Multiple trajectories of land-cover change: influence of disturbance severity over four decades after the eruption of Mount St. Helens

Abstract Context To understand land-cover change, it is necessary to monitor landscapes over long temporal extents. The 1980 eruption of Mount St. Helens (MSH) provides an opportunity to analyze vegetation recovery trajectories over four decades after disturbances of different severities. Objectives We aim to enhance understanding of land-cover changes after major disturbances to inform management of the processes influencing ecosystem reestablishment. Methods We assessed land-cover change using Land Cover Mapping and Projection data (derived from Landsat) at 5-year intervals from 1985 to 2020. Non-water pixels within the MSH National Volcanic Monument were classified as Barren, Grass/Shrubland, or Forest. We calculated land-cover changes for severely disturbed (undergoing secondary succession) areas created by a lateral blast and mudflows and for extremely disturbed (undergoing primary succession) areas created by pyroclastic flows and a debris avalanche. Results Disturbance severity strongly influenced which land-cover conversion was likely to occur and the extent of land surface converted. However, severity had little influence on when change occurred or on starting land cover (except for Forest, which was initially only found in secondary successional areas). Grass/Shrubland-to-Forest conversion is accelerating across all disturbances 40 years after the eruption. Alternative successional pathways occurred in all disturbance zones. Conclusions Contrary to conclusions of earlier studies at Mount St. Helens using normalized difference vegetation index, land-cover change is ongoing 40 years after the eruption. Many aspects of land cover and land-cover change are unique to individual disturbances. Hence land managers should carefully assess the initial conditions of their systems before undertaking any management planning.