Multiple assessments of succession rates on Mount St. Helens

Abstract

Understanding what regulates successional recovery improves ecosystem management, but since facets of vegetation structure respond differentially to regulatory mechanisms and do not develop concurrently, multiple measures of succession rates are needed to fully describe recovery. Using permanent plots located across a disturbance gradient on Mount St. Helens, we explore factors that regulate succession rates and methods to assess them. Rates were assessed with eight criteria. Structural recovery was determined by cover accumulation. Floristic turnover was evaluated by trajectory complexity (community type turnover), time to reach 20 % similarity to initial conditions, mean plot similarity, regression of the similarity decay, Euclidean distance (ED) in detrended correspondence analysis (DCA) space, principal components analysis of the DCA space, and the slope of this regression. Cover recovery is related to impact intensity, while floristic changes were inversely related to impact intensity, modified by local conditions. Scour sites soon achieved maximum cover, but stress delayed floristic recovery. Development on lahars was inhibited by isolation. Pumice and protected blasted ridge sites developed quickly; rates were slowed by disturbance and enhanced in protected sites and where Lupinus lepidus thrived. Comparisons of plots in different habitats confirmed that recovery rates are slowed by stresses imposed by elevation, exposure, and infertility, by isolation and by disturbance. No single criterion can characterize succession rates comprehensively, but trajectory complexity, matrix similarity, and ED in DCA space offer critical insight to understand the interactive effects of environment and disturbance history on rates of successional recovery.