Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians
The loss of Tsuga Canadensis from invasion by hemlock woolly adelgid (Adelges tsugae, HWA) has altered ecosystem structure and funcition in forests across the eastern United States. In southern Appalachian forests, T. Canadensis co-occurred with hardwood species and an evergreen Rhododendron maximum shrub layer in riparian and cove positions. In this region, HWA infestation was detected in 2003, with mortality reaching 97% by 2014. In this study we examined responses of light, soil moisture, tee seedling density, and overstory and understory vegetation growth from 2004 to 2014 following HWA infestation and mortality of T. Canadensis. We hypothesized that seedling recruitment and vegetation growth would continue to invrease over time as observed with initial trends reported through 2009, and the species that associate with ectomycorrhizal (ECM) fungi my benefit more than those that do not due to the pulse of organic material in these stands from dead T. Canadensis trees. Light transmission measure at 1 m above the ground increased from 2006 to 2009, but gradually decreased from 2015 to 2017. Basal area of overstory non-Tsuga trees increased only marginally, and there was no recruitment of tree seedlings to the overstory size class, even though seedling density of deciduous species increased initially. Increased basal area and stem density of R. maximum may explain the light and seedling responses, as this species can inhibit tree seedling recruitment by limiting light and nutrients. Overstory species with the highest basal area increment (BAI) in the post-T. Canadensis stands were Pinu regida, Betula lenta and Quercus coccinea which all associate wit ECM fungi. However, not all ECM tree species grew significantly more following T. Canadensis mortality compared to pre-mortality growth rates- only thos ECM species that had high growth rates prior to mortality did. After a decade, growth of both overstory trees and R. maximum has not compensated for the loss of T. Canadensis. Active management of R. maximum, which may involve the removal of the evergreen shrub and soil organic layer, may required to allow for diverse tree seedling recruitment; and subsequently, restor riparian forest structure, diversity, and function.