Abiotic Controls on Long-Term Windthrow Disturbance and Temperate Rain Forest Dynamics in Southeast Alaska

Abstract

We investigated the role of abiotic factors in controlling patterns of long-term windthrow in the pristine coastal temperate rain forests of southeast Alaska. Our objectives were to test the extent to which long-term patterns of windthrow can be predicted spatially at the landscape scale by using four abiotic factors (slope, elevation, soil stability, and exposure to prevailing storm winds), evaluate landform influence on windthrow, and compare stand age and structural characteristics in areas prone to and protected from windthrow. On Kuiu Island, southeast Alaska, we used field validation photo-interpretation procedures to identify forest patches likely to be of windthrow origin. A spatially explicit logistic model was then built from the windthrow data and other GIS data layers, based on slope, elevation, soil type, and exposure to prevailing storm winds. Landform influence on patterns of windthrow was examined by evaluating correct model classification by landform type. The model was cross-validated by extrapolating the Kuiu model coefficients to nearby Zarembo Island, and comparing model predictions to an independent large-scale windthrow data set. The model correctly classified 72% of both windthrown and nonwindthrown forest. Field data collected in areas most and least prone to windthrow on Kuiu suggest that structural and age characteristics, as well as forest development stages, vary with the probability of windthrow across the landscape. We conclude that small-scale (partial-canopy) disturbance processes predominate in areas least prone to windthrow, and that large-scale stand-replacement disturbance processes predominate in areas most prone to windthrow. Our work suggests that a spatially predictable long-term wind-damage gradient exists on Kuiu Island. Before this research, gap-phase disturbances have been emphasized as the dominant disturbance process controlling forest dynamics in North American coastal temperate rain forests. We conclude that there is less naturally occurring old-growth forest regulated by gap-phase succession than previously believed, and that catastrophic windthrow is an important process driving forest development in southeast Alaska. To date, most timber harvest on Kuiu Island has been concentrated in storm-protected areas where gap-phase processes (old-growth forests) predominate; future management activities could be tailored to consider long-term natural disturbance patterns to better maintain historical ecosystem function.