Herbaceous-layer diversity and tree seedling recruitment are enhanced following Rhododendron maximum shrub removal

Abstract

Forest ecosystems dominated by Tsuga canadensis are undergoing fundamental changes in function and composition from infestations by hemlock woolly adelgid (Adelges tsugae). We proposed that the first step to restoring southern Appalachian riparian forests following T. canadensis mortality would be eliminating the evergreen shrub, Rhododendron maximum. We hypothesized that removing R. maximum would increase light transmittance, soil moisture and temperature; and subsequently, enhance herbaceous-layer diversity and promote tree seedling recruitment and survival. We tested these hypotheses at two locations, (CWT, Coweeta Hydrologic Laboratory; WOC, White Oak Creek) in the Nantahala Mountain Range of western North Carolina, both with heavy T. canadensis mortality and a dense R. maximum subcanopy. The treatments were designed to remove only soil O-horizon (FF), remove only R. maximum (CR), remove R. maximum and soil O-horizon (CFFR), and untreated, reference (REF). We installed permanent plots across treatments and locations and measured light transmittance (Qi/Qo), soil water content (θ), herbaceous-layer cover and diversity (Shannon’s index (H′cover) and species richness), and tree seedling recruitment.As expected, cutting the R. maximum subcanopy (CR and CFFR) immediately increased Qi/Qo in the spring months across locations, and it was sustained through the first growing season. θ was generally high across plots, averaging 26% during the growing season, and didn’t vary over time. By the second growing season (2017) after treatments, herbaceous-layer cover and diversity increased on CR and CFFR. Herbaceous-layer cover was significantly related to Qi/Qo (r2 = 0.22, p < 0.001) and θ (r2 = 0.13, p = 0.009), while diversity was only related to Qi/Qo (H′cover, r2 = 0.14, p < 0.001; species richness, r2 = 0.21, p < 0.001). Tree seedling density was related to Qi/Qo (r2 = 0.10, p = 0.001) and θ (r2 = 0.26, p < 0.001). Tree seedling density was low before treatment (1.4 ± 0.3 seedlings m−2) and increased by 10-fold in CR and CFFR two growing seasons after treatment. In CR, species with the highest density ranked Betula spp. > Acer rubrum > Quercus coccinea > Liriodendron tulipifera > Q. rubra. In CFFR, tree seedling recruitment ranked Betula spp. > A. rubrum > L. tulipifera. These vegetation responses have important implications for potential recovery of riparian forests following T. canadensis mortality.