No abstract

Seasonally dry tropical forests (SDTF) are located in regions with alternating wet and dry seasons, with dry seasons that last several months or more. By the end of the 21st century, climate models predict substantial changes in rainfall regimes across these regions, but little is known about how individuals, species, and communities in SDTF will cope with the hotter, drier conditions predicted by climate models. In this review, we explore different rainfall scenarios that may result in ecological drought in SDTF through the lens of two alternative hypotheses: 1) these forests will be sensitive to drought because they are already limited by water and close to climatic thresholds, or 2) they will be resistant/resilient to intra-and inter-annual changes in rainfall because they are adapted to predictable, seasonal drought. In our review of literature that spans microbial to ecosystem processes, a majority of the available studies suggests that increasing frequency and intensity of droughts in SDTF will likely alter species distributions and ecosystem processes. Though we conclude that SDTF will be sensitive to altered rainfall regimes, many gaps in the literature remain. Future research should focus on geographically comparative studies and well-replicated drought experiments that can provide empirical evidence to improve simulation models used to forecast SDTF responses to future climate change at coarser spatial and temporal scales.

Much ecological research aims to explain how climate impacts biodiversity and ecosystem-level processes through functional traits that link environment with individual performance. However, the specific climatic drivers of functional diversity across space and time remain unclear due largely to limitations in the availability of paired trait and climate data. We compile and analyze a global forest dataset using a method based on abundance-weighted trait moments to assess how climate influences the shapes of whole-community trait distributions. Our approach combines abundance-weighted metrics with diverse climate factors to produce a comprehensive catalog of trait–climate relationships that differ dramatically—27% of significant results change in sign and 71% disagree on sign, significance, or both—from traditional species-weighted methods. We find that (i) functional diversity generally declines with increasing latitude and elevation, (ii) temperature variability and vapor pressure are the strongest drivers of geographic shifts in functional composition and ecological strategies, and (iii) functional composition may currently be shifting over time due to rapid climate warming. Our analysis demonstrates that climate strongly governs functional diversity and provides essential information needed to predict how biodiversity and ecosystem function will respond to climate change.

Questions Are patterns of intra‐ and inter‐specific functional trait variation consistent with greater abiotic filtering on community assembly at high latitudes and elevations, and greater biotic filtering at low latitudes and elevations? Locations Area de Conservación Guanacaste, Costa Rica; Santa Catalina Mountains, Arizona; Siskiyou Mountains, Oregon. Methods We measured woody plant species abundance and a key functional trait associated with competition for resources and environmental tolerance (specific leaf area, SLA) along elevational gradients in low‐latitude tropical (Costa Rica), mid‐latitude desert (Arizona) and high latitude mediterranean (southern Oregon) biomes. We explored patterns of abiotic and biotic filtering by comparing observed patterns of community‐weighted means and variances along elevational and latitudinal gradients to those expected under random assembly. In addition, we related trait variability to niches and explored how total trait space and breadth vary across broad spatial gradients by quantifying the ratio of intra‐ to inter‐specific variation. Results Both the community‐wide mean and variance of SLA decreased with increasing latitude, consistent with greater abiotic filtering at higher latitudes. Further, low‐elevation communities had higher trait variation than expected by chance, consistent with greater biotic filtering at low elevations. Finally, in the tropics and across latitude the ratio of intra‐ to inter‐specific variation was negatively correlated to species richness, which further suggests that biotic interactions influence plant assembly at low latitudes. Conclusions Intra‐ and inter‐specific patterns of SLA variation appeared broadly consistent with the idea that the relative strength of biotic and abiotic drivers on community assembly changes along elevational and latitudinal gradients; evidence for biotic drivers appeared more prominent at low latitudes and elevations and evidence for abiotic drivers appeared more prominent at high latitudes and elevations.

Summary1. Patterns of species co-existence and species diversity in plant communities remain an important research area despite over a century of intensive scrutiny. To provide mechanistic insight into the rules governing plant species co-existence and diversity, plant community ecologists are increasingly quantifying functional trait values for the species found in a wide range of communities. 2. Despite the promise of a quantitative functional trait approach to plant community ecology, we suggest that, along with examining trait variation across species, an assessment of trait variation within species should also be a key component of a trait-based approach to community ecology. Variability within and between individuals and populations is likely widespread due to plastic responses to highly localized abiotic and biotic interactions. 3. In this study, we quantify leaf trait variation within and across ten co-existing tree species in a dry tropical forest in Costa Rica to ask: (i) whether the majority of trait variation is located between species, within species, within individuals or within the leaves themselves; (ii) whether trait values collected using standardized methods correlate with those collected using unstandardized methods; and (iii) to what extent can we differentiate plant species on the basis of their traits? 4. We find that the majority of variation in traits was often explained by between species differences; however, between leaflet trait variation was very high for compound-leaved species. We also show that many species are difficult to reliably differentiate on the basis of functional traits even when sampling many individuals. 5. We suggest an ideal sample size of at least 10, and ideally 20, individuals be used when calculating mean trait values for individual species for entire communities, though even at large sample sizes, it remains unclear if community level trait values will allow comparisons on a larger geographic scale or if species traits are generally similar across scales. It will thus be critical to account for intraspecific variation by comparing species mean trait values across space in multiple microclimatic environments within local communities and along environmental gradients. Further, quantifying trait variability due to plasticity and inheritance will provide a better understanding of the underlying patterns and drivers of trait variation as well as the application of functional traits in outlining mechanisms of species co-existence.