Decoupled phylogenetic and functional diversity in European grasslands

The relationship between phylogenetic diversity (PD) and functional diversity (FD) is important for understanding the mechanisms of community assembly. The traditional view assumes a coupled (positively correlated) relationship between these two diversity measures, suggesting that competitive exclusion and environmental filtering are important drivers of both phylogenetic and functional structure of communities. In contrast, there is evidence that communities might deviate from this pattern, exhibiting either phylogenetic overdispersion connected with trait convergence (decoupled PD) or functional overdispersion connected with phylogenetic clustering (decoupled FD). In this study, we examined the relationship between PD and FD within vascular-plant communities in European grasslands, focusing on decoupled PD-FD patterns. We hypothesized that the decoupled patterns are connected with past or current environmental changes and are rarer in comparison with the coupled PD-FD pattern, reflecting long-term relatively stable environments. We used 81,484 plots (communities) of European dry, mesic, wet and alpine grasslands, containing 4,119 angiosperm species, and data on six functional traits relevant for different plant functions and habitats (plant height, leaf area, specific leaf area, leaf nitrogen content, seed mass and lateral spreading distance). Functional diversity was evaluated in two ways" as a single combined measure and as variability in each trait separately. We found various PD-FD patterns across different habitats, traits and regions, with the coupled pattern widespread but not universal. In many communities, we detected the tendency towards decoupled PD, likely caused by environmental filtering of phylogenetically diverse species pools. This was most pronounced in dry grasslands, and also in wet and alpine grasslands when FD based on plant height, leaf area or seed mass was considered. In contrast, the tendency towards decoupled FD was detected only in mesic and wet grasslands for leaf nitrogen content and lateral spreading distance, possibly due to competitive interactions among species interplaying with land-use history. Decoupled PD is relatively common in European grasslands, especially in mountainous and hilly areas of central and southern Europe and in parts of western Europe with a mild climate. This likely results from refugial effects that have preserved many distinct phylogenetic lineages, but their species are functionally similar due to environmental filters that affect the assembly of present-day grassland communities. We demonstrate that PD and FD may reflect different aspects of community structure and assembly mechanisms, and suggest that the phenomenon of decoupled PD and FD deserves more systematic study.