The direct and indirect effects of hemlock woolly adelgid infestation on eastern hemlock forests.
Orwig, David *,1, 1 Harvard Forest, Harvard University, Petersham, MA
ABSTRACT- The introduced hemlock woolly adelgid (Adelges tsugae;HWA) is causing decline and mortality of eastern hemlock (Tsuga canadensis) across the eastern U.S. The adelgid directly impacts forests by causing foliar loss and eventual hemlock mortality, typically over a 5 to 15 year period. Since 1995, hemlock mortality in permanent plots in southern New England has increased 5 to 15 % annually to overall values of 80 to 99%. HWA continues to spread throughout Connecticut and Massachusetts, but a recent resampling of 141 stands has documented a drop in HWA densities, probably due to cold winter temperatures. The structural loss of this foundation species leads to dramatic compositional changes, as hardwood species such as black birch (Betula lenta), red maple (Acer rubrum), and red oak (Quercus rubra) rapidly replace hemlock. HWA also indirectly impacts forests by changing microenvironmental conditions, altering ecosystem function, and influencing management decisions. Hemlock stands in Connecticut infested with HWA typically have higher organic and mineral soil temperatures, lower organic soil moisture and higher mineral soil moisture levels. Infested stands also have higher nitrogen mineralization and nitrification rates and overall nitrogen availability than uninfested stands. The primary management response to HWA is logging, and its effect on forest vegetation and ecosystem dynamics can often be more dramatic than HWA impacts alone. Logged hemlock sites in Connecticut had more abundant and diverse understory vegetation when compared to uncut and uninfested stands. In addition, some logged stands had nitrogen availability levels that were 3 to 15 times higher than uninfested or infested stands.
Key words: hemlock woolly adelgid, Tsuga canadensis, forest dynamics
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