In 2023, more than half of olive harvests ( Olea europaea ) across Spain, Greece, and Türkiye were lost to drought. The same year late freeze destroyed 90% of the peach crop ( Prunus persica ) on the Georgia Piedmont and the apple crop ( Malus domestica ) in central New York, Vermont, and southern Quebec. Climate extremes now rank with the costliest threats to agriculture, but their role in forest recovery from diebacks that are happening globally is unknown for lack of tree fecundity estimates in forests. Tolerance of climate extremes could depend on past exposure but constrained by phylogenetic conservatism. We report a continental scale analysis of climate extremes and forest fecundity across North America and Europe showing that responses to late freeze and drought are happening now. Species differences are not explained by the traits typically included in ecological studies and they are weakly associated with phylogeny. Late freeze, that is, freezing temperatures that follow the onset of flower development in spring, is shown to be “normal” in North America, but not Europe, potentially explaining failed seed production due to delayed onset and the resultant shorter growing period by North American transplants dating back at least to the 18th century. Drought has thus far had the greatest impacts in dry forested regions, but here too, species differences are not explained by traditional trait values. If responses have been buffered from drought and late freeze by past exposure, acclimation and local adaptation prove inadequate as extremes intensify.

Tree-related microhabitats (TreMs) are key features for forest biodiversity, and knowing their accumulation rate is essential to design integrative management strategies. Many types of TreMs are associated to large old trees and show slow ontogenical processes. The rarity of such TreMs (particularly in intensively managed forests) hinder the estimation of their occurrence rate along tree growth. Here, we used a continental meta-analysis on TreMs occurrence rate along tree growth to build informative priors for a model of trunk-base rot-hole occurrence on oaks within the Gr & eacute;signe forest, France - a context where stand management and tree DBH were confounded. We explored whether the use of informative priors could improve the identifiability, the precision of estimates and the predictive abilities of the model. Without prior information, the low variance of tree DBH within management modalities rendered the model poorly identifiable and prevented the detection of an effect of tree DBH per se across the range of explored tree DBH. By contrast, using informative priors contributed to improve the precision of estimates and lead to detecting a positive effect of tree DBH per se. Informative priors did not degrade the model fit and clearly improved predictive abilities on new stands. In particular, while the model without prior information did not predict the occurrence of trunk-base rot-holes significantly better than a purely random guess, the model with informative priors did. Irrespective of the prior used, models suggested that the high recruitment of trunk-base rot-holes in Gr & eacute;signe may be a temporary management effect in stands undergoing conversion from coppice-with-standards to high forest through sprout thinning, which will lead to conservation issues for cavicolous saproxylic species when all conversions are complete. Because using informative priors was simple and beneficial in our study, it should be further explored in other local applied contexts to orientate forest management.

When an even-aged forest stand reaches maturity, it can be renewed within a limited period to maintain the even-aged structure or gradually transformed to an uneven-aged stand. However, there is still debate as to which silvicultural approach is more profitable, conducive to carbon storage, favourable to biodiversity or resilient. In this study, we simulated the evolution of fifteen stands representative of the Walloon forest, whose initial structure was even-aged. The stands were managed according to two contrasting silvicultural approaches (continuation of the even-aged system vs transformation to an uneven-aged one), and the simulations were run with the SSP3-7.0 climate projections produced by five global circulation models. Our simulations indicate that even-aged and uneven-aged silviculture yield similar outcomes in terms of carbon storage. Financial indicators were likewise largely unaffected, except in the oak-beech mixture, where uneven-aged silviculture increased profitability through the substitution of oak by beech. This shift reduced tree species diversity in uneven-aged oak-beech stands. Tree microhabitats, except in beech stands, were more abundant under uneven-aged silviculture. While mean values of forest ecosystem functioning indicators are largely comparable between the two approaches, uneven-aged stands exhibit higher temporal stability. Uneven-aged silviculture produces stands that are more wind-resistant and avoid periods of extreme vulnerability. Our study shows that the most appropriate silviculture may vary depending on the aspect considered. Uneven-aged silviculture has a definite advantage in terms of stability, risk management, and tree microhabitats. However, maintaining even-aged patches at the landscape scale remains important to facilitate the regeneration of shade-intolerant species.

The size of a tree and its crown are key drivers of its ability to intercept light. These characteristics are also constrained by biomechanical and hydraulic limitations, leading to tremendous variations in crown size and tree height between species. To date, there is no consensus on how covariation between crown characteristics (i.e. tree height, crown diameter and relative crown depth) control the spectrum of crown sizes, nor on the species characteristics (i.e. species climate niche or functional traits) that govern their variation. Using species‐specific allometric relationships accounting for local competition data on individual crown characteristics of tree species across Europe and North America, we showed that the spectrum of tree crown size is mainly driven by two independent axes: one correlating tree height with relative crown depth, and another linking crown diameter to crown volume. While both angiosperms and gymnosperms exhibited similar patterns of covariation for these crown characteristics, distinct ecological strategies emerged for maximum tree height. We also highlighted that shade‐tolerant species were characterized by wider and deeper crowns and showed reduced sensitivity to local competition in relative crown depth, indicating enhanced competitive resilience. Finally, simulations of light interception in virtual stands performed using a ray tracing algorithm highlighted the importance of different crown characteristics in various competitive contexts. Tree height was the primary determinant of light interception in low competitive contexts, common in early successional stages, whereas relative crown depth and crown diameter played a more significant role in high‐competitive contexts typical of late succession. These findings suggest that shade‐tolerant species, with their deeper crowns, are better equipped to intercept light under competition and ultimately emphasize the ability of shade tolerance to improve the realism of forest dynamic models as it can be used as an indicator of crown size and its response to competition.

Tree-related Microhabitats (TreMs) are of prime concern for biodiversity since they host thousands of taxa. TreMs are discrete habitat patches borne by trees and cover a wide range of lifespans, from days to decades to centuries. The taxa associated with such discrete and sometimes highly ephemeral habitat patches are likely to be sensitive to spatial distribution because they need to search for new habitats after the occupied one disappears. Although many studies have recently been dedicated to TreMs, only very few have investigated their spatial distribution. Focusing on European beech-dominated forests, we used a European TreM database with 12,362 trees and 296 plots (ranging from local (0.1-1 ha) to landscape scale (10,000 ha)) to assess TreM spatial distribution patterns in long unmanaged forests. Then, with a TreM sub-database with 6828 trees and 21 plots, we assessed whether and if so, how harvesting changes spatial patterns at the plot scale. In long unmanaged forests, most TreMs showed a regular pattern at the plot scale and an aggregated pattern at larger scales. Tree diameter was the most influential factor for spatial patterns in TreMs. Spatial patterns at the plot scale in harvested forests differed slightly from those observed in unmanaged forests. To favor TreM-dwelling taxa in harvested stands, our results suggest retaining habitat trees in a regular spatial pattern to mimic the natural pattern. However, some TreMs should be conserved in an aggregate pattern; we specify the spatial scale at which this should be done.

Competition between individuals is a key process that drives tree growth and survival in forests. Ecological theories predict that the effect of competition should be weaker in stressful environments. However, quantitative studies have failed to reach a consensus on the direction of the interaction between climate and competition. In this study, we demonstrate that this interaction appears clearly when we explicitly focus on light competition. We analysed the effect of light competition on tree growth and survival along both temperature and aridity gradients for the 33 major European tree species. We collected forest inventories from nine European countries, encompassing over 1 million trees from Spain to Scandinavia. We used species-specific crown allometric equations to connect this extensive database to the SamsaraLight ray tracing model and to calculate a tree-based light competition index from the light intercepted by the tree crown. Within a given species' climatic niche, the effect of light competition on tree growth and survival decreased towards both the dry and cold margins, supporting the stress gradient hypothesis. Climate mainly affected tree growth in light, with slower growth in drier or colder conditions. In contrast, for survival, climate mainly affected trees in shade, with better survival in the dry or cold stress margins. Among species, the mean sensitivity of tree growth and survival to light competition decreased with increasing mean aridity niche and shade tolerance of the species. Synthesis. Our study emphasises the importance of considering species-specific interactions between light competition and climate on tree growth and survival. The impact of climate change on an individual tree is likely to depend on its light competition status within the forest stand, as well as its species-specific climatic niche and shade tolerance. La comp & eacute;tition entre individus est un processus cl & eacute; qui d & eacute;termine la croissance et la survie des arbres des for & ecirc;ts. Les th & eacute;ories & eacute;cologiques pr & eacute;voient que l'effet de la comp & eacute;tition devrait & ecirc;tre plus faible dans les environnements stressants. Cependant, les & eacute;tudes quantitatives ne sont pas parvenues & agrave; un consensus sur la direction de l'interaction entre climat et comp & eacute;tition. Dans cette & eacute;tude, nous d & eacute;montrons que cette interaction appara & icirc;t clairement lorsque nous nous concentrons explicitement sur la comp & eacute;tition pour la lumi & egrave;re. Nous avons analys & eacute; l'effet de la comp & eacute;tition pour la lumi & egrave;re sur la croissance et la survie des arbres le long des gradients de temp & eacute;rature et d'aridit & eacute; pour les 33 principales esp & egrave;ces d'arbres europ & eacute;ennes. Nous avons rassembl & eacute; des inventaires forestiers de neuf pays europ & eacute;ens, couvrant plus d'un million d'arbres de l'Espagne & agrave; la Scandinavie. Nous avons utilis & eacute; des & eacute;quations allom & eacute;triques de houppiers sp & eacute;cifiques & agrave; chaque esp & egrave;ce pour relier cette vaste base de donn & eacute;es au mod & egrave;le de tra & ccedil;age de rayons lumineux SamsaraLight. Cela nous a permis de calculer un indice de comp & eacute;tition pour la lumi & egrave;re, bas & eacute; sur la lumi & egrave;re intercept & eacute;e par le houppier de l'arbre. Au sein de la niche climatique d'une esp & egrave;ce, l'effet de la comp & eacute;tition pour la lumi & egrave;re sur la croissance et la survie des arbres diminue vers la marge aride et la marge froide, soutenant l'hypoth & egrave;se du gradient de stress. Le climat affecte principalement la croissance des arbres en pleine lumi & egrave;re, avec une croissance plus lente dans des conditions plus arides ou plus froides. En revanche, pour la survie, le climat affecte principalement les arbres & agrave; l'ombre, avec une meilleure survie dans les marges stressantes aride ou froide. Lorsque l'on compare les esp & egrave;ces entre elles, la sensibilit & eacute; moyenne de la croissance et de la survie des arbres & agrave; la comp & eacute;tition pour la lumi & egrave;re diminue avec l'augmentation de l'aridit & eacute; de la niche climatique moyenne et de la tol & eacute;rance & agrave; l'ombre des esp & egrave;ces. Synth & egrave;se. Notre & eacute;tude souligne l'importance de prendre en compte l'interaction entre comp & eacute;tition pour la lumi & egrave;re et climat pour mieux pr & eacute;dire la croissance et la survie des arbres. L'impact du changement climatique sur un arbre d & eacute;pendra probablement de son statut de comp & eacute;tition pour la lumi & egrave;re au sein du peuplement forestier, ainsi que de la niche climatique et de la tol & eacute;rance & agrave; l'ombre de l'esp & egrave;ce.

The constraint caused by wild ungulates on forest regeneration is increasing worldwide. Hypotheses for plant association effects predict that species susceptible to herbivory can gain protection from other neighbouring plant species. In theory, such interactions could help limit the impact of browsing on the regeneration of specific tree species. However, the presence of neighbouring species can also result in increasing competition for resources between species. The resultant effects on forest regeneration of these interactions, both positive (protection against herbivores) and negative (inter-specific competition) are still unclear. To gain insight, we coupled models of browsing by roe deer (Capreolus capreolus) and of forest dynamics to simulate trajectories of oak (Quercus petraea (Matt.) Liebl.) regeneration admixed with species of contrasted palatability and growth rate under different scenarios of browsing pressure and initial sapling density. We also investigated how releasing oak saplings from all or specific neighbours during the simulation affect regeneration. We found that admixed species composition had a relatively weak effect on the density of oak recruits, but a strong effect on the duration of the regeneration phase. Oak regenerated faster when admixed with species of intermediate growth and low palatability (Fagus sylvatica) than with species of fast growth and high palatability (Carpinus betulus L.), except at intermediate sapling density and high browsing pressure where we found the opposite. Releasing oak from all competitors was most effective in promoting oak regeneration when admixed with both species together, although the benefit of competition release was much weaker at high browsing pressure. Lastly, we found that at low initial sapling density (i.e., 10 saplings/m2), oak regeneration was driven only by browsing and the effect of admixing species became negligible. Our study showed that admixing oak with palatable neighbours impedes rather than improves oak regeneration due to increased competition for resources. As such, we suggest that the benefits of herbivore diversion can be off-set by increased inter-specific competition.

The role of intraspecific variability (IV) in shaping community dynamics and species coexistence has been intensively discussed over the past decade and modelling studies have played an important role in that respect. However, these studies often implicitly assume that IV can be represented by independent random draws around speciesspecific mean parameters. This major assumption has largely remained undiscussed, although a great part of observed IV is structured in space or time, in particular when environmental dimensions that influence individual performance are imperfectly characterised or unobserved in the field. To test the impact of this strong assumption on the outcome of community dynamics models, we designed a simulation experiment where we varied the level of knowledge of the environment in virtual communities, resulting in different relative importance of explained vs unexplained spatial individual variation in performance. We used a community dynamics simulator to generate communities where the unexplained individual variation is, or is not, added as an unstructured random noise. Communities simulated with unstructured IV never reached the community diversity and composition of those where all the variation was explained and structured (perfect knowledge model). This highlights that incorporating unstructured IV ( i.e . a random noise) to account for unexplained (but structured) variation can lead to incorrect simulations of community dynamics. In addition, the effects of unstructured IV on community diversity and composition depended on the relative importance of structured vs unstructured IV, i.e . on the level of knowledge of the environment, which may partly explain the contrasting results of previous studies on the effect of IV on species coexistence. In particular, the effect of unstructured IV on community diversity was positive when the proportion of structured IV vs unstructured IV in the model was low, but negative when this proportion was high. This is because unstructured random noise can either limit the competitive exclusion of inferior competitors in low dimensions or destabilise tight niche partitioning in high dimension. Our study suggests that it is crucial to account for the sources and structure of observed IV in real communities to better understand its effect on community assembly and properly include it in community dynamics models.

The fundamental trade-off between current and future reproduction has long been considered to result in a tendency for species that can grow large to begin reproduction at a larger size. Due to the prolonged time required to reach maturity, estimates of tree maturation size remain very rare and we lack a global view on the generality and the shape of this trade-off. Using seed production from five continents, we estimate tree maturation sizes for 486 tree species spanning tropical to boreal climates. Results show that a species' maturation size increases with maximum size, but in a non-proportional way: the largest species begin reproduction at smaller sizes than would be expected if maturation were simply proportional to maximum size. Furthermore, the decrease in relative maturation size is steepest in cold climates. These findings on maturation size drivers are key to accurately represent forests' responses to disturbance and climate change.

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands. A new method to quantify three masting components from individual tree-years has revealed that globally, masting is uncommon in tree species that depend on mutualist dispersers, with its distribution further mediated by climate and nutrient availability.

Aim Our understanding of the mechanisms that maintain forest diversity under changing climate can benefit from knowledge about traits that are closely linked to fitness. We tested whether the link between traits and seed number and seed size is consistent with two hypotheses, termed the leaf economics spectrum and the plant size syndrome, or whether reproduction represents an independent dimension related to a seed size-seed number trade-off.Location Most of the data come from Europe, North and Central America and East Asia. A minority of the data come from South America, Africa and Australia.Time period 1960-2022.Major taxa studied Trees.Methods We gathered 12 million observations of the number of seeds produced in 784 tree species. We estimated the number of seeds produced by individual trees and scaled it up to the species level. Next, we used principal components analysis and generalized joint attribute modelling (GJAM) to map seed number and size on the tree traits spectrum.Results Incorporating seed size and number into trait analysis while controlling for environment and phylogeny with GJAM exposes relationships in trees that might otherwise remain hidden. Production of the large total biomass of seeds [product of seed number and seed size; hereafter, species seed productivity (SSP)] is associated with high leaf area, low foliar nitrogen, low specific leaf area (SLA) and dense wood. Production of high seed numbers is associated with small seeds produced by nutrient-demanding species with softwood, small leaves and high SLA. Trait covariation is consistent with opposing strategies: one fast-growing, early successional, with high dispersal, and the other slow-growing, stress-tolerant, that recruit in shaded conditions.Main conclusions Earth system models currently assume that reproductive allocation is indifferent among plant functional types. Easily measurable seed size is a strong predictor of the seed number and species seed productivity. The connection of SSP with the functional traits can form the first basis of improved fecundity prediction across global forests.

Intraspecific variability (IV) has been proposed to explain species coexistence in diverse communities. Assuming, sometimes implicitly, that conspecific individuals can perform differently in the same environment and that IV increases niche overlap, previous studies have found contrasting results regarding the effect of IV on species coexistence. We aim at showing that the large IV observed in data does not mean that conspecific individuals are necessarily different in their response to the environment and that the role of high-dimensional environmental variation in determining IV has largely remained unexplored in forest plant communities. We first used a simulation experiment where an individual attribute is derived from a high-dimensional model, representing "perfect knowledge " of individual response to the environment, to illustrate how large observed IV can result from "imperfect knowledge " of the environment. Second, using growth data from clonal Eucalyptus plantations in Brazil, we estimated a major contribution of the environment in determining individual growth. Third, using tree growth data from long-term tropical forest inventories in French Guiana, Panama and India, we showed that tree growth in tropical forests is structured spatially and that despite a large observed IV at the population level, conspecific individuals perform more similarly locally than compared with heterospecific individuals. As the number of environmental dimensions that are well quantified at fine scale is generally lower than the actual number of dimensions influencing individual attributes, a great part of observed IV might be represented as random variation across individuals when in fact it is environmentally driven. This mis-representation has important consequences for inference about community dynamics. We emphasize that observed IV does not necessarily impact species coexistence per se but can reveal species response to high-dimensional environment, which is consistent with niche theory and the observation of the many differences between species in nature.

The periodic production of large seed crops, or masting, is a widespread phenomenon in perennial plants. This behavior can enhance the reproductive efficiency of plants, leading to increased fitness, and produce ripple effects on food webs. While variability from year to year is a defining characteristic of masting, the methods used to quantify this variability are highly debated. The commonly used coefficient of variation lacks the ability to account for the serial dependence in mast data and can be influenced by zeros, making it a less suitable choice for various applications based on individual-level observations, such as phenotypic selection, heritability, and climate change studies, which rely on individual-plant-level datasets that often contain numerous zeros. To address these limitations, we present three case studies and introduce volatility and periodicity, which account for the variance in the frequency domain by emphasizing the significance of long intervals in masting. By utilizing examples of Sorbus aucuparia, Pinus pinea, Quercus robur, Quercus pubescens, and Fagus sylvatica, we demonstrate how volatility captures the effects of variance at both high and low frequencies, even in the presence of zeros, leading to improved ecological interpretations of the results. The growing availability of long-term, individual-plant datasets promises significant advancements in the field, but requires appropriate tools for analysis, which the new metrics provide.

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.