Landscape scale effects of disturbance on riparian vegetation

SUMMARY 1. Differing responses in riparian species richness and composition to disturbance have been reported as a possible explanation for the differences along and between rivers. This paper explores the role of physical disturbance in shaping landscape-scale patterns of species distribution in riparian vegetation along a free-flowing river in northern Sweden. 2. To test whether sensitivity to disturbance varies across large landscapes, we experimentally disturbed riparian vegetation along an entire, free-flowing river by scouring the soil and the vegetation turf, cutting vegetation, applying waterborne plant litter, and after a period of recovery we measured vegetation responses. The experiment was repeated for two consecutive years. 3. We found no significant effect of disturbance on species composition, but all three forms of disturbance significantly reduced species richness. There was no downstream variation in community responses to disturbance but morphological groups of species responded differently to different kinds of disturbance. Graminoids were most resistant, suppressed only by litter burial. All forms of disturbance except cutting reduced the density of herbaceous species, and species density of trees + shrubs and dwarf shrubs was negatively affected by both scouring and cutting. We also evaluated the effects of disturbance in relation to varying levels of species richness. In nearly all cases, responses were significantly negatively correlated with control plot species richness, and relative responses indicated that species-rich plots were less resistant to scouring and cutting. 4. Our results suggest that although all disturbance treatments had an effect on species richness, variation in sensitivity to disturbance is not the most important factor shaping landscape-scale patterns of riparian plant species richness along rivers.