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Title Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians
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Abstract

The loss of Tsuga canadensis from invasion by hemlock woolly adelgid (Adelges tsugae, HWA) has altered ecosystem

structure and function 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, tree 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 increase over time as observed with

initial trends reported through 2009, and that species that associate with ectomycorrhizal (ECM) fungi may

benefit more than those that do not due to the pulse of organic material in these stands from dead T. canadensis

trees. Light transmission measured at 1m 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 Pinus rigida, Betula lenta and

Quercus coccinea, which all associate with ECM fungi. However, not all ECM tree species grew significantly more

following T. canadensis mortality compared to pre-mortality growth rates—only those 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 be required to allow for diverse tree seedling recruitment; and

subsequently, restore riparian forest structure, diversity, and function.

Contributors Sandra N. Dharmadi, Katherine J. Elliott and Chelcy F. Miniat
Citation

Dharmadi, Sandra N., Elliott, Katherine J., Miniat, Chelcy F. (2019) Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians, Forest Ecology and Management. Forest Ecology and Management, 440:122-130, https://doi.org/10.1016/j.foreco.2019.03.024

Key Words Eastern hemlock, Forest disturbance, Hemlock wooly adelgid, Rhododendron, Seedling recruitment
File Date 2019
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