| The Mount St. Helens eruption of May 18th, 1980, involved perhaps the most widely investigated geologic event in the United States during the 20th century. The combination of remote sensing and GIS provide an opportunity to study vegetation changes throughout the entire eruption blast zone. Landsat TM data for 1984, 1989, 1992, 1993, 1994, 1996, and 1998 were used in this analysis. This is the first study to compare the results of digital image analysis of Landsat TM data for both managed and natural re-vegetated areas within the 1980 blast zone. Image analysis using NDVI data was used to assess temporal and spatial differences in vegetation changes. Environmental modeling, using multiple linear regression, enabled NDVI values for single dates and NDVI changes to be assessed against related human and environmental variables.; Analysis using the seven Landsat TM images indicates that reforested land planted soon after the 1980 eruption had the greatest increases in NDVI and vegetation cover immediately following the volcanic disturbance. Other interrelated factors that image analysis revealed as being important to the reestablishment of vegetation include proximity to streams, elevation, slope, and aspect. In the National Volcanic Monument, where vegetation reestablishment is occurring by natural ecological processes, image analysis indicates that most substantial NDVI gains in the early periods happened at higher elevations in areas where surviving vegetation was likely protected from the blast by topography, snowpack, or a combination of both. In the more recent time periods, lower elevations adjacent to streams exhibit the greatest NDVI gain.; Regression models suggest that, in areas of natural regrowth, lower ash depths and steeper slopes were the most influential factors to the reestablishment and growth of vegetation. In areas where human influences were important to vegetation reestablishment and growth, factors favoring replanting were the most influential. Slope and ash depth were important for explaining NDVI variation in Gifford Pinchot National Forest through 1994, whereas on Weyerhaeuser property lower elevations, species type, and distance to roads explained most NDVI variation. |
