Functional traits during succession: is plant community assembly trait-driven?

Little is known about changes in the significance of environmental filtering and neutral processes during the succession of plant communities. Generally, a succession is expected to be trait-driven and a shift is predicted from high importance of seed dispersal ability in initial phases to importance of strong competitive ability in later stages. In this study, we consider a community assembly trait-driven in the case of a shift in functional traits during succession and randomly assembled in the case there is no shift in functional traits during succession. We therefore tested whether urban plant communities show trait convergence or trait divergence in relation of successional stage, water and nutrient availability. At two scales (100 x 100 cm and 10 x 10 cm), we analysed traits relevant to seed dispersal, stress tolerance and competitive ability and compared real data with null models. We subsequently used regression trees to associate convergence and divergence to plot age and stress due to low water and soil nutrient availability. Most traits were neither significantly convergent nor divergent and the variance explained by the regression trees was often lower than 20%. We found divergence in seed number instead of the expected convergence towards high seed number. In accord with expectations we found low seed terminal velocity and annual life span in early succession. Convergence in seed bank longevity occurred at intermediate plot age, and convergence in the combination of life span and lateral spread showed the relevance of competition filtering in the most fertile plots. On the other hand, competition-induced convergence in canopy height or specific leaf area was not supported by our results. We concluded that, based on the traits considered in this study, the overwhelming evidence was that community assembly during the first 40 years of succession was driven by trait-neutral mechanisms.