Forest Community Dynamics After Widespread Die-Off From an Invasive Insect
Forested watersheds are an especially important regulator of the Nation's water supply because managed and unmanaged forests provide the cleanest and most stable water supplies for drinking water, aquatic habitat, and groundwater recharge compared with all other land uses. Understanding how disturbances, such as the widespread die-off of a foundation tree species like eastern hemlock, restructure the forest community is critical to understanding the controls and partitioning of water in these headwater catchments. Understanding changes in community composition caused by invasive species is critical for predicting probable effects on ecosystem function.
A Forest Service study quantified changes in microclimate, community composition, and growth in southern Appalachian eastern hemlock forests during 7 years of infestation by the invasive hemlock woolly adelgid. A separate, simultaneous treatment mimicked infestation by girdling hemlock trees. Mortality was rapid, with 50-percent hemlock tree mortality occurring after 6 years of invasion, and 2 years after girdling. Because the leaf area lost by infested hemlock was similar to that of girdled trees during the study, the changes in light, growth, and soil moisture were identical in the infested and girdled stands.
Increased growth of co-occurring canopy trees was limited to the first few years, while increased growth of the evergreen shrub, rhododendron, continued over time. After 7 years, it had a 2.6-fold higher growth rate than expected, responding to increased light during leaf-off periods of the deciduous species. Increased growth and dominance of rhododendron may be a major determinant of future responses in southern Appalachian ecosystems; however, the results suggest a mix of maple, birch, beech, and oak canopy genera will replace hemlock where establishment is not limited by rhododendron.