|Abstract or Summary
- I used satellite remote sensing to examine the key factors controlling the natural revegetation of Mount St. Helens since its eruption in 1980. The study required three stages: determining the amount of vegetation present; characterizing vegetation change; and analyzing the influence of factors affecting vegetation change. To determine the amount of vegetation present, I compared various vegetation indices and multiple regression using raw spectral bands. Reference data were from interpretation of aerial photographs. A model using multiple regression on raw spectral bands provided the best results and explained 75% of the variability in vegetation cover. To characterize vegetation change during the study period, eight Landsat Thematic Mapper satellite scenes from 1984 to 1995 were geometrically and radiometrically corrected to each other. They were then transformed to estimate green vegetation cover for each scene using the model developed in the first stage. I fit growth curves to each pixel using a combination of a non-hierarchical clustering algorithm and polynomial growth curves. To represent response variables, I extracted four parameters from the growth curves: number of years to reach 10% vegetation cover; maximum rate of increase in vegetation cover; time-integrated vegetation cover; and maximum estimated cover reached. Regression tree analysis explained 50%, 31%, 57%, and 51% of the variation in
these response variables, respectively. Remaining variability was a function of other variables, stochastic factors and image processing. The most important determinant of revegetation was the presence or absence of biotic legacies, as evidenced by volcanic disturbance impacts. This stratified the study area into primary and secondary succession areas. Under secondary succession conditions, gradients of biotic legacies were evident.
Factors affecting the establishment and growth of survivors were important, including original tephra thickness and erosional processes, evidenced by the importance of slope gradient. In primary succession areas, erosional processes were important in mitigating site conditions for colonizing seeds. Distance from seed sources was important primarily near forested edges. Additional topographic variables, including elevation, aspect, and
slope curvature, had limited importance. I was not able to detect significant effects of streams, roads, or pre-eruption vegetation conditions.