Fine-scale coexistence patterns along a productivity gradient in wet meadows: shifts from trait convergence to divergence

Semi-natural meadows host a great number of species coexisting at fine spatial scales. Different assembly mechanisms, related to differences in functional traits between species, can influence such coexistence. Coexisting species could be either functionally dissimilar to occupy different niches (divergence') or functionally similar due to exclusion of species with traits less adapted to the prevailing abiotic and biotic conditions (convergence'). Various theories differently predict how trait convergence and divergence should differ due to disturbance, along productivity gradients, and across different functional traits. We tested such theories in 21 wet meadows of different productivity in central Europe. In each meadow, four 1 x 1 m plots were established in which disturbance by mowing was combined with fertilization. Species presence was recorded in 100 quadrats 10 x 10 cm in size within the plots over five years. Convergence and divergence were assessed at very fine spatial scales (10 x 10 cm) to focus on the processes driven by the interactions for similar resources. Convergence emerged as the dominant pattern for all traits and across all years. It was particularly strong in the least productive conditions while divergence emerged in some of the most productive meadows. Mowing increased convergence in meadows with low productivity, but increased divergence in productive meadows. Fertilization generally increased convergence, with this increase being more pronounced in mown plots. Convergence in unproductive conditions could be caused by either higher fitness of stress-tolerant species (more abundant in the species pool of these sites) or by functionally similar species sharing similar patches within fine-scale heterogeneous plots. This outcome also suggests abiotic filters can have an important role at fine scales, where plant-ecological theory usually predicts the prevalence of biotic processes.