial stand biomass and soil nutrients were the three most important individual predictors for CWP in secondary forest. Instead, initial stand biomass, rather than diversity and functional trait composition (vegetation quality) was the most parsimonious predictor of CWP in old growth forest. By comparing the results from secondary and old growth forest, the summed relative contribution of trait composition and soil nutrients on productivity decreased as those of diversity indices and initial biomass increased, suggesting the stronger effect of diversity and vegetation quantity over time. Vegetation quantity, rather than diversity and soil nutrients, is the main driver of forest productivity in temperate mixed forest. Our results imply that diversity effect for productivity in natural forests may not be so important as often suggested, at least not during the later stage of forest succession. This finding suggests that as a change of the importance of different divers of productivity, the environmentally driven filtering decreases and competitively driven niche differentiation increases with forest succession., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Mediterranean climate promotes two distinct growth peaks separated by summer quiescence in trees. This bimodal pattern has been associated to favourable growing conditions during spring and autumn when mild temperatures and soil-water availability enhance cambial activity. Climatic models predict progressive warming and drying for the Mediterranean Basin, which could shorten or shift the spring and autumn growing seasons. We explored this idea by comparing two sites with different Mediterranean climate types (continental/dry and coastal/wet) and studied how climate drives the bimodal growth pattern in Aleppo pine (Pinus halepensis). Specifically we investigated the intra-annual changes in wood anatomy and the corresponding formation of density fluctuations (IADF). Trees on both sites were analyzed by dendrometer monitoring and by developing chronologies of wood anatomical traits. Radial-increment dynamics followed a similar bimodal pattern in both sites but coastal trees showed higher increments during the spring and autumn growth peaks, especially in autumn. The summer rest of cambium activity occurs almost one month earlier in the coastal than in the inland site. Lumen area and cell-wall thickness were significantly smaller in the continental site, while the increment rate of cell-wall thickness during an IADF event was much higher in the coastal pines. The accumulated soil moisture deficit was the main climatic constraint of tracheid enlargement in continental pines. Intra-annual density fluctuations were more frequent in the coastal trees where wood anatomy features recover to average values after such events, meanwhile inland trees presented a much lower recovery rate. Growth bimodality and the formation of density fluctuations were linked, but mild climate of the coastal site allows a longer growing season, which explains why trees in this area showed higher and more variable growth rates., (Copyright © 2017 Elsevier B.V. All rights reserved.)