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Summary The digitisation of plant collections is bringing large quantities of information into accessible electronic databases. However, in recent decades, traditional taxonomic work in collections has declined, meaning that more specimens are only determined to family or genus, particularly when lacking key identification structures. If unaddressed, large‐scale digitisation risks widening the gap between well‐studied species and those lacking data.Hyperspectral reflectance and computer vision are two emerging approaches for identifying species, but these have yet to be cross‐compared for herbarium‐based taxonomy. UsingLithocarpusspecies as a case study, we compared classification accuracy obtained from leaf reflectance spectra with computer vision (implemented via Pl@ntNet), a RGB (red, green, blue) image‐based approach known to work well on specimens presenting reproductive structures. In the spectral approach, we assessed how much data are needed to optimise classification accuracy, how many species could be discriminated between, and whether close relatives were more frequently confounded.We found thatLithocarpusherbarium specimens were accurately identified to species from relatively small spectral datasets. Despite not incorporating reproductive structures, this was only 14% less accurate than Pl@ntNet.We suggest these rapid, nondestructive leaf reflectance measurements, paired with computer vision, could fill identification gaps in collections, particularly for specimens lacking reproductive features. 

Summary Leaf deciduousness is a key drought‐avoidance strategy in tropical flora, reducing water loss during seasonal dry periods. While winter‐deciduousness in temperate regions is well‐understood, the evolutionary and environmental drivers of dry‐season deciduousness remain poorly explored.Using the genusMimosa,a species‐rich and morphologically diverse lineage, we applied an eco‐evolutionary framework to investigate the role of dry‐season deciduousness across time and space. We combined a time‐sliced trait‐dependent diversification model, analyses of joint evolution of environmental niches in relation to leaf habit (deciduous vs evergreen), and phylogenetic multilevel models to test whether deciduousness influenced diversification and to identify its environmental drivers.Lineages switch from evergreen to deciduous habit more frequently beforec.7 Ma, whereas deciduous to evergreen transitions and increased speciation rates of evergreen lineages became dominant after this time. Deciduous lineages exhibited faster evolutionary rates along gradients of vapor pressure deficit, which also emerged as the strongest environmental predictor of deciduousness. However, most variation in leaf habit was explained by species‐level (nonphylogenetic) variation and phylogeny.Although deciduous lineages respond rapidly to atmospheric dryness, dry‐season deciduousness appears to be primarily shaped by the interplay between species identity and shared ancestry rather than by environmental conditions alone. 

Abstract Community‐wide efforts to digitize herbarium specimens have facilitated novel uses of specimen data across scales. However, the need for destructive sampling has prevented large‐scale examination of foliar functional traits. We demonstrate that nondestructive reflectance spectra are an effective tool for estimating leaf traits from herbarium collections.Using partial least squares regression (PLSR), we developed and evaluated trait estimation models for leaf mass per area (LMA), nitrogen content (N) and carbon content (C) from nondestructive leaf‐level spectra and destructive trait measurements from herbarium specimens. Our modelling dataset consisted of temperate species collected in the Minneapolis‐St. Paul area (Minnesota, USA) between 1876 and 2022. We also trained biased models excluding sample subsets corresponding to specimen age, habit, growth form and species categories to assess the transferability of PLSR trait estimation models. Finally, we evaluated whether leaf traits estimated from spectra recover the temporal trends observed in destructively measured traits.Our PLSR models accurately estimate LMA and N, and reasonably estimate C. They are robust to sample greenness, specimen age and species, but sensitive to mismatches in functional group categories between the training and target data. Furthermore, spectrally estimated LMA and N reproduce temporal trends observed in measured traits over the 146‐year time series; estimated C reproduces most temporal trends despite poorer model performance than the other traits.Given the limitations of broad destructive sampling from herbarium collections, we show that PLSR models trained on younger specimens can reliably estimate leaf traits from older specimens. While we caution that trait estimation models are only as good as their training data, we are optimistic that spectroscopy can capture the extensive functional data preserved in herbarium collections in a largely nondestructive manner. 

ABSTRACT Over a century of research has revealed an amazing complexity of behaviours and physiological adaptations that allow tiny bark beetles to overcome large trees, sometimes resulting in outbreaks that kill millions of trees. Turning a tree into a home and successfully raising offspring involves constant interactions among the beetles, the tree, its microbiome, and the beetles' associated microbes, all influenced by abiotic factors that can determine success or failure. While we have learned much about these systems, substantial knowledge gaps remain. This synthesis aims to clarify and integrate current understanding, identify gaps, challenge long‐held assumptions, and address interpretative issues that impede progress toward a holistic understanding of these systems. We advocate for expanding perspectives using synecological approaches to understand these complex systems better. We encourage expanding research into how colonization by the bark beetle–fungi complex influences subsequent tree decay and forest carbon dynamics. An explicit goal is to provide a comprehensive resource for new researchers while encouraging them to question established hypotheses and to explore new avenues of enquiry. 

Summary With current climate trajectories, tree populations will encounter novel selection pressures that risk local extinction if they are unable to acclimate or adapt. Within a reciprocal transplant experiment withQuercus macrocarpaL. established across a latitudinal gradient, we asked: (1) Is there genetic variation within populations? (2) Are there differences in the direction and strength of selection? (3) Do traits within populations differ in adaptation potential in response to future climate conditions?Within each population in each of three gardens (Minnesota, Illinois, and Oklahoma), we estimated genetic variance for nine traits grouped in three realms: physiology, spectral reflectance features, and morphology/growth. We also analyzed selection on these traits and assessed their potential adaptive response to selection.Our results indicate that traits related to morphology and growth have high genetic variance and are under strong directional selection in warmer gardens. The populations that represent extreme ends of the climatic gradient have high potential to adapt to climate change, based on their responses to selection in the warmest garden (Oklahoma).These results inform strategies to improve species resilience by providing seed source information relevant to managers planning assisted migration to promote climate change adaptation. 

Abstract Global environmental change is causing a decline in biodiversity with profound implications for ecosystem functioning and stability. It remains unclear how global change factors interact to influence the effects of biodiversity on ecosystem functioning and stability. Here, using data from a 24-year experiment, we investigate the impacts of nitrogen (N) addition, enriched CO₂(eCO₂), and their interactions on the biodiversity-ecosystem functioning relationship (complementarity effects and selection effects), the biodiversity-ecosystem stability relationship (species asynchrony and species stability), and their connections. We show that biodiversity remains positively related to both ecosystem productivity (functioning) and its stability under N addition and eCO₂. However, the combination of N addition and eCO₂diminishes the effects of biodiversity on complementarity and selection effects. In contrast, N addition and eCO₂do not alter the relationship between biodiversity and either species asynchrony or species stability. Under ambient conditions, both complementarity and selection effects are negatively related to species asynchrony, but neither are related to species stability; these links persist under N addition and eCO₂. Our study offers insights into the underlying processes that sustain functioning and stability of biodiverse ecosystems in the face of global change. 

Abstract Constrained carbon allocation toward secondary metabolites involved in chemical defense is a common explanation for widespread drought-related beetle-kill in conifers—we challenge the generality of this explanation. While monitoring drought stress (ψpd), we tracked both carbon reserves (non-structural carbohydrates) and chemical defenses (terpenes, phenolics, resin flow) in mature Pinus edulis Englem. trees experiencing either short-term (3-year) or a ‘legacy’ long-term (13-year) throughfall exclusion treatments, plus a control. We also quantified the Δ14C-age of resin to measure past allocation to current defense. While 72% of trees in short-term throughfall exclusion plots died (attacked by bark beetles, Ips confusus LeConte), mortality patterns were unrelated to throughfall exclusion intensity and all ‘legacy’ trees survived. We thus assessed trees in four survivorship categories: control, ‘legacy’, surviving, and dying trees. We found concentrations of certain defense compounds (leaf phenolics, twig monoterpenes) increased with drought stress, particularly in dying trees. In the main stem, dying trees exhibited similar terpene concentrations (94%) and phenolic concentrations (139%) relative to control trees. Compared with control trees, only ‘legacy’ trees had reduced stem terpenes (−49%, P < 0.05) after a decade of drought. Δ14C-age of resin could be up to 10.2 ± 0.5 years old, where the oldest resin was exuded from trees with low sugar concentrations and more negative Ψpd. Our results suggest that drought imposes a weak constraint on carbon allocation to resin-based defense. Instead, we primarily found evidence of increased concentrations of terpene and phenolic compounds under drought, even in dying trees, and only observed reductions in resin-based defenses after 10+ years of drought. Δ14C-ages demonstrate limited resin turnover and/or synthesis of resin from old reserves, suggesting that long-term drought is required to reduce resin-based defenses. Persistent allocation coupled with past investments appears to preserve or enhance concentrations of resin-based defenses even under lethal drought stress in P. edulis. 

Abstract Background and AimsTropical forests exchange more carbon dioxide (CO2) with the atmosphere than any other terrestrial biome. Yet, uncertainty in the projected carbon balance over the next century is roughly three times greater for the tropics than other for ecosystems. Our limited knowledge of tropical plant physiological responses, including photosynthetic, to climate change is a substantial source of uncertainty in our ability to forecast the global terrestrial carbon sink. MethodsWe used a meta-analytic approach, focusing on tropical photosynthetic temperature responses, to address this knowledge gap. Our dataset, gleaned from 18 independent studies, included leaf-level light-saturated photosynthetic (Asat) temperature responses from 108 woody species, with additional temperature parameters (35 species) and rates (250 species) of both maximum rates of electron transport (Jmax) and Rubisco carboxylation (Vcmax). We investigated how these parameters responded to mean annual temperature (MAT), temperature variability, aridity and elevation, as well as also how responses differed among successional strategy, leaf habit and light environment. Key ResultsOptimum temperatures for Asat (ToptA) and Jmax (ToptJ) increased with MAT but not for Vcmax (ToptV). Although photosynthetic rates were higher for ‘light’ than ‘shaded’ leaves, light conditions did not generate differences in temperature response parameters. ToptA did not differ with successional strategy, but early successional species had ~4 °C wider thermal niches than mid/late species. Semi-deciduous species had ~1 °C higher ToptA than broadleaf evergreen species. Most global modelling efforts consider all tropical forests as a single ‘broadleaf evergreen’ functional type, but our data show that tropical species with different leaf habits display distinct temperature responses that should be included in modelling efforts. ConclusionsThis novel research will inform modelling efforts to quantify tropical ecosystem carbon cycling and provide more accurate representations of how these key ecosystems will respond to altered temperature patterns in the face of climate warming. 

Abstract AimUnderstanding the mechanisms promoting resilience in plant communities is crucial in times of increasing disturbance and global environmental change. Here, we present the first meta‐analysis evaluating the relationship between functional diversity and resilience of plant communities. Specifically, we tested whether the resilience of plant communities is positively correlated with interspecific trait variation (following the niche complementarity hypothesis) and the dominance of acquisitive and small‐size species (following the mass ratio hypothesis), and for the context‐dependent effects of ecological and methodological differences across studies. LocationGlobal. Time Period2004–2021. Major Taxa StudiedVascular plants. MethodsWe compiled a dataset of 69 independent sites from 26 studies that have quantified resilience. For each site, we calculated functional diversity indices based on the floristic composition and functional traits of the plant community (obtained from the TRY database) which we correlated with resilience of biomass and floristic composition. After transforming correlation coefficients to Fisher'sZ‐scores, we conducted a hierarchical meta‐analysis, using a multilevel random‐effects model that accounted for the non‐independence of multiple effect sizes and the effects of ecological and methodological moderators. ResultsIn general, we found no positive functional diversity–resilience relationships of grand mean effect sizes. In contrast to our expectations, we encountered a negative relationship between resilience and trait variety, especially in woody ecosystems, whereas there was a positive relationship between resilience and the dominance of acquisitive species in herbaceous ecosystems. Finally, the functional diversity–resilience relationships were strongly affected by both ecological (biome and disturbance properties) and methodological (temporal scale, study design and resilience metric) characteristics. Main ConclusionsWe rejected our hypothesis of a general positive functional diversity–resilience relationship. In addition to strong context dependency, we propose that idiosyncratic effects of single resident species present in the communities before the disturbances and biological legacies could play major roles in the resilience of terrestrial plant communities. 

ABSTRACT Biodiversity promotes ecosystem productivity and stability, positive impacts that often strengthen over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO₂) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over time. Using a quarter‐century grassland biodiversity‐global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over 25 years, CO₂and N enrichment gradually eroded some of the positive effects of biodiversity on ecosystem stability. As elevated CO₂, N eutrophication, and biodiversity loss increasingly co‐occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long‐term grassland stability.