Seed bank dynamics under rhododendron maximum: implications for the restoration of southern Appalachian forests affected by hemlock mortality
This study investigates the potential of the soil seed bank in restoring riparian forests once dominated by eastern hemlock (Tsuga canadensis (L.) Carr). Eastern forests have undergone fundamental changes in composition and function from infestation by hemlock woolly adelgid ( Adelges tsugae Annand). Eastern hemlock and rosebay rhododendron (Rhododendron maximum L.) often co-occur in these forests with rhododendron forming a dense continuous subcanopy that inhibits seedling recruitment and alters forest community structure. Consequently, the removal of rhododendron may be critical to restoring riparian areas affected by eastern hemlock loss. To investigate the contribution of the soil seed bank as a recruitment source following rhododendrons removal, I characterized the size and composition of the seed bank in riparian forests with and without rhododendron subcanopies. Further, I examined how microclimate and edaphic conditions beneath rhododendron may affect seed bank dynamics and the potential for seed germination. I found fewer species in the soil seed bank within rhododendron thickets (n = 43) relative to open forests (n = 64). Betula was the most abundant species in the seed bank under rhododendron, while herbaceous species were exceedingly rare. Other woody species found in both forest types were Rubus and Acer. Soil water content and temperature were significantly lower (P < 0.02) in plots with rhododendron than plots without rhododendron. Light transmittance was relatively low (118 ± 7.9 µmol m-2 s -1) in these closed canopy forests with no significant difference (F = 0.57, P = 0.45) between the two forest types. Forest floor depth was greater (F = 64.5, P < 0.0001) in plots with rhododendron (75 mm) than those without rhododendron (21 mm). My results suggest that the soil seed bank may not be the primary mode of recruitment to establish a diverse vegetative community after rhododendron is removed from these forests.