Distribution and Dynamics of the Evergreen Understory Layer in Central Appalachian Highland Forests

Abstract

Evergreen understory layer communities dominated by Rhododendron maximum L. and/or Kalmia latifolia L. may exert significant controls on regeneration of overstory trees, carbon sequestration, and nutrient retention in central Appalachian forests, but their distribution and ecological importance are poorly understood at the regional scale. The distribution, temporal dynamics, and biomass of the evergreen understory layer were examined in the Ridge and Valley and Allegheny Plateau physiographic provinces in the mid-Atlantic Highlands using plot data, remote sensing, dendrochronology, and modeling. First, leaf-off satellite remote sensing and topographic data were applied to map the spatial extent and distribution of R. maximum and K. latifolia with better than 80 percent accuracy. Second, plot data were used to determine the relevant environmental factors and species associations related to the distributions of K. latifolia and R. maximum and assess their influence on forest vertical structure. Cluster analysis and ordination revealed that topo-edaphic gradients and intensity of gypsy moth defoliation were associated with differences in the distribution of these two shrub species within and between the two study areas, and midstory volume was significantly lower where evergreen understory coverage was continuous. Third, variation in K. latifolia and R. maximum growth rates were examined using remote sensing change detection and dendrochronology, and trends were compared to the timing of climatic fluctuations and gypsy moth defoliation of canopy trees. Remote sensing showed that patterns of evergreen understory growth vigor correlated with both defoliation and topographically mediated drought stress. Dendrochronology revealed considerable within-site variability among individual shrubs. However, both releases and suppressions in growth were associated with the timing of gypsy moth defoliation for K. latifolia in both provinces and for R. maximum in the Allegheny Plateau. Finally, carbon sequestration and nutrient storage impacts of these species were estimated by modeling their current aboveground biomass and ecosystem storage influences on several test watersheds. The inclusion of K. latifolia and R. maximum in the ecosystem model NuCSS indicated increases of up to 4825 kg/ha of carbon and 224 kg/ha of nitrogen storage, including notable increases of carbon and nitrogen in the forest floor and soil pools.