Vegetational heterogeneity along elevational gradients

How vegetation recovers from disturbance is a central ecological question. We tested the hypothesis that plant spatial heterogeneity declines with increasing functional vegetation age. As pioneers expand and more species invade, we expect heterogeneity to decline. Biomass accumulation and seedling recruitment both drive succession and proceed more slowly at higher elevation. Therefore, we predicted that higher elevation sites would be more heterogeneous than lower sites. We sampled vegetation in five habitats on Mount St. Helens to study recovery 17 years after its most recent eruption. Two lahars were undergoing primary succession and one ridge was recovering from nearly total destruction. A second ridge had been greatly impacted by the blast, while a fifth site was little damaged. Plant cover was analyzed using percentage similarity (PS) and detrended correspondence analysis (DCA). PS decreased with elevation on lahars and on the devastated ridge, but no clear trend was found on less impacted sites. The variation in DCA scores of samples at a site suggested increasing heterogeneity with elevation on one lahar and the strongly disturbed secondary site. The other transects did not show this pattern. These patterns support the hypothesis that heterogeneity declines with increasing successional age. Implicit in the hypothesis is that stochastic factors are important to the establishment of vegetation. High variability among samples of stable vegetation at lower elevation sites suggests that the effects of stochastic establishment can persist.