Characterization of Understory Shrub Expansion in a West Virginia Watershed from 1986 – 2011 Using Landsat Derived Vegetation Indices
Abstract:Mid and southern Appalachian forests have been heavily influenced by human intervention, with much of the current forest area covered by secondary or tertiary growth following significant past logging or fire. The pre-logging forests of mid Appalachia were mainly comprised of large Quercus spp. and Liriodendron tulipiferia with Pinus rubens and Tsuga canadensis at higher elevations. These species have been supplanted by more mesic species such as Betula alleghaniensis and Acer rubrum. Within these forests, Rhododendron maximum is an abundant evergreen shrub that grows in dense thickets that can alter forest community structure, affect species diversity, lower decomposition rates, and affect forest carbon and nitrogen cycling through altering soil chemistry and physics. The spatial patterns and temporal dynamics of R. maximum within these forests, especially in the mid Appalachians, is not fully conceptualized. An increase in R. maximumspatial coverage could significantly affect basic forest ecosystem processes and be of interest to researchers and forest managers.
Using Landsat derived vegetation indices--including Normalized Difference Vegetation Index (NDVI), Soil Adjusted Vegetation Index (SAVI), and Tasseled Cap Transformations--we quantified the expansion of R. maximum within a topographically complex watershed in West Virginia from 1986-2011. Our array of models show an initial shrub coverage (1986) in our target watershed of between 27.7 - 36.6% and a present-day shrub coverage (2011) of between 41.2 - 42.8%--with a range from 10.2 - 15.1% increase in shrub coverage over the 25 year study window. Averaged model output suggests an increase of 38.4 ha from 1986 to 2011 and a mean NDVI increase of 0.076 for the entire watershed. Furhter spatial analysis will elucidate possible connections and patterns related to distance-from-streams and/or elevation.