Contribution of molehill disturbances to grassland community composition along a productivity gradient

Site productivity and disturbances are among the main factors determining plant community composition. With increasing site productivity, plant diversity is often reduced and the importance of traits associated with light competition increases. Small-scale disturbances created by mole activity are assumed to affect this dynamics by two factors: increasing site heterogeneity and providing survival opportunity for weaker light competitors. These effects are expected to become stronger with increasing site productivity. To test this hypothesis we compared species composition on molehills and in undisturbed plots within grasslands and along a productivity gradient. The differences in species composition were evaluated both by species diversity and by traits representing trade-offs between species' response to competition and disturbance. Species diversity generally decreased with site productivity, while disturbances had no significant contribution to the variation among samples. When analyzing shifts in species traits, we found a central role of site productivity, but also a significant, though secondary, effect of molehill disturbances. In particular, communities showed increased potential height and specific leaf area and decreased seed weight and Ellenberg's light indicator values with increasing site productivity, supporting the assumption of increasing light limitation along the gradient. Species observed on molehills were generally less shade tolerant, especially in the more productive grasslands. We conclude that, although overall plant community composition is mainly shaped by large environmental gradients such as site productivity, small-scale disturbances may contribute to local environmental heterogeneity and trait variation by enabling species less adapted to light competition to survive.