Relating aspen defoliation to changes in leaf area derived from field and satellite remote sensing data

Trembling aspen is the most important deciduous species in the North American boreal forest for which methods are needed to consistently map and monitor insect defoliation patterns to assess impacts on aspen health and productivity. The loss of foliage due to insect defoliation results in changes to leaf area index (DeltaLAI) that were studied from both field and satellite image data for a large aspen tortrix (Choristoneura conflictana (Wlk.)) outbreak in boreal forest aspen stands in northern Alberta. Field estimates of defoliation for stands undergoing various levels of defoliation severity and both allometric and optical LAI-2000 measurements of leaf area index were collected and related to Landsat enhanced thematic mapper plus (ETM+) image data representing before- and after-defoliation events. This study was undertaken as part of the Climate Change Impact on the Productivity and Health of Aspen (CIPHA) research initiative that is taking place in western Canada. Field estimates of DeltaLAI followed defoliation trends but with substantial scatter that may be attributed to the use of allometric estimates of predefoliation LAI and the subjective nature of visual defoliation assessment. Change estimates derived from both Landsat ETM+ scenes were statistically correlated (correlation r = 0.84, probability P = 0.001) with percent defoliation. Such an approach may be used in the future to calibrate LAI algorithms to other defoliated areas or to map the temporal intensity of these events as they occur.