Contrasting patterns of trait-based community assembly in lianas and trees from temperate Australia

Trait variation in plant communities is thought to be constrained by two opposing community assembly processes operating at discrete spatial scales: habitat filtering and limiting similarity between coexisting species. Filtering processes cause convergence in ecological strategy as species are excluded from unsuitable sites, whilst limiting similarity leads to the divergence of trait values between co-occurring species in order to alleviate competition for finite resources. Levels of alpha (within-site) and beta (among-site) trait variation can be indicative of the strength of these community assembly processes. We used trait-gradient analysis to explicitly compare evidence of community assembly patterns in lianas (woody vines) and trees. These two growth forms exhibit striking differences in carbon capture and regeneration strategies, yet trait-based mechanisms that maintain their coexistence remain understudied. Using data for four functional traits - leaf mass per area, leaf nitrogen content (N_mass), leaf area and seed mass - we partitioned interspecific trait variation in lianas and trees into alpha and beta components. Our three key findings were: 1) lianas and trees exhibit divergent patterns of trait-based habitat filtering, due to differences in the relationship between leaf size and the other three traits examined (LMA, N_mass and seed mass), 2) on average, liana species possess smaller seeds, lower LMA and higher N_mass than do trees, but there was no clear difference in leaf area between the two growth forms, and 3) soil fertility was correlated with trait variation (leaf area, seed mass) at the site-level in trees, but not in lianas. These results provide evidence that dominant growth forms can be filtered into the same habitat on the basis of different combinations of traits. Our findings have important implications for community assembly and co-existence theory and for more pragmatic matters such as using trait-based principles to inform community restoration.