Worldwide decomposition rates depend both on climate and the legacy of plant functional traits as litter quality. To quantify the degree to which functional differentiation among species affects their litter decomposition rates, we brought together leaf trait and litter mass loss data for 818 species from 66 decomposition experiments on six continents. We show that: (i) the magnitude of species-driven differences is much larger than previously thought and greater than climate-driven variation; (ii) the decomposability of a species' litter is consistently correlated with that species' ecological strategy within different ecosystems globally, representing a new connection between whole plant carbon strategy and biogeochemical cycling. This connection between plant strategies and decomposability is crucial for both understanding vegetation-soil feedbacks, and for improving forecasts of the global carbon cycle.

Environmental conditions, dispersal lags, and interactions among species are major factors structuring communities through time and across space. Ecologists have emphasized the importance of biotic interactions in determining local patterns of species association. In contrast, abiotic limits, dispersal limitation, and historical factors have commonly been invoked to explain community structure patterns at larger spatiotemporal scales, such as the appearance of late Pleistocene no‐analog communities or latitudinal gradients of species richness in both modern and fossil assemblages. Quantifying the relative influence of these processes on species co‐occurrence patterns is not straightforward. We provide a framework for assessing causes of species associations by combining a null‐model analysis of co‐occurrence with additional analyses of climatic differences and spatial pattern for pairs of pollen taxa that are significantly associated across geographic space. We tested this framework with data on associations among 106 fossil pollen taxa and paleoclimate simulations from eastern North America across the late Quaternary. The number and proportion of significantly associated taxon pairs increased over time, but only 449 of 56 194 taxon pairs were significantly different from random. Within this significant subset of pollen taxa, biotic interactions were rarely the exclusive cause of associations. Instead, climatic or spatial differences among sites were most frequently associated with significant patterns of taxon association. Most taxon pairs that exhibited co‐occurrence patterns indicative of biotic interactions at one time did not exhibit significant associations at other times. Evidence for environmental filtering and dispersal limitation was weakest for aggregated pairs between 16 and 11 kyr BP, suggesting enhanced importance of positive species interactions during this interval. The framework can thus be used to identify species associations that may reflect biotic interactions because these associations are not tied to environmental or spatial differences. Furthermore, temporally repeated analyses of spatial associations can reveal whether such associations persist through time.

Aim Biotic homogenization – the tendency for communities to converge in species composition – has occurred in many ecosystems, creating management challenges. The extent to which this convergence in species composition is related to convergence in trait composition (‘functional homogenization’), however, remains unresolved. Location North America, Wisconsin. Methods Using extensive plant community survey data from the 1950s and 2000s, and values for 11 traits measured on 169 species, we examined changes in functional beta diversity across 151 upland forest stands distributed across southern and northern Wisconsin. To estimate functional beta diversity, we used two recently developed pairwise functional dissimilarity metrics, plus an additive partitioning of functional diversity approach. Results Using pairwise functional dissimilarity metrics, we found no significant changes in functional beta diversity through time in either southern or northern upland forests. Under additive partitioning, species alpha diversity was lower than species beta diversity; whereas functional alpha diversity was much higher than functional beta diversity in both time periods and across all forest types. This suggests a high turnover of species but a low turnover of traits among communities. Main conclusions Although upland forests in Wisconsin have experienced taxonomic homogenization, they have not undergone functional homogenization, which may reflect a high functional redundancy among Wisconsin forest plants. As species decline further or disappear in response to habitat fragmentation and other global changes, functional redundancy may decline in a way that could diminish the functional diversity of Wisconsin's forests at both local and regional scales.

Question: What are the physical and chemical effects of plant litter on annual grassland community composition, aboveground net primary production (ANPP), and density? Location: California annual grassland. Methods: We manipulated litter and light levels independently and in concert. Litter removal and litter addition treatments tested both the physical and chemical impacts of litter's presence. We additionally simulated the effect of litter physical shading by using shade cloth, and added powdered litter to test for the chemical impacts of decomposing litter. Results: Increased whole litter and shading decreased grass germination and establishment, but not that of forbs or legumes. Species shifts occurred within all groups across treatments, including a transition from small-seeded to large-seeded grass and legume species with increased shading. ANPP was highest in control plots (473 ± 59 g/m 2 ), and species richness was highest in litter removal plots. While the physical effects of litter via shading were significant, the chemical effects of adding powdered litter were negligible. Conclusions: This work suggests that over one growing season, the physical impacts of litter are more important than chemical impacts in shaping community structure and ANPP in annual grasslands. Changes in light availability with altered litter inputs drive shifts in species and functional group composition. Litter feedbacks to ANPP and species composition of local patches may help maintain diversity and stabilize ANPP in this grassland.

Temperate North American forest communities have changed considerably in response to logging, fragmentation, herbivory, and other global change factors. Significant changes in the structure and composition of seemingly undisturbed Wisconsin forest communities have occurred over the past 50 years, including widespread declines in alpha and beta species diversity. To investigate how shifts in species composition have affected distributions of plant functional traits, we first compiled extensive data on understory plant species traits. We then computed community-weighted trait means and functional diversity metrics for communities in both the 1950s and 2000s. We examined how trait values and diversity varied across environmental gradients and among Wisconsin's four main ecoregions. Trait means and diversity values reflect conspicuous gradients in species composition, soils, and climatic conditions. Over the past 50 years, values of most traits have changed as communities shifted toward species with higher leaf nutrient levels and specific leaf area, particularly in the southern ecoregions. Trait richness and diversity have declined, particularly in historically species- and trait-rich unglaciated southwestern Wisconsin. Reductions in within-site trait diversity may be diminishing the ability of these forest communities to resist or resiliently respond to shifts in environmental conditions. Despite changes in trait and community composition, trait-environment relationships measured directly via fourth-corner analysis remain strong for most plant traits. Nevertheless, accelerating ecological change (including climate change) could outstrip the ability of plant species and traits to match their environment, particularly in more fragmented landscapes.