30 Years of Forest Change in the Eastern United States Highlands

Abstract:

Forest disturbances drive successional changes, impact management, redistribute carbon among ecosystem pools and the atmosphere, and alter nutrient cycling. Detecting these important ecological dynamics using remotely sensed methods allows for a consistent and comprehensive census of vegetation change over the study area, unlike in traditional plot-based methods. However, in the species-rich and structurally complex forests of the eastern United States, disturbance events caused by low-intensity fires or species-specific insects and disease are often partial, and therefore difficult to detect using satellite-based methods that rely only on total vegetation amount. We developed a set of new algorithms, collectively called VeRDET (Vegetation Regeneration and Disturbance Estimates through Time), which use a novel patch-based approach to incorporate spatial information from Landsat TM to detect disturbance, stable, and regeneration periods in a time-series of imagery. VeRDET uses SPARCS to identify clear-sky probabilities for each pixel, generates a yearly clear-sky composite of those images, calculates a vegetation index using that composite, spatially segments the vegetation index into patches using total variation regularized denoising, and then temporally segments the time-series of each pixel into a piecewise linear function. For each pixel, the slopes of the segments in the piecewise linear function are interpreted as disturbed, stable, or regenerating. We present a map of cumulative forest disturbance and regeneration over the Landsat 5 record in the eastern United States. We detect higher total rates of forest disturbance than previous studies, likely because we include stress and non-mortality declines in vegetation cover. Additionally, ecoregions predict major differences in forest change, with the Piedmont and Southeastern Plains having upwards of 7% of total forest lands affected by disturbance each year on the high end and the Blue Ridge Mountains having a low of 1.9% annual area affected. Across all ecoregions, the predominant trend is for dynamic equilibrium or afforestation. Overall, VeRDET is a promising new tool to answer spatially and temporally explicit questions about vegetation change, and can open new avenues of inquiry into forest monitoring and assessment.