Detecting Hemlock Woolly Adelgid (Adeleges tsugae) Impacts on Eastern Hemlock (Tsuga canadensis) Stands in the Quabbin Reservoir Watershed Using AIMS-1 Imagery
The infestation of indigenous forest species by foreign pests plays an important role in ecosystem change, often altering stable stand composition and vigor. A recent and pressing infestation in the United States by a foreign insect is the hemlock woolly adelgid (Adeleges tsugae: HWA), attacking eastern (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana). The eastern hemlock is a shade tolerant, late succession conifer and key component of Northeastern mixed forest stands. The adelgid kills eastern hemlocks by feeding on the xylem ray parenchyma cells and disabling the tree from producing allelochemicals which chemically defend plant tissue walls. Feeding changes chlorophyll production, with needles eventually turning gray and dropping. It was this change in chlorophyll vigor that this study sought to detect and characterize. As hemlocks die due to adelgid infestation, gaps in the canopy aid the growth and density of already abundant hardwoods, altering stand dynamics. Additionally, nitrogen held in the root systems of hemlocks is released into soils as death occurs. Nitrate leaching is a problem within the Quabbin Watershed because it is a public water supply watershed, and higher nitrogen rates degrade the quality of water. Hemlocks and adelgid infestation were analyzed in images captured by the Airborne Imaging Multispectral Sensor (AIMS-1) in April of 2004 during leaf senescence of hardwoods. AIMS-1 collects data within the visible spectrum and near infrared spectrum of light using 3 multispectral bands: near infrared, red and green. Healthy vegetation reflects light in the near-infrared (NIR) band between 0.62 nm and 0.7 nm and absorbs red energy between 0.45 nm and 0.66 nm. As parenchyma cells accumulate, the chlorophyll content of plants increases, causing plants to absorb more light energy in the red band and reflect more energy in the near infrared band. The reduction of parenchyma cells should provide a direct link between infestation and corresponding changes in the absorption and reflectance properties of damaged hemlocks. The plots used in this study had previously been classified by the Division of Water Supply Protection, which is part of the Massachusetts Department of Conservation and Recreation, as possessing adelgid damage, so tests focused on classifying hemlock vigor tree by tree. This was done in order to first assess if AIMS-1 data preserved spectral differentiation within two broad classes of health: damaged and healthy. Tests show a statistically significant difference in spectral response between the two classes in the 8-bit data. The vegetation class with the tightest spectral fit is the Transform Normalized Difference Vegetation Index (TNDVI). Percentages of stands can be delineated manually due to the high spatial resolution of the image; however supervised classification does not provide any information about individual tree health. 10-bit data used in this study does not lend any more useful information about the spectral differences of damaged hemlocks. Future work will attempt to characterize entire stands of damage rather than individual trees so foresters may be able to implement treatment options typically conducted at the stand level.