Using Landsat Time Series to Evaluate Hemlock Woolly Adelgid Infestation over Southern Appalachian Mountains

Abstract: Hemlock Woolly Adelgid (Adelges tsugae Annand, HWA) is a non-native insect that causes widespread foliar damage and tree mortality in eastern (Tsuga Canadensis L.Carr.) and Carolina (Tsuaga caroliniana Engelm.) hemlocks throughout eastern North America. To analyze the implications of hemlock decline in biogeochemical cycles in those infested forests, it is critical to map the hemlock morbidity distribution and its regeneration patterns over time. Recently, change detection techniques using remote-sensing data have been widely used to improve the hemlock infestation monitoring; however, mapping spatial and temporal trajectories of HWA-related forest cover change, from pre-infestation to regeneration stage, remains poorly investigated. In this study, our objective is to use multi-temporal Landsat satellite images to investigate the performance of Tasseled Cap Transformation (TCT) indices time series in capturing spatial and temporal trajectory of HWA-induced hemlock mortality and following sub-canopy regeneration. This study was performed over the Linville River Gorge Wilderness area in Southern Appalachian Mountains. The time series of three TCT indices - brightness, greenness, and wetness - were produced by using all available Landsat images from 1990 to 2017 (1032 images). In addition, the time series of TCT indices were estimated by a time series analyses which can separate inter-annual (low frequency) disturbance patterns from seasonal phenology (high frequency) signals. Analyzing the anomalies of observed and predicted (without HWA infestation) TCT indices time series suggested that wetness index is more accurate in determining the initial timing of both HWA infestation and regeneration process, compared to brightness and greenness. We found out that wetness index can detect the HWA infestation-induced hemlock decline distribution as well as the long-term regeneration patterns of undercanopy deciduous trees in previously hemlock dominated forests.

Keywords: HWA, hemlock infestation, Landsat, time series, TCT.