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Plant functional diversity (FD) is an important component of biodiversity that characterizes the variability of functional traits within a community, landscape, or even large spatial scales. It can influence ecosystem processes and stability. Hence, it is important to understand how and why FD varies within and between ecosystems, along resources availability gradients and climate gradients, and across vegetation successional stages. Usually, FD is assessed through labor-intensive field measurements, while assessing FD from space may provide a way to monitor global FD changes in a consistent, time and resource efficient way. The potential of operational satellites for inferring FD, however, remains to be demonstrated. Here we studied the relationships between FD and spectral reflectance measurements taken by ESA's Sentinel-2 satellite over 117 field plots located in 6 European countries, with 46 plots having in-situ sampled leaf traits and the other 71 using traits from the TRY database. These field plots represent major European forest types, from boreal forests in Finland to Mediterranean mixed forests in Spain. Based on in-situ data collected in 2013 we computed functional dispersion (FDis), a measure of FD, using foliar and whole-plant traits of known ecological significance. These included five foliar traits: leaf nitrogen concentration (N%), leaf carbon concentration (%C), specific leaf area (SLA), leaf dry matter content (LDMC), leaf area (LA). In addition they included three whole-plant traits: tree height (H), crown cross-sectional area (CCSA), and diameter-at-breast-height (DBH). We applied partial least squares regression using Sentinel-2 surface reflectance measured in 2015 as predictive variables to model in-situ FDis measurements. We predicted, in cross-validation, 55% of the variation in the observed FDis. We also showed that the red-edge, near infrared and shortwave infrared regions of Sentinel-2 are more important than the visible region for predict

Motivation: Trait variation within species can reveal plastic and/or genetic responses to environmental gradients, and may indicate where local adaptation has occurred. Here, we present a dataset of rangewide variation in leaf traits from seven of the most ecologically and economically important tree species in Europe. Sample collection and trait assessment are embedded in the GenTree project (EU‐Horizon 2020), which aims at characterizing the genetic and phenotypic variability of forest tree species to optimize the management and sustainable use of forest genetic resources. Our dataset captures substantial intra‐ and interspecific leaf phenotypic variability, and provides valuable information for studying the relationship between ecosystem functioning and trait variability of individuals, and the response and resilience of species to environmental changes. Main types of variable contained: We chose morphological and chemical characters linked to trade‐offs between acquisition and conservation of resources and water use, namely specific leaf area, leaf size, carbon and nitrogen content and their ratio, and the isotopic signature of stable isotope 13C and 15N in leaves. Spatial location and grain: We surveyed between 18 and 22 populations per species, 141 in total, across Europe. Time period: Leaf sampling took place between 2016 and 2017. Major taxa and level of measurement: We sampled at least 25 individuals in each population, 3,569 trees in total, and measured traits in 35,755 leaves from seven European tree species, i.e. the conifers Picea abies, Pinus pinaster and Pinus sylvestris, and the broadleaves Betula pendula, Fagus sylvatica, Populus nigra and Quercus petraea. Software format: The data files are in ASCII text, tab delimited, not compressed.

1. Plant species that grow across environmental gradients show a range of trait expression, but traits do not vary independently. In fact, phenotypes are integrated expressions of multiple traits that covary due to trade-offs among functions and processes. Understanding trait covariation structures will ultimately help with predicting species' responses to change and developing management actions. 2. We measured variation and covariation (a proxy of phenotypic integration) among functional traits of Pinus sylvestris from paired populations across its European distribution. Populations within a pair were close enough to be in gene flow contact but located in contrasting environmental conditions across a local gradient. Measured traits represented three axes of variation (groups of traits) related to a tree's competitive ability and the trade-off between resource acquisition and conservation, namely plant size measures and stem and foliar traits. 3. Results revealed important intra- and inter-population trait variability. In particular, at the population level, trait means shifted across the climatic gradient mainly described by mean annual temperature. Moreover, we found a higher degree of trait covariation in populations under harsher environments (i.e. lower environmental suitability for the species). This pattern was consistent within population pairs, suggesting that higher trait covariation may be adaptive, being more coordinated in sites with harsher conditions. At larger spatial scales, we found a less conclusive pattern with a trend of increasing covariation at the northern edge of the species distribution. This result suggests that at larger scales different processes may be involved in the trade-off between the adaptive value of phenotypic covariation versus its constraints on trait combinations that may limit plant's response capability. 4. Synthesis. Trait covariation varies at different spatial scales, increasing under harsher conditions, and the robustn

1. Plant species that grow across environmental gradients show a range of trait ex-pression, but traits do not vary independently. In fact, phenotypes are integrated expressions of multiple traits that covary due to trade-offs among functions and processes. Understanding trait covariation structures will ultimately help with predicting species' responses to change and developing management actions., 2. We measured variation and covariation (a proxy of phenotypic integration) among functional traits of Pinus sylvestris from paired populations across its European distribution. Populations within a pair were close enough to be in gene flow con-tact but located in contrasting environmental conditions across a local gradient. Measured traits represented three axes of variation (groups of traits) related to a tree's competitive ability and the trade-off between resource acquisition and conservation, namely plant size measures and stem and foliar traits., 3. Results revealed important intra- and inter-population trait variability. In particular, at the population level, trait means shifted across the climatic gradient mainly de-scribed by mean annual temperature. Moreover, we found a higher degree of trait covariation in populations under harsher environments (i.e. lower environmental suitability for the species). This pattern was consistent within population pairs, suggesting that higher trait covariation may be adaptive, being more coordinated in sites with harsher conditions. At larger spatial scales, we found a less conclusive pattern with a trend of increasing covariation at the northern edge of the species distribution. This result suggests that at larger scales different processes may be in-volved in the trade-off between the adaptive value of phenotypic covariation ver-sus its constraints on trait combinations that may limit plant's response capability., 4. Synthesis. Trait covariation varies at different spatial scales, increasing under harsher conditions, and the robustness and repeatability of this pattern suggests its adaptive role for the species' responses to different environments.

[Background]: Progress in the field of evolutionary forest ecology has been hampered by the huge challenge of phenotyping trees across their ranges in their natural environments, and the limitation in high-resolution environmental information., [Findings]: The GenTree Platform contains phenotypic and environmental data from 4,959 trees from 12 ecologically and economically important European forest tree species: Abies alba Mill. (silver fir), Betula pendula Roth. (silver birch), Fagus sylvatica L. (European beech), Picea abies (L.) H. Karst (Norway spruce), Pinus cembra L. (Swiss stone pine), Pinus halepensis Mill. (Aleppo pine), Pinus nigra Arnold (European black pine), Pinus pinaster Aiton (maritime pine), Pinus sylvestris L. (Scots pine), Populus nigra L. (European black poplar), Taxus baccata L. (English yew), and Quercus petraea (Matt.) Liebl. (sessile oak). Phenotypic (height, diameter at breast height, crown size, bark thickness, biomass, straightness, forking, branch angle, fructification), regeneration, environmental in situ measurements (soil depth, vegetation cover, competition indices), and environmental modeling data extracted by using bilinear interpolation accounting for surrounding conditions of each tree (precipitation, temperature, insolation, drought indices) were obtained from trees in 194 sites covering the species’ geographic ranges and reflecting local environmental gradients., [Conclusion]: The GenTree Platform is a new resource for investigating ecological and evolutionary processes in forest trees. The coherent phenotyping and environmental characterization across 12 species in their European ranges allow for a wide range of analyses from forest ecologists, conservationists, and macro-ecologists. Also, the data here presented can be linked to the GenTree Dendroecological collection, the GenTree Leaf Trait collection, and the GenTree Genomic collection presented elsewhere, which together build the largest evolutionary forest ecology data collection available.

[Motivation] Trait variation within species can reveal plastic and/or genetic responses to environmental gradients, and may indicate where local adaptation has occurred. Here, we present a dataset of rangewide variation in leaf traits from seven of the most ecologically and economically important tree species in Europe. Sample collection and trait assessment are embedded in the GenTree project (EU-Horizon 2020), which aims at characterizing the genetic and phenotypic variability of forest tree species to optimize the management and sustainable use of forest genetic resources. Our dataset captures substantial intra- and interspecific leaf phenotypic variability, and provides valuable information for studying the relationship between ecosystem functioning and trait variability of individuals, and the response and resilience of species to environmental changes., [Main types of variable contained] We chose morphological and chemical characters linked to trade-offs between acquisition and conservation of resources and water use, namely specific leaf area, leaf size, carbon and nitrogen content and their ratio, and the isotopic signature of stable isotope 13C and 15N in leaves., [Spatial location and grain] We surveyed between 18 and 22 populations per species, 141 in total, across Europe., [Time period] Leaf sampling took place between 2016 and 2017., [Major taxa and level of measurement] We sampled at least 25 individuals in each population, 3,569 trees in total, and measured traits in 35,755 leaves from seven European tree species, i.e. the conifers Picea abies, Pinus pinaster and Pinus sylvestris, and the broadleaves Betula pendula, Fagus sylvatica, Populus nigra and Quercus petraea., [Software format] The data files are in ASCII text, tab delimited, not compressed.

[Questions]: While positive effects of tree diversity on tree community biomass have often been reported in mature stands, the debate on whether diversity effects may be detectable at the seedling level persists, with opposing outcomes found so far. We still lack a comprehensive evaluation of the biodiversity effects (so- called ‘comple-mentarity’ and ‘selection’ effects), as well as the phenotypic drivers at play, underly-ing early-community biomass. Even less is known about whether such biodiversity effects may change under water- limited conditions., [Location]: Seeds from four tree species coexisting in a Mediterranean forest (Spain)., [Methods]: We built experimental tree seedling assemblages with three diversity lev-els — monocultures, two- species and four- species mixtures — and under two soil moisture conditions. We quantified the extent to which species richness, species identity, community-weighted mean (CWM) and functional dissimilarity (FD) influ-ence complementarity and selection effects. We computed CWM and FD for seven functional traits related to water and light acquisition; and we calculated the comple-mentarity and selection effects from above- and below- ground biomass measures at the community level., [Results]: Our results showed that complementarity drove the greater biomass in more diverse assemblages at the seedling stage. This pattern was largely favored by a par-ticular species, Quercus faginea, with distinct phenotypic traits (great height, lateral ramification and root biomass with high dry matter content), which induced a positive effect of CWM on community biomass. Moreover, our study showed that the water deficit limited the production of above- ground biomass without interacting with the community’s species richness., [Conclusion]: Our study provides evidence that positive biodiversity effects on com-munity biomass occur early, at the seedling stage, and it highlights the essential role that certain tree species play from their initial development stages by favoring spatresource partitioning. Our work motivates future studies to apply integrated ap-proaches in assessing both the community-wide and species-specific effects to understand the biodiversity–biomass relationship.

Deciduous seasonal forests (DSFs) have a peculiar floristic composition, with species capable of surviving periods of high water deficit in a year. Our goal was to demonstrate that abiotic filters lead to the assembly of two DSF communities that have high floristic dissimilarity. For this, we characterized the environment of the areas and used the community-weighted mean (CWM), functional richness (FRic) and functional dispersion (FDis) indices for regional recognition of functional patterns. The local assessment of assembly rules was carried out using null models. We found differences in the FRic and FDis between the areas, which was attributed to the different floristic influences exercised on the communities. However, in both, the typical attributes of dry formations were dominant (CWM), indicating that, on the regional scale of study, the dry season acts as a filter in the composition of species in the communities. On a local scale, stochastic dispersion was identified as the mos influential mechanism in the assembly of communities. We conclude that deterministic and stochastic processes act in the assembly of the studied communities, and the proportion of each of these depends on the scale, with abiotic filtration predominating on a regional scale and stochastic dispersion events on a local scale.

Understanding the dynamics of selection is key to predicting the response of tree species to new environmental conditions in the current context of climate change. However, selection patterns acting on early recruitment stages and their climatic drivers remain largely unknown in most tree species, despite being a critical period of their life cycle. We measured phenotypic selection on Pinus sylvestris seed mass, emergence time and early growth rate over 2 yr in four common garden experiments established along the latitudinal gradient of the species in Europe. Significant phenotypic plasticity and among-population genetic variation were found for all measured phenotypic traits. Heat and drought negatively affected fitness in the southern sites, but heavy rainfalls also decreased early survival in middle latitudes. Climate-driven directional selection was found for higher seed mass and earlier emergence time, while the form of selection on seedling growth rates differed among sites and populations. Evidence of adaptive and maladaptive phenotypic plasticity was found for emergence time and early growth rate, respectively. Seed mass, emergence time and early growth rate have an adaptive role in the early stages of P. sylvestris and climate strongly influences the patterns of selection on these fitness-related traits.

Understanding constraints to phenotypic plasticity is key given its role on the response of organisms to environmental change. It has been suggested that phenotypic integration, the structure of trait covariation, could limit trait plasticity. However, the relationship between plasticity and integration is far from resolved. Using a database of functional plasticity to drought of a Mediterranean shrub that included 20 ecophysiological traits, we assessed environmentally-induced changes in phenotypic integration and whether integration constrained the expression of plasticity, accounting for thewithin-environment phenotypic variation of traits. Furthermore, we provide the first test of the association between differential trait plasticity and trait integration across an optimumand a stressful environment. Phenotypic plasticity was positively associated with phenotypic integration in both environments, but this relationship was lost when phenotypic variation was considered. The similarityin the plastic response of two traits predicted their integration across environments, with integrated traits having more similar plasticity. Such variation in the plasticity of traits partly explained the lower phenotypic integration found in the stressful environment. We found no evidence that integration may constitute an internal constraint to plasticity. Rather, we present the first empirical demonstration that differences in plastic responses mayinvolve a major reorganization of the relationships among traits, and challenge the notion thatstress generally induces a tighter phenotype.

[EN] The functional traits reflect the ecological differences between species and are considered indicators of the species ecological role and realized niche within plant communities. Thus, they emerge as an important tool to understand the individual responses of species to environmental conditions. In this context, decipheringpatternsin the variability of functional traits implies a better understanding of underlying mechanisms that regulate the species assembly. The maingoal of this doctoral thesis was to quantify the variability of key functional traits in individuals of Pinus sylvestrisacrossdifferent spatial scales in European forests. We chose Pinus sylvestrisas a study system because of its wide distributionand its relevance in European forests. Analysing phenotypes spatial distribution we found that competition arises asan important factor that determines the spatial structure of functional traits within the population. Our results showedthat basal area increments of treesdepend mainly on functional traits related to the growth performance and its competitive capacity such as thetreeheight and crown size. The growth of the pine trees was affected by the change in temperature over the last c.a. 50years, together with the changes in the drought regime in our study area. The response of growth to climate change is age-dependent, and the youngest pines (≤50 years) depictthe best resilience in this warming scenario. We found coordinated phenotypes, where the covariation of traits involveddifferent parts of the plant (stem and leaf traits) and plant size. In addition, our results demonstratedenvironmental-induced differences in the structure of covariation of functional traits, with greatercovariation of traitsfound in more stressful conditions within a local gradient, while at larger scaleswe found no clear pattern in the degree of trait covariation., [ES] Los rasgos funcionales reflejan las diferencias ecológicas entre las especies yse consideran indicadores del papel ecológico y del nicho realizado de cada especie dentro de las comunidades vegetales. Por este motivo han despertado gran interés comoherramienta clavepara comprender las respuestas individualesde las especiesa las condiciones ambientales. En este contexto, profundizar en el estudio de patrones de variabilidad de los valores de rasgos funcionales puede suponer un avance en lacapacidad para entender los mecanismos que regulan el ensamblaje de las especies. El objetivo principal de esta tesis doctoral fue cuantificar la variabilidad de rasgos funcionales claves enindividuos de Pinus sylvestrisa lo largo de distintas escalas espaciales en bosques europeos. Escogimos el pino silvestre como sistema de estudio por su amplia distribución y su papel destacado en la vegetaciónforestaleuropea. Nuestros resultados mostraran que el incremento en área basal de los árboles depende principalmente de rasgos funcionales relacionados con el rendimiento del crecimiento y su capacidad competitiva como la altura del árbol y tamañode copa.Además, la competencia entre los individuos aparece como un importante factor que determina la estructura espacial de los rasgos funcionales dentro de la población. El crecimiento de los pinos se vio afectado por el cambio en la temperatura a lo largo de los últimos años, junto con los cambios enel régimen de sequía en nuestra área de estudio. La respuesta en el crecimiento frente al calentamiento difirió con la edad, de manera que los pinos más jóvenes (≤50 años) presentaron mejor capacidad de recuperación en ese escenario de calentamiento. Finalmente,al abordar el espacio fenotípico en su conjunto analizando la covariaciónde rasgos,encontramos un fenotipo coordinado, dónde la covariación de rasgos relacionados condiferentes partes de la planta (rasgos de tallo y hojas)y el tamaño de la planta es significativa. Además, nuestros resulta

The functioning of plant communities is strongly influenced by the number of species in the community and their spatial arrangement. This is because plants interact with their nearest neighbors and this interaction is expected to be stronger when the interacting individuals are ecologically similar in terms of resource use. Recent evidence shows that species richness alters the balance of intra- versus interspecific competition, but the effect of species richness, and phylogenetic and functional diversity on the spatial pattern of the plant communities remain less studied. Even far, how forest stand structure derived from past management practices can influence the relationship between species richness and spatial pattern is still unknown. Here, we evaluate the spatial distribution of woody individuals (DBH >7.5 cm) in 209 forest stands (i.e. plots) with an increasing level of species richness (from 1 up to 10 species) in six forest types along a latitudinal gradient in Europe. We used completely mapped plots to investigate the spatial pattern in each forest stand with point pattern techniques. We fitted linear models to analyze the effect of species richness (positively correlated with phylogenetic diversity) and functional diversity on tree spatial arrangements. We also controled this relationship by forest type and stand structure as a proxy of the management legacy. Our results showed a generalized positive effect of species richness and functional diversity on the degree of spatial clustering of trees, and on the spatial independence of tree sizes regardless of the forest type. Moreover, current tree spatial arrangements were still conditioned by its history of management; however its effect was independent of the number of species in the community. Our study showed that species richness and functional diversity are relevant attributes of forests influencing the spatial pattern of plant communities, and consequently forest functioning.

[Background]: In contrast with the negligible contribution of the forest understorey to the total aboveground phytobiomass of a forest, its share in annual litter production and nutrient cycling may be more important. Whether and how this functional role of the understorey differs across forest types and depends upon overstorey characteristics remains to be investigated., [Methods]: We sampled 209 plots of the FunDivEUROPE Exploratory Platform, a network of study plots covering local gradients of tree diversity spread over six contrasting forest types in Europe. To estimate the relative contribution of the understorey to carbon and nutrient cycling, we sampled non-lignified aboveground understorey biomass and overstorey leaf litterfall in all plots. Understorey samples were analysed for C, N and P concentrations, overstorey leaf litterfall for C and N concentrations. We additionally quantified a set of overstorey attributes, including species richness, proportion of evergreen species, light availability (representing crown density) and litter quality, and investigated whether they drive the understorey’s contribution to carbon and nutrient cycling. [Results and conclusions]: Overstorey litter production and nutrient stocks in litterfall clearly exceeded the contribution of the understorey for all forest types, and the share of the understorey was higher in forests at the extremes of the climatic gradient. In most of the investigated forest types, it was mainly light availability that determined the contribution of the understorey to yearly carbon and nutrient cycling. Overstorey species richness did not affect the contribution of the understorey to carbon and nutrient cycling in any of the investigated forest types., [Results and conclusions]: Overstorey litter production and nutrient stocks in litterfall clearly exceeded the contribution of the understorey for all forest types, and the share of the understorey was higher in forests at the extremes of the climatic gradient. In most of the investigated forest types, it was mainly light availability that determined the contribution of the understorey to yearly carbon and nutrient cycling. Overstorey species richness did not affect the contribution of the understorey to carbon and nutrient cycling in any of the investigated forest types.

Increasing evidence supports that canopy richness favors multifunctionality in European forests. Given that recruitment represents one of the main demographic bottlenecks in plant communities, the provision of similar levels of forest functioning in the future will depend on successful natural regeneration in the present. Here, we evaluated whether canopy diversity enhances recruitment (abundance and richness) by providing a more diverse seed supply and a more heterogeneous environmental context; or, alternatively, if post-dispersal environmental filters outweigh these expected positive effects. For this purpose, we used a large-scale European platform comprising 173 forest plots that follow a canopy species richness gradient (from monoculture to mixed plots up to 5 tree species) in five forest types from Mediterranean to boreal forests. Despite that canopy diversity promoted a more diverse seed supply and a more heterogeneous light environment, this did not translate into a richer or more abundant community of recruits. In fact, we found an important uncoupling in species composition between recruitment and canopy layers. Our results suggest that strong post-dispersal filters, mediated by an interaction between seed size and latitude, drive this mismatch. Large-seeded species were over-represented in the recruitment at both extremes of the European gradient (Mediterranean and boreal forests), while the opposite trend occurred at intermediate latitudes. Altogether, our work shows that preserving a more diverse canopy may not ensure successful recruitment, and advocates future studies analyzing the role of post-dispersal filters on the assembly of European forests.

Functional diversity informs about biodiversity-ecosystem functioning relationships. The intraspecific component of functional diversity (i.e. the phenotypic space of each species) depicts individual differences in the resource use and fitness among conspecifics, and gives valuable information about the functional similarity (competition) or dissimilarity (complementarity) of coexisting species. Here, we quantified trait differences within tree species along local diversity gradients to shed light on the role that this intraspecific variability exerts on functional complementarity of tree species. We measured architectural traits in 5,036 individuals and leaf traits in 1,403 individuals from nine dominant tree species, surveyed in 92 plots located in three major European forest types (Mediterranean, temperate and boreal forests). In each forest type, plots were positioned along a canopy richness gradient, with every study species present in different species richness levels, including monocultures. Our results showed that the relative magnitude of intraspecific trait variability to community-level variability is high in these forests. At the species level, we found adjustments of species leaf traits (mean shifts) in response to neighbouring trees, suggesting the existence of processes that limit niche overlap. We also found higher variability in architectural traits of conspecific individuals in more diverse canopies, suggesting greater niche packing and a more efficient use of available space as the number of species in the canopy increases. Altogether, our results support the hypothesis that differential responses of individuals within a species promote species complementarity, suggesting that biodiversity-ecosystem functioning relationships cannot be properly estimated without accounting for the intraspecific level of functional variation.

Many tree species from Mediterranean regions have started to show increased rates of crown defoliation, reduced growth, and dieback associated with the increase in temperatures and changes in the frequency and intensity of drought events experienced during the last decades. In this regard, Quercus ilex L. subsp. ballota [Desf.] (Holm oak), despite being a drought-tolerant species widely distributed in the Mediterranean basin, it has recently started to show acute signs of decline, extended areas from Spain being affected. However, few studies have assessed the role of climatic variability (i.e., temperature, precipitation, and drought) on the decline and resilience of Holm oak. Here, we measured secondary growth of seventy Holm oaks from a coppice stand located in central Spain. Sampled trees had different stages of decline, so they were classified into four vigour groups considering their crown foliar lost: healthy (0%), low defoliated (<25%), highly defoliated (25–70%), and dying (70–100%). Our results showed that during the study period (1980–2009) the highly defoliated and dying Holm oaks grew significantly less than their healthy and low defoliated neighbours, suggesting permanent growth reduction in the less vigorous individuals. Despite these differences, all four vigour groups showed similar responses to climatic variations, especially during winter and late spring – early summer seasons, and similar resilience after severe drought events, managing to significantly recover to pre-drought growth rates after only two years. Our findings, hence, illustrate that tree vigour influences secondary growth but not responsiveness to climatic variability in Holm oak. Still, as reduced growth rates are frequently associated with the process of tree mortality, we conclude that the less vigorous Holm oaks might not be able to cope with future water stress conditions, leading to increased mortality rates among this emblematic Mediterranean species.

Humans require multiple services from ecosystems, but it is largely unknown whether trade‐offs between ecosystem functions prevent the realisation of high ecosystem multifunctionality across spatial scales. Here, we combined a comprehensive dataset (28 ecosystem functions measured on 209 forest plots) with a forest inventory dataset (105,316 plots) to extrapolate and map relationships between various ecosystem multifunctionality measures across Europe. These multifunctionality measures reflected different management objectives, related to timber production, climate regulation and biodiversity conservation/recreation. We found that trade‐offs among them were rare across Europe, at both local and continental scales. This suggests a high potential for ‘win‐win’ forest management strategies, where overall multifunctionality is maximised. However, across sites, multifunctionality was on average 45.8‐49.8% below maximum levels and not necessarily highest in protected areas. Therefore, using one of the most comprehensive assessments so far, our study suggests a high but largely unrealised potential for management to promote multifunctional forests.

Many tree species from Mediterranean regions have started to show increased rates of crown defoliation, reduced growth, and dieback associated with the increase in temperatures and changes in the frequency and intensity of drought events experienced during the last decades. In this regard, Quercus ilex L. subsp. ballota [Desf.] (Holm oak), despite being a drought-tolerant species widely distributed in the Mediterranean basin, it has recently started to show acute signs of decline, extended areas from Spain being affected. However, few studies have assessed the role of climatic variability (i.e., temperature, precipitation, and drought) on the decline and resilience of Holm oak. Here, we measured secondary growth of seventy Holm oaks from a coppice stand located in central Spain. Sampled trees had different stages of decline, so they were classified into four vigour groups considering their crown foliar lost: healthy (0%), low defoliated (<25%), highly defoliated (25–70%), and dying (70–100%). Our results showed that during the study period (1980–2009) the highly defoliated and dying Holm oaks grew significantly less than their healthy and low defoliated neighbours, suggesting permanent growth reduction in the less vigorous individuals. Despite these differences, all four vigour groups showed similar responses to climatic variations, especially during winter and late spring – early summer seasons, and similar resilience after severe drought events, managing to significantly recover to pre-drought growth rates after only two years. Our findings, hence, illustrate that tree vigour influences secondary growth but not responsiveness to climatic variability in Holm oak. Still, as reduced growth rates are frequently associated with the process of tree mortality, we conclude that the less vigorous Holm oaks might not be able to cope with future water stress conditions, leading to increased mortality rates among this emblematic Mediterranean species.

Species diversity seems to favour the environmental heterogeneity and habitat complexity, which in turn favour the selection of greater levels of phenotypic plasticity of plant species. While recent evidence suggested that local species diversity and environmental heterogeneity act as selective pressures over mean traits and plasticity for herbs, which adjust their traits to the identity and species richness of their neighbors, little is known for tree species. Here, we investigated shifts in the phenotypic expression and plasticity of seedlings of two functional groups (broadleaved and conifers) coming from communities with different canopy species richness in two contrasting forest types (a hemiboreal and a Mediterranean forest). We carried out a greenhouse experiment to test the response of seedlings to different light and nutrient availability, measuring different functional traits related to resource acquisition. Our results indicated that seedlings coming from monospecific and more diverse communities had similar phenotypic expression and response capacity to the study treatments. This finding suggests that canopy species richness did not exert a detectable selective pressure on tree phenotypes and plasticity at this early stage. Additionally, we found great differences in the phenotypic expression and plasticity between the two major functional groups (conifers and broadleaved species). Specifically, we found that broadleaves reached higher mean values in key functional traits by a greater plasticity to both light and nutrient treatment than coniferous species, mainly in the Mediterranean forest. This finding suggests that broadleaved species potentially have a higher capacity to respond to future changing environmental conditions than conifers at early stages, conferring an advantage that can be crucial for the species competitive ability for resources and, therefore, for survival.

The importance of biodiversity in supporting ecosystem functioning is generally well accepted. However, most evidence comes from small-scale studies, and scaling-up patterns of biodiversity-ecosystem functioning (B-EF) remains challenging, in part because the importance of environmental factors in shaping B-EF relations is poorly understood. Using a forest research platform in which 26 ecosystem functions were measured along gradients of tree species richness in six regions across Europe, we investigated the extent and the potential drivers of context dependency of B-EF relations. Despite considerable variation in species richness effects across the continent, we found a tendency for stronger B-EF relations in drier climates as well as in areas with longer growing seasons and more functionally diverse tree species. The importance of water availability in driving context dependency suggests that as water limitation increases under climate change, biodiversity may become even more important to support high levels of functioning in European forests.

The importance of biodiversity in supporting ecosystem functioning is generally well accepted. However, most evidence comes from small‐scale studies, and scaling‐up patterns of biodiversity–ecosystem functioning (B‐EF) remains challenging, in part because the importance of environmental factors in shaping B‐EF relations is poorly understood. Using a forest research platform in which 26 ecosystem functions were measured along gradients of tree species richness in six regions across Europe, we investigated the extent and the potential drivers of context dependency of B‐EF relations. Despite considerable variation in species richness effects across the continent, we found a tendency for stronger B‐EF relations in drier climates as well as in areas with longer growing seasons and more functionally diverse tree species. The importance of water availability in driving context dependency suggests that as water limitation increases under climate change, biodiversity may become even more important to support high levels of functioning in European forests.

Disentangling the mechanisms that shape community assembly across diversity gradients is a central matter in ecology. While many studies have explored community assembly through species average trait values, there is a growing understanding that intraspecific trait variation (ITV) can also play a critical role in species coexistence. Classic biodiversity theory hypothesizes that higher diversity at species-rich sites can arise from narrower niches relative to species-poor sites, which would be reflected in reduced ITV as species richness increases. To explore how ITV in woody plant communities changes with species richness, we compiled leaf trait data (leaf size and specific leaf area) in a total of 521 woody plant species from 21 forest communities that differed dramatically in species richness, ranging from boreal to tropical rainforests. At each forest, we assessed ITV as an estimate of species niche breadth and we quantified the degree of trait overlap among co-occurring species as a measure of species functional similarity. We found ITV was relatively invariant across the species richness gradient. In addition, we found that species functional similarity increased with diversity. Contrary to the expectation from classic biodiversity theory, our results rather suggest that neutral processes or equalizing mechanisms can be acting as potential drivers shaping community assembly in hyperdiverse forests.

Evidence of tree regeneration failure of some species in the Iberian Peninsula forests warns us about the impact that the global change may exert on the preservation of Mediterranean forests, such as we know them. Predictions agree about an exacerbation of the summer drought there, acknowledged as the main limiting factor for the recruits’ survival. On the other hand, many studies have also proved the relevant role that local heterogeneity has over the spatial distribution of forest species recruitment by providing safe sites. Therefore, to unravel how climate interacts with local factors over juveniles’ performance seems crucial for the design of successful management strategies that allow facing the global warming. Here, we surveyed the natural recruitment of four dominant tree species in seven mountainous regions in the Iberian Peninsula, along entire elevational ranges as surrogates of their climatic ranges. Two of them have alpine and temperate distributions with populations at their rear edge in the Spanish mountains: Fagus sylvatica and Pinus uncinata; and the other two have a genuine Mediterranean distribution: Quercus ilex and Pinus nigra. Our main goal was to analyze for each species the effect of climate, local factors (i.e. light availability, stand structure and ground cover) and the interactions among them to identify the main drivers leading the regeneration process, assessed in terms of presence, abundance and mean annual growth of juveniles. The results showed different environmental factors determining the recruitment patterns of each species. Nevertheless, they highlighted the pervasive role exerted by both climate and fine scale factors, particularly the co-occurring vegetation on recruits’ abundance, and the light availability on their growth. Moreover, we found some interactions among annual mean temperature and local factors, suggesting that climate and local heterogeneity act hierarchically, i.e. the local conditions may mitigate or exacerbat

The long lifespan of trees makes them sensitive to climate warming, particularly when abrupt changes in climatic conditions occur and when trees are already growing near to their climatic tolerance thresholds. In many Mediterranean pine forests, drought stress induced by warmer temperatures and increased aridification compromise tree survival, causing growth decline and, eventually, triggering tree mortality. We evaluated the effect of climate on radial growth and mortality comparing dead and living trees of two pine species, Pinus sylvestrisand. Pinus nigrasubsp.salzmannii, in a continental Mediterranean area over a 50-year long period. We used linear mixed models and correlations to evaluate the effect of climate on basal area increment (BAI) and on ring-width indices, respectively. In P. nigra, growth was enhanced by wet and cold conditions, whilst growth increased with temperatures in. P. sylvestris. A gradual BAI reduction was detected in P. nigratrees since the late 1970s, becoming more pronounced in recently dead trees. Contrarily, P. sylvestris growth did not show such decline, whilst death events in this species were linked to severe and punctual droughts. Our findings show that warming and drought stress trigger contrasting responses in functionally and phylogenetically similar tree species, and suggest climate-mediated important changes on competitive dominance in Mediterranean forests.