Estimating Rhododendron maximum L. (Ericaceae) Canopy Cover Using GPS/GIS Technology
In the southern Appalachians, Rhododendron maximum L. (Ericaceae) is a key evergreen understory species, often forming a subcanopy in forest stands. Little is known about the significance of R. maximum cover in relation to other forest structural variables. Only recently have studies used Global Positioning System (GPS) technology as a field-based method to map the perimeter of shrub patches as a means of estimating canopy cover. We assessed the viability of using GPS technology to accurately measure R. maximum canopy cover in mountainous terrain; and we compared canopy cover to other R. maximum abundance variables, forest structural attributes, and environmental factors. We selected forty 20 x 40 m permanent plots at Coweeta Hydrologic Laboratory in western North Carolina to employ a variety of methods (visual estimates, GPS, and x-y coordinate measurements) to estimate canopy cover of R. maximum within each plot. We found a positive relationship between the GPS method and the more accurate x-y coordinate measurements (r = 0.967, p < 0.001). We compared the GPS-derived estimates to other measures of R. maximum abundance and found positive relationships between cover and density (r2 = 0.800, p < 0.001), basal area (r2 = 0.747, p < 0.001), total biomass (r2 = 0.761, p < 0.001), and leaf area index (r2 = 0.761, p < 0.001). The GPS method is a reliable field-based technology to estimate evergreen canopy cover and it could be used to estimate more difficult to measure parameters of R. maximum, given the significant relationships found in this study.