Larger hardwood trees benefit from removing Rhododendron maximum following Tsuga canadensis mortality

Abstract

Forest disturbance to overstory vegetation can result in increased abundance of understory vegetation, potentially affecting various ecosystem processes. Habitat expansion of a native evergreen woody shrub, Rhododendron maximum (rosebay rhododendron), in southern Appalachian Mountain riparian forests of the eastern United States has occurred following the mortality of Castanea dentata (American chesnut) in the early 20th century and more recently Tsuga canadensis (Eastern hemlock). Increased abundance of R. maximum has been associated with reduced tree seedling recruitment and survival, as well as shorter overstory tree height compared to where R. maximum understory was not present. As part of a larger investigation into the removal of R. maximum to restore vegetation structure and composition, this study examined the effects of removing R. maximum understory (by cutting) and soil O-horizon (by prescribed fire) on microclimate, whole tree and plot-level transpiration during 2016 and 2017 growing seasons. We found increased soil moisture in the wetter year (2017) and greater variation in light transmission to the forest floor after the treatment. Larger trees were able to increase their water use in the drier year (2016), resulting in similar plot-level transpiration of treatment and reference plots. Removal of R. maximum understory is not expected to significantly alter streamflow or stream chemistry during the growing season; however, it can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.