Experimental evidence for disturbance as key to the conservation of dune grassland

Coastal dunes are dynamic systems maintained and renewed by disturbance. Concerns have been raised over shrub and tree encroachment, changes in species composition and decreasing species richness in coastal dunes with nitrogen (N) deposition and loss of natural dynamics acknowledged as pressures. We tested the effects of N-deposition and disturbance on Danish dune grassland vegetation. We applied simulated trampling, grazing, blowouts and N-deposition in a randomized design to ten sites along the major natural gradient in the area. After three years we recorded plant, bryophyte and lichen biomass as well as species frequency, foliar N and P, soil pH and soil organic matter content. We hypothesized that species richness would increase with disturbance and decrease with N-addition while biomass was expected to increase with N-addition. Disturbance was expected to counteract the effects of N-addition. The hypotheses were tested using linear mixed effects models on species richness, biomass and phylogenetic community structure with treatments and interactions among treatments as explanatory variables and site as a random effect. Although N-deposition affected plant nutrient balance, the effect of N-addition on vegetation was consistently smaller than the effect of disturbance, especially cutting. Disturbances all had the opposite effect to N-addition causing an increase in species richness and decrease of biomass. The subordinate effect of N-addition likely reflects that growth is limited by moisture rather than nitrogen. Disturbances apparently relaxed the ecological filtering during community assembly, resulting in a more diverse community of less related species. Anthropogenic suppression of disturbances by wind, coastal erosion or grazing animals may potentially be a larger threat to dune biodiversity than increased N-deposition.