FIFTEEN YEARS OF REVEGETATION OF MOUNT ST. HELENS: A LANDSCAPE-SCALE ANALYSIS

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Understanding vegetation responses to landscape-scale disturbance often is critical for understanding ecosystem structure and function. Satellite remote sensing and geographic information systems can be used to determine whether response patterns observed in fine-scale studies are present at landscape scales. We used these technologies to examine key factors correlated with revegetation of Mount St. Helens in the first 15 yr following its catastrophic eruption in 1980. To measure revegetation, we used eight Landsat satellite scenes from 1984 to 1995 to estimate for each pixel (1) the time (in years) to reach an estimated 10% vegetation cover (EC10), (2) maximum rate of increase in vegetation cover (MR), (3) time-integrated vegetation cover (TIC), and (4) maximum estimated cover reached during the study period (MEC). Explanatory variables included type of volcanic disturbance, distance from the eruption, initial tephra thickness, distance from surviving forests, and topographic variables. Regression tree analysis (RTA) was used to model the response variables with the explanatory variables. RTA explained 50% of the variation in EC10, 57% of the variation in TIC, 31% of the variation in MR, and 51% of the variability in MEC. Remaining variability was a function of other variables, stochastic factors, and image processing. The greatest amount of variability in revegetation was explained by type of volcanic disturbance, which stratified the study area into primary and secondary successional areas and revealed previously undocumented patterns of where each successional type was present. Under secondary successional conditions, distance from the eruption and original tephra thickness were important. For primary successional areas, proximity to forest edges was important only at the edges of mudflows. Slope gradient was important for both secondary and primary successional areas. Landscape-scale patterns of revegetation were consistent with field studies of the importance of biotic legacies, colonizing vegetation, and topography. However, the importance of slope gradient for revegetation in primary successional areas has not been previously reported.