Forest development following mudflow deposition, Mount St. Helens, Washington

Abstract

Volcanic mudflows are locally important disturbance agents in the Pacific Northwest rarely studied within the context of forest succession. We describe 18 years (1981–1999) of forest development on the Muddy River mudflow deposit following the 1980 eruption of Mount St. Helens using permanent plot data collected along two transects traversing the Cedar Flats river terrace. We analyze changing forest structure over the study period and compare results with mudflow deposition using correlation and pairwise comparisons, as well as ordination (detrended correspondence analysis) and cluster analysis. Our results show a statistically significant relationship between mudflow deposition and forest change. Following mudflow deposition, the site consisted of patches of high tree mortality caused by deep mudflow deposits in abandoned river channels as well as patches of accelerated regeneration of surviving understory trees in areas of more shallow mudflow deposition and partial overstory mortality. Mudflow deposition at the site initiated multiple stages of stand development with (1) early-colonizing red alder (Alnus rubra Bong.) dominating deep deposition sites with fewer surviving trees, (2) gap recruitment and establishment by western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) on intermediate to deep deposition sites with more postburial surviving trees, and (3) late-seral conditions and accelerated succession on shallow burial sites, where tree mortality was low. The initiation of differing succession trajectories, as well as variability in the extent and timing of tree mortality following mudflow deposition, demonstrates a dynamic response to disturbance in relation to small-scale gradients of mudflow deposition.