Understanding processes responsible for vegetation dynamics is a fundamental goal of ecology. Investigating the biophysical factors supporting resiliency or predisposing factors favoring stochastic vs. deterministic configurations provides a framework for anticipating when disturbances may trigger abrupt shifts in forest ecosystems, when forests are likely to be resilient, and what biophysical factors are scale dependent. The Mount St. Helens (Washington, USA) volcano eruption in 1980 created a unique ecological opportunity to examine how scale mediates post-disturbance regrowth, as directed by biophysical factors. Specifically, we identified underlying physical, ecological, and climatic factors responsible for the vegetative spatial patterns seen on Mount St. Helens since the 1980 eruption. Using a top down-bottom up scale approach, we modeled vegetation encroachment (Landsat-derived NDVI data) across the entirety of the Mount St. Helens National Monument using topographic predictors, and then mapped and modeled successful post-eruption tree recruitment at a fine-scale on the Muddy River Lahar (on the SE flank of the volcano) using GIS, GLMMs and dendroecological techniques. Indeed, the contingency on which vegetative establishment relies on these biophysical factors may vary with scale, and the discrepancies between scales can have large impacts on how humans view and manage forest growth.
Results/Conclusions
At the coarse scale, vegetation encroachment occurred most rapidly on low to moderate elevations and on gentle to moderate NE facing slopes over the last 35 years. We suggest that these areas proved adept for colonization as there was a balance of legacy individuals to propagate seeds into the disturbed areas and competition amongst those individuals for the remaining resources. However, at a finer-scale, micro-topography has played a minor role and instead ecological processes, like legacies and life history traits, have become driving forces of current compositional patterns and overall successional change. In contrast, interannual-scale climate variability showed a similarly strong effect on vegetation regrowth at both spatial scales, particularly during teleconnected to PDO and ENSO events. Furthermore, at the fine scale, micro-topography interacted with micro-climatic conditions that resulted in pulses of tree recruitment in safe sites, during time periods when temperature and precipitation extremes were not conducive, allowing for tree recruitment in otherwise stressful environments.