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Recent drought-induced dieback alters forest dynamics, which are also shaped by past management. In western Pyrenean silver fir (Abies alba) stands, dieback concurs in space and time with the legacies of past management, but the impacts on forest growth, structure and composition are unknown. We aim to disentangle how dieback interacts with the legacies of past human use and modulates the recent dynamics of silver fir forests. To this end, we sampled eleven silver fir forests across wide climatic gradients and included declining and non-declining sites. We measured radial growth, structure, composition, understory cover and type and amount of deadwood. Silver fir growth declines in response to late-summer drought. In declining sites, most defoliated stands showed the lowest silver fir density and were those where growth depended more on water availability. Tree death enhanced the cover of dominant understory plants such as Buxus sempervirens. Past management activities leave an imprint in the growth of silver fir, such as releases due to past logging, but also affect the number of stumps and snags and the current tree density. A more extensive monitoring will be required to fully disentangle the multiple influences of past management legacies and current climate change on forest dynamics.

[Purpose of Review] The capacity of woody plants to cope with climate change depends on their adjustments to changing environmental conditions by phenotypic plasticity or by genotypic changes (i.e., local adaptation). To determine whether intraspecific trait variation (ITV) translates into resistance or tolerance to drought and eventually how it relates to performance when facing drought, we reviewed the recent literature on the intraspecific variation in woody plant functional traits along aridity gradients., [Recent Findings] At the intraspecific level, functional traits vary along aridity gradients, but this variation depends on the considered trait and species. While ITV is ubiquitous and of relevant magnitude, its relationship with tree performance in the case of growth or survival is unclear and very idiosyncratic., [Summary] ITV varies along aridity gradients and, for several traits, is of comparable magnitude as interspecific trait variation. However, the relationship between ITV and tree performance is not consistent among species. This makes the use of ITV challenging when quantifying its contribution to drought tolerance.

Increased winter and early spring temperatures due to climate change can enhance forest productivity due to earlier growth onset in temperate regions. However, drought-prone forests can be highly vulnerable to the combined effect of high summer temperatures and water stress. Understanding how water stress and rising temperatures along the growing season control tree growth in co-occurring species with contrasting drought tolerances is key to project climate change effects on forest dynamics. Here, we evaluated the interactive effect of seasonal temperature and water availability on annual radial growth and climate–growth instability over six decades (1951–2015) in three pine species with contrasting drought tolerances (Pinus pinaster, Pinus nigra, and Pinus sylvestris). For this, we retrospectively observed radial tree growth using dendroecological methods and evaluated seasonal temperature and water availability effects by using linear mixed models. Early growing season temperature and water availability had a positive effect on tree growth, but the positive effect of late season temperature was modulated by water availability. Moving time-window analyses revealed temporal instability in climate–growth relationships. Since the 1980s, pine species showed a higher growth sensitivity to both seasonal temperatures and annual water availability. Furthermore, growth reductions were more pronounced due to increased summer temperatures and reduced precipitation. Our results were similar for the three studied pine species despite their contrasting tolerance to drought. Overall, climate warming effects on pine growth are contingent upon water availability in Mediterranean continental forests. Synchronization among species, climate–growth instability, and negative growth trends suggests an increased vulnerability to drought of Mediterranean pine species in response to ongoing climate change.

Tree phenology is sensitive to climate warming and changes in seasonal precipitation. Long xylogenesis records are scarce, thus limiting our ability to analyse how radial growth responds to climate variability. Alternatively, process-based growth models can be used to simulate intra-annual growth dynamics and to better understand why growth bimodality varies along temperature and precipitation gradients. We used the Vaganov-Shashkin (VS) growth model to analyse the main climatic drivers of growth bimodality in eight trees and shrubs conifers (four pines and four junipers) across Spain. We selected eleven sites with different continentality degree and spring/autumn precipitation ratios since we expected to find pronounced bimodal growth in less continental sites with spring and autumn precipitation peaks. The VS model successfully simulated annual growth rates at all sites as a function of daily temperature and soil moisture data. Bimodal growth patterns clustered into less continental sites showing low spring/autumn precipitation ratios. This finding agrees with observed climate-growth associations showing that growth was enhanced by wet-cool winter-to-spring conditions, but also by wet autumn conditions in the most bimodal sites. We observed a stronger growth bimodality in pines compared to junipers. We discuss the spatial variability of climate drivers in bimodality growth pattern and how increasing continentality and shifts in seasonal precipitation could affect growth patterns. Bimodality could be an advantageous response to overcome summer drought in Mediterranean forests. The ability of some species to reactivate growth during autumn might determine their capacity to withstand increasing summer aridity.

During the past years, growth and productivity of different oak species have been constrained by water shortage in seasonally dry regions such as the Mediterranean Basin. Thinning could improve oak radial growth in these drought-prone regions through the reduction of tree competition for soil water in summer. However, we still lack adequate, long-term assessments on how lasting are thinning treatments effects and to what extent they contribute to oak growth recovery after drought. Here we aim: (i) to study the radial growth sensitivity to drought of Quercus subpyrenaica in the Spanish Pre-Pyrenees, and (ii) to verify if thinning represents a suitable option to enhance growth resistance to drought and post-drought growth recovery. We analysed basal area increment (BAI) trends in the period 1960–2020 of formerly coppiced oak stands thinned in 1984 and compared them with unthinned plots and also with coexisting Scots pine (Pinus sylvestris) growing in thinned plots. We used the Standardized Precipitation Evapotranspiration Index (SPEI) to estimate the severity of droughts and we also assessed climate-growth relationships. Oaks in thinned plots showed higher BAI (369 mm2) than those in unthinned plots (221 mm2). Growth rates remained higher in thinned than in unthinned plots also under severe drought stress. A severe summer drought in 1986 caused abrupt BAI reductions in both oaks (- 40.5%) and pines (- 40.1%). The positive effect of thinning on growth lasted for over 20 years and slightly declined as canopies closed. In the thinned plots, trees with smaller diameter showed the greatest growth release. Oaks in unthinned plots and Scots pine were more sensitive to short-term droughts in terms of growth reduction than oaks in thinned plots, while long term droughts have similar effects on oaks from both thinned and unthinned plots. Oaks were resilient to drought, showing recovery periods lasting from 1 to 2 years in both thinned and unthinned plots. However, intense and p

Impacts of climate warming on forests vigour are forecasted to increase in magnitude. Yet it remains unclear how stand characteristics and competition modulate the relationship between tree growth and gross primary production with drought. Here, we studied how the spatial variation in stand density, basal area and height modulates tree growth (Basal Area Increment, BAI and stand growth), summer NDVI, as well as their responses to drought (Standardized Precipitation-Evapotranspiration Index, SPEI) in 56 Aleppo pine (Pinus halepensis Mill.) planted forests located in Northeast Spain. Long-term BAI responses to SPEI were strongly determined by stand density, suggesting that competition modulates Aleppo pine growth responses to drought. Along this, summer NDVI also displayed strong associations with SPEI. NDVI was mostly related with stand growth, suggesting canopy densification drives NDVI pattern and trends. Short-term BAI and NDVI responses to severe droughts were mainly independent of stand characteristics. In the studied region, drought is a universal factor limiting Aleppo pine secondary growth and canopy greening. However, the results suggest that stand density modulates Aleppo pine growth responses to drought on the long-term, reducing the growth in densest stands. Denser stands with larger trees are the ones that present higher NDVI values, suggesting that canopy activity depends more on stand canopy coverage than on secondary growth rate and its response to drought. In these Mediterranean pines, canopy activity and secondary growth are temporally coupled but spatially decoupled.

Forest growth is driven by climate variability at continental to regional scales, but other factors play major roles at local scales (0.1–1 ha). Topography impacts on tree growth responses to climate stressors, including drought, by modifying radiation, evapotranspiration rates and the access to soil moisture. However, there is a lack of studies investigating how topographical factors (elevation, aspect, slope) affect climate-growth relationships considering both continuous (tree-ring width) and discrete wood-anatomical features (e.g., intra-annual density fluctuations–IADFs). Here, we investigated how topography modulated the influences of climate and drought on semi-arid Juniperus thurifera forests from north-eastern Spain. We compared two stands located in valley bottoms and two stands located in steep slopes. Radial growth was measured using dendrochronology which also allowed quantifying latewood IADFs. A proxy of topographic influence (incident radiation) was calculated. In addition, the Vaganov-Shashkin (VS) model was used to infer the main climatic constraints of growth at intra-annual scales. We found that junipers growing in valley bottoms were taller and produced more IADFs than junipers growing in steep slopes, but responded less to precipitation variability. This was confirmed by the VS model which showed that low soil moisture in June limited growth, particularly in steep-slope sites and during dry periods. Wet-cool late-summer conditions induced the formation of IADFs. The topoclimatic modulation of growth was explained by the interaction between incident radiation and tree age. Furthermore, climate-growth associations are changing as temperatures rise. Drought impacts on growth are strengthening, but less responsive junipers growing in valley bottoms may be buffered against intensified aridification.

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

The time required for an ecosystem to recover from severe drought is a key component of ecological resilience. The phenology effects on drought recovery are, however, poorly understood. These effects centre on how phenology variations impact biophysical feedbacks, vegetation growth and, ultimately, recovery itself. Using multiple remotely sensed datasets, we found that more than half of ecosystems in mid- and high-latitudinal Northern Hemisphere failed to recover from extreme droughts within a single growing season. Earlier spring phenology in the drought year slowed drought recovery when extreme droughts occurred in mid-growing season. Delayed spring phenology in the subsequent year slowed drought recovery for all vegetation types (with importance of spring phenology ranging from 46% to 58%). The phenology effects on drought recovery were comparable to or larger than other well-known postdrought climatic factors. These results strongly suggest that the interactions between vegetation phenology and drought must be incorporated into Earth system models to accurately quantify ecosystem resilience.

[Background] Climate and land-use changes, which include the application of various types of organic and inorganic fertilisers, have been reducing the species diversity of Mediterranean grasslands and threatening its conservation. Annual plants are one of the most diverse functional groups of species in these grasslands, despite suffering competitive pressure from perennial herbaceous and woody species, and they are essential for ecosystem functioning and stability., [Aims] To quantify how fertilisation modulates the impact of plant-to-plant interactions and climate fluctuations on the dynamics of annuals in Mediterranean grasslands. We hypothesised that the application of sewage sludge would increase competition between functional groups, reducing the abundance of annuals in the long-term, but would buffer the negative impacts of drought on the year-to-year fluctuation of the diversity of annuals., [Methods] In a semi-natural species-rich Mediterranean grassland in northern Spain, we analysed the changes in the taxonomical and functional composition and diversity of annuals over 14 years in response to variations in the abundance of perennial herbaceous and woody species, climate fluctuations and fertilisation with sewage sludge. We quantified separately the patterns of year-to-year fluctuations and long-term trends., [Results] The frequency and diversity of annuals decreased with higher abundance of perennial herbaceous species, drought in June and cold winters. The addition of sewage sludge decreased the abundance of annuals in the long-term, seemed to promote competition between annuals and other functional groups at an interannual scale, and mitigated the negative effects of drought and cold., [Conclusions] Fertilisation influences differently the temporal response of annuals to climate fluctuations and plant-to-plant interactions.

[Background] Climate and land-use changes, which include the application of various types of organic and inorganic fertilisers, have been reducing the species diversity of Mediterranean grasslands and threatening their conservation. Annual plants are one of the most diverse functional groups of species in these grasslands, despite suffering competitive pressure from perennial herbaceous and woody species, and they are essential for ecosystem functioning and stability., [Aims] To quantify how fertilisation modulates the impact of plant-to-plant interactions and climate fluctuations on the dynamics of annuals in Mediterranean grasslands. We hypothesised that the application of sewage sludge would increase competition between functional groups, reducing the abundance of annuals in the long-term, but would buffer the negative impacts of drought on the year-to-year fluctuation of the diversity of annuals., [Methods] In a semi-natural species-rich Mediterranean grassland in northern Spain, we analysed the changes in the taxonomical and functional composition and diversity of annuals over 14 years in response to variations in the abundance of perennial herbaceous and woody species, climate fluctuations, and fertilisation with sewage sludge. We quantified separately the patterns of year-to-year fluctuations and long-term trends., [Results] The frequency and diversity of annuals decreased with a higher abundance of perennial herbaceous species, drought in June, and cold winters. The addition of sewage sludge decreased the abundance of annuals in the long-term, seemed to promote competition between annuals and other functional groups at an interannual scale, and mitigated the negative effects of drought and cold., [Conclusions] Fertilisation influences differently the temporal response of annuals to climate fluctuations and plant-to-plant interactions.

Understanding the mechanisms underlying community stability has become an urgent need in order to protect ecosystems from global change and resulting biodiversity loss. While community stability can be influenced by richness, synchrony in annual fluctuations of species, species stability and functional traits, the relative contributions of these drivers to stability are still unclear. In semi-natural grasslands, land-use changes such as fertilization might affect stability by decreasing richness and influencing year-to-year fluctuations. In addition, they can promote long-term directional trends, shifting community composition and influencing grassland maintenance. Thus, it is important to consider how species and community stability vary year-to-year but also in the long term. Using a 14-year vegetation time series of a species-rich semi-natural Mediterranean grassland, we studied the relative importance of richness, synchrony, species stability and functional traits on community stability. To assess land-use change effects on stability, we applied a fertilization treatment. To distinguish stability patterns produced by year-to-year fluctuations from those caused by long-term trends, we compared the results obtained using a detrending approach from those without detrending. Stability is influenced by richness, synchrony and functional traits. Fertilization decreases species and community stability by promoting long-term trends in species composition, favouring competitive species and decreasing richness. Studying stability at the community and species level, and accounting for the effect of trends is essential to understand stability and its drivers more comprehensively.

There is large uncertainty on the future of European silver fir (Abies alba) forests and how they will respond to climate warming. Many studies, such as Walder et al. (2021), suggest they will show positive growth trends and a larger distribution area in response to warmer conditions whenever summer drought is not severe and long. This contradicts localized dieback events in continental, drought-prone Mediterranean sites mainly located in the southwestern margin of the current species distribution range. We discuss these disagreements and propose compound climate extremes and elevated atmospheric drought as drivers of dieback in similar marginal silver fir populations.

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

Understanding the mechanisms underlying community stability has become an urgent need in order to protect ecosystems from global change and resulting biodiversity loss. While community stability can be influenced by richness, synchrony in annual fluctuations of species, species stability and functional traits, the relative contributions of these drivers to stability are still unclear. In semi-natural grasslands, land-use changes such as fertilization might affect stability by decreasing richness and influencing year-to-year fluctuations. In addition, they can promote long-term directional trends, shifting community composition and influencing grassland maintenance. Thus, it is important to consider how species and community stability vary year-to-year but also in the long term. Using a 14-year vegetation time series of a species-rich semi-natural Mediterranean grassland, we studied the relative importance of richness, synchrony, species stability and functional traits on community stability. To assess land-use change effects on stability, we applied a fertilization treatment. To distinguish stability patterns produced by year-to-year fluctuations from those caused by long-term trends, we compared the results obtained using a detrending approach from those without detrending. Stability is influenced by richness, synchrony and functional traits. Fertilization decreases species and community stability by promoting long-term trends in species composition, favouring competitive species and decreasing richness. Studying stability at the community and species level, and accounting for the effect of trends is essential to understand stability and its drivers more comprehensively., [Methods] Study site and experimental design: In 2003, 12 plots of 15x5 m (hereafter called macro-plots) were established inside an area of 5500 m2. Half of the macro-plots (six) were used as control plots and half were fertilized with sewage sludge in a single event in 2003, applying manually to the soil surface 5 kg m-2 . The sludge came from a municipal urban wastewater treatment plant located in Tudela (Navarra, Spain), and it was sludge previously dried to 28% dry matter by centrifugation. To accurately assess vegetation changes, a 1x1 m permanent plot was placed in the centre of each macro-plot. Every year for 14 consecutive years (from 2004 to 2017), at the end of June, vegetation was sampled by R. Ibáñez, who identified and recorded every vascular plant species present in each of the permanent plots. The 1x1 m permanent plots were divided into 100 10x10 cm subplots to measure species abundance (frequency) by counting the number of 10x10 cm subplots in which the species was present (presence was recorded if shoots overlapped with the sampling unit/subplot, not according to rooted plants). Richness, synchrony and stability measures: Species richness in each permanent plot was measured both as cumulative species richness, counting the number of species found at least once in a permanent plot during the 14 years of the study, and as mean species richness, averaging the number of species found in a permanent plot over the 14 years (Lepš et al., 2018). Community-level synchrony for each permanent plot was calculated using the log variance ratio index (“Logvar”), which is the log-transformation of the ratio of observed to expected variance (i.e. the ratio of variance of the total community abundance to the sum of variances of the abundance of each species; Lepš et al., 2018; Roscher et al., 2011). Stability at both the community and the species level was calculated as the inverse of the coefficient of variation (CV-1) across years of cumulative or individual specie

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.

1. Understanding the mechanisms underlying community stability has become an urgent need to protect ecosystems from global change and resulting biodiversity loss. While community stability can be influenced by species richness, synchrony in annual fluctuations of species, species stability and functional traits, the relative contributions of these drivers to stability are still unclear. In semi-natural grasslands, land-use changes such as fertilization might affect stability by decreasing richness and influencing year-to-year fluctuations. In addition, they can promote long-term directional trends, shifting community composition and influencing grassland maintenance. Thus, it is important to consider how species and community stability vary year-to-year but also in the long term., 2. Using a 14-year vegetation time series of a species-rich semi-natural Mediterranean grassland, we studied the relative importance of richness, synchrony, species stability and functional traits on community stability. To assess land-use change effects on stability, we applied a fertilization treatment. To distinguish stability patterns produced by year-to-year fluctuations from those caused by long-term trends, we compared the results obtained using a detrending approach from those without detrending., 3. Independently of the treatment and approach applied, the most stable communities were those composed of asynchronous species with low specific leaf area. Fertilization decreased year-to-year and long-term community stability by increasing community-weighted mean of specific leaf area, decreasing species stability or also reducing richness in the case of year-to-year stability. Additionally, traits such as seed mass had an indirect effect on stability through synchrony. Long-term trends appeared in control and fertilized plots (due to fertilization), decreasing community and species stability and leading to differences in the relationships found between community stability and some of its drivers. This reflects the importance of accounting for the effect of temporal trends on community and species stability using both a long-term and a year-to-year approach., 4. Synthesis. Stability is influenced by richness, synchrony and functional traits. Fertilization decreases species and community stability by promoting long-term trends in species composition, favouring competitive species and decreasing richness. Studying stability at the community level and species level, and accounting for the effect of trends is essential to understand stability and its drivers more comprehensively.

Drought is an important driver of forest dynamics in the Mediterranean region. The forecasted increase in drought frequency and severity can notably influence tree growth, forest structure, composition and productivity. Understanding how coexisting tree species respond to drought is thus crucial to understand which are less vulnerable and will perform better in a warmer and drier world. To assess drought vulnerability, we used dendrochronology to study the radial growth trends and responses to a drought index of four pine species (Pinus halepensis, Pinus pinea, Pinus nigra, and Pinus sylvestris) coexisting in North-eastern Spain. We reconstructed the growth of each species and evaluated their short- and long-term growth response to drought for the common period 1980–2017. The growth of the four pine species depended on water availability and high early spring temperatures impacted the growth of P. nigra and P. sylvestris negatively. The occurrence of a severe drought between 2005 and 2007 lead to marked growth reductions in the four species, but it was greater in magnitude in P. pinea and P. halepensis in 2005, and in P. nigra in 2007. The results of basal area increment models at the individual tree level suggested that P. halepensis trees grow more than the rest of species. After accounting for age and drought effects, P. nigra and P. sylvestris displayed negative growth trends in the 2008–2017 period while P. pinea and P. halepensis displayed positive growth trends. P. sylvestris was the most resistant species and P. pinea the less resistant. Conversely, P. halepensis and P. pinea were slightly more resilient than P. sylvestris. Moreover, P. sylvestris was the species displaying the highest autocorrelation and the lowest coefficient of variation in ring-width indices. A marked drop in the autocorrelation of P. pinea ring-width index was observed in response to the 2005 drought. These results indicate that all study species are vulnerable to drought but in differe

Drought is an important driver of forest dynamics in the Mediterranean region. The forecasted increase in drought frequency and severity can notably influence tree growth, forest structure, composition and productivity. Understanding how coexisting tree species respond to drought is thus crucial to understand which are less vulnerable and will perform better in a warmer and drier world. To assess drought vulnerability, we used dendrochronology to study the radial growth trends and responses to a drought index of four pine species (Pinus halepensis, Pinus pinea, Pinus nigra, and Pinus sylvestris) coexisting in North-eastern Spain. We reconstructed the growth of each species and evaluated their short- and long-term growth response to drought for the common period 1980–2017. The growth of the four pine species depended on water availability and high early spring temperatures impacted the growth of P. nigra and P. sylvestris negatively. The occurrence of a severe drought between 2005 and 2007 lead to marked growth reductions in the four species, but it was greater in magnitude in P. pinea and P. halepensis in 2005, and in P. nigra in 2007. The results of basal area increment models at the individual tree level suggested that P. halepensis trees grow more than the rest of species. After accounting for age and drought effects, P. nigra and P. sylvestris displayed negative growth trends in the 2008–2017 period while P. pinea and P. halepensis displayed positive growth trends. P. sylvestris was the most resistant species and P. pinea the less resistant. Conversely, P. halepensis and P. pinea were slightly more resilient than P. sylvestris. Moreover, P. sylvestris was the species displaying the highest autocorrelation and the lowest coefficient of variation in ring-width indices. A marked drop in the autocorrelation of P. pinea ring-width index was observed in response to the 2005 drought. These results indicate that all study species are vulnerable to drought but in differe

There is debate on which tree species can sustain forest ecosystem services in a drier and warmer future. In Europe, the use of non-native timber species, such as Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), is suggested as a solution to mitigate climate change impacts because of their high growth resilience to drought. However, the biogeographical, climatic and ecological limits for widely planted timber species still need to be defined. Here, we study the growth response to climate variables and drought of four Douglas fir plantations in northern Spain subjected to contrasting climate conditions. Further, we measure wood density in one of the sites to obtain a better understanding of growth responses to climate. Correlative analyses and simulations based on the Vaganov–Shaskin process-based model confirm that growth of Douglas fir is constrained by warm and dry conditions during summer and early autumn, particularly in the driest study site. Minimum wood density increased in response to dry spring conditions. Therefore, planting Douglas fir in sites with a marked summer drought will result in reduced growth but a dense earlywood. Stands inhabiting dry sites are vulnerable to late-summer drought stress and can act as “sentinel plantations”, delineating the tolerance climate limits of timber species.

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.

Wood formation during the growing season is shaped by the intra-annual variability of climatic conditions. In the Mediterranean, the cambial activity is seasonally constrained by winter low temperature and summer drought, resulting in bimodal growth patterns. Although bimodal growth is an ecologically important adaptation of woody species to seasonally dry environments, its variability across space and among species remains unclear. We combined direct monitoring of intra-annual radial growth using dendrometers and xylogenesis micro-sampling with indirect wood formation modeling to assess growth bimodality for two coexisting oak species across six sites in North-Eastern Spain. We focused on species with two contrasting life strategies, evergreen with diffuse-porous wood (Quercus ilex) and winter deciduous with ring-porous wood (Q. faginea/Q. pubescens). Study sites spanned from oceanic areas with high autumn precipitation to dry continental inland regions. We hypothesized that growth bimodality reflects the interaction between local precipitation patterns and species-specific traits. Our results revealed cambial activity in autumn after summer quiescence across the entire region and for both oak species. However, growth bimodality and the relative contribution of the autumn growth peak to the total annual growth was geographically structured and species-specific. Growth was unimodal under continental and Atlantic conditions. By contrast, growth bimodality was significant along the Mediterranean coast, where precipitation showed a bimodal pattern. Moreover, evergreen Q. ilex showed more growth bimodality compared with deciduous Q. faginea/Q. pubescens at Mediterranean sites. Different intra-annual distribution of radial growth could facilitate coexistence of both oak species through temporal niche partitioning. The growth bimodality helps to compensate for reduced growth in summer by autumn cambial resumption and, therefore, the differences in its intensity between si

Climate warming and the occurrence of more severe dry spells are causing widespread drought-induced forest die-off events. Despite research on drought-triggered die-off processes is rapidly increasing, little is known on how soil conditions and rhizosphere features are affected by canopy dieback and tree death. We studied the soils in the rhizosphere of three coniferous forests where die-off was induced by a severe drought in 2012. We selected three forest types subjected to contrasting climatic and edaphic conditions dominated by three different tree species: silver fir (Abies alba; temperate conditions), Scots pine (Pinus sylvestris; continental and Mediterranean conditions) and Aleppo pine (Pinus halepensis; semi-arid and Mediterranean conditions). In each forest, we analyzed soil physical characteristics such as water retention capacity and soil texture, nutrient availability and microbial community structure (Phospholipid fatty acids, PLFA) below non-declining and declining or dying trees. We did not observe differences in nutrient availability between the two vigor classes. Conversely, we found strong differences in soil microbial community structure below non-declining and declining trees in the Silver fir and Aleppo pine stands. Soils in the Scots pine stand presented extremely low values of soil saturated sorptivity indicating a reduction of soil water infiltration after prolonged dry periods which could exacerbate drought stress. We conclude that forest dieback impacts the soil microbial community structure in the short term. Further research is required to understand the linkages between a reduced capacity of soil water infiltration after prolonged droughts, short-term changes in the soil microbiota, long-term nutrient imbalances and tree death. Soil conditions shall be considered as an important part of forest management strategies after drought-induced die off.