Contrasting effects of productivity and disturbance on plant functional diversity at local and metacommunity scales

Questions Is trait convergence more intense when soil resource availability and disturbance constrain productivity and limit above-ground competition? Do the effects of productivity and disturbance on functional diversity differ between the local and metacommunity scales? Location Semi-arid grasslands in New Zealand (43 degrees 59S, 170 degrees 27E). Methods We measured trait convergence and divergence in grasslands along gradients of primary productivity and disturbance at local (i.e. 1mx1m) and metacommunity (8mx50m) scales, using long-term (27-yr) manipulations of soil resource availability and grazing intensity. We compared trait dispersion metrics to those expected under different null models. Results At the metacommunity scale, we found stronger trait convergence with increasing productivity and grazing intensity, where all short, slow-growing species were excluded from the potential species pool. However, once this broad-scale filter on species pool was taken into account, we found that at the local scale, abundance-weighted functional dispersion of co-occurring species was stronger than expected under our null model, thereby suggesting limiting similarity. Moreover, trait divergence became stronger at higher productivity and lower grazing intensity, where size-asymmetric competition for light is likely to have been more intense. Conclusions At the metacommunity scale, environmental filtering led to species with particular traits being excluded from the species pool. In contrast, at the local community scale where individuals interact, there was evidence of limiting similarity. Our results suggest that environmental filtering and limiting similarity are not mutually exclusive and jointly determine community structure, but can operate at different spatial scales.