LAUSR

Abstract Wind is an important driver of forest dynamics in eastern Canada, but knowledge of variables that predispose forest stands to windthrow remains unclear. This is of particular concern as climate change is expected to alter the frequency of strong wind events that affect eastern Canada. In this study, we used widescale forest survey data from Nova Scotia, Canada, of wind damage caused by Hurricane Juan, to investigate variables that influence stand vulnerability to windthrow. Juan made landfall as a category SS2 hurricane with sustained winds of 158 km/h and damaged over 600,000 ha of forest. The damage zone was surveyed using aerial photography and satellite imagery, delineated according to level of wind damage, and digitized as a 15 × 15 m resolution spatial raster layer. We selected a random sample of 50,000 cells classified as intact forest and 50,000 cells classified as stand-replacing windthrow from the raster layer and used boosted regression tree analysis to explore the influence of various meteorological, topographic, soil, and forest structural variables on the occurrence of windthrow. Wind speed and forest structure, specifically stand height and species composition, were most influential in determining windthrow. Sustained winds of at least 95 km/h or gusts of 130 km/h caused >50% probability of windthrow. Taller stands were most vulnerable, especially those dominated by spruce (Picea spp.) and balsam fir (Abies balsamea), whereas higher hardwood and pine abundance reduced windthrow. Interestingly, topographical exposure (Topex) ranked low in overall influence; however, a clear relationship between increased exposure and windthrow was observed.