Fifteen years of revegetation of Mount St. Helens: A landscape-scale analysis

Understanding vegetation responses to landscape-scale disturbance often is critical for understanding ecosystem structure and function. Satellite remo te sensing and geographic information systems can be used to determine whet her response patterns observed in fine-scale studies are present at landsca pe scales. We used these technologies to examine key factors correlated wit h revegetation of Mount St. Helens in the first 15 yr following its catastr ophic eruption in 1980. To measure revegetation, we used eight Landsat sate llite scenes from 1984 to 1995 to estimate for each pixel (1) the time tin 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 fr om the eruption, initial tephra thickness, distance from surviving forests, and topographic variables. Regression tree analysis (RTA) was used to mode l the response variables with the explanatory variables. RTA explained 50% of the variation in EC10, 57% of the variation in TIG, 31 % of the variation in MR, and 51% of the variability in MEG. Remaining vari ability was a function of other variables, stochastic factors, and image pr ocessing. The greatest amount of variability in revegetation was explained by type of volcanic disturbance, which stratified the study area into prima ry and secondary successional areas and revealed previously undocumented pa tterns of where each successional type was present. Under secondary successional conditions, distance from the eruption and ori ginal tephra thickness were important. For primary successional areas, prox imity to forest edges was important only at the edges of mudflows. Slope gr adient was important for both secondary and primary successional areas. Lan dscape-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 succe ssional areas has not been previously reported.