Local trampling disturbance effects on alpine plant populations and communities: Negative implications for climate change vulnerability

Abstract

Global change is modifying species communities from local to landscape scales, withalterations in the abiotic and biotic determinants of geographic range limits causingspecies range shifts along both latitudinal and elevational gradients. An importantbut often overlooked component of global change is the effect of anthropogenicdisturbance, and how it interacts with the effects of climate to affect both speciesand communities, as well as interspecies interactions, such as facilitation and competition. We examined the effects of frequent human trampling disturbances on alpineplant communities in Switzerland, focusing on the elevational range of the widelydistributed cushion plant Silene acaulis and the interactions of this facilitator specieswith other plants. Examining size distributions and densities, we found that disturbance appears to favor individual Silene growth at middle elevations. However, it hasnegative effects at the population level, as evidenced by a reduction in populationdensity and reproductive indices. Disturbance synergistically interacts with the effects of elevation to reduce species richness at low and high elevations, an effect notmitigated by Silene. In fact, we find predominantly competitive interactions, both bySilene on its hosted and neighboring species and by neighboring (but not hosted) species on Silene. Our results indicate that disturbance can be beneficial for Silene individual performance, potentially through changes in its neighboring speciescommunity. However, possible reduced recruitment in disturbed areas could eventually lead to population declines. While other studies have shown that light to moderate disturbances can maintain high species diversity, our results emphasize thatheavier disturbance reduces species richness, diversity, as well as percent cover, andadversely affects cushion plants and that these effects are not substantially reducedby plant-plant interactions. Heavily disturbed alpine systems could therefore be atgreater risk for upward encroachment of lower elevation species in a warming world.