Quantifying the transition in composition and function in eastern hemlock forests impacted by hemlock woolly adelgid

Background/Question/Methods

An invasive insect pest, hemlock woolly adelgid, (Adelges tsugae, HWA), is causing the loss of a unique ecosystem across an expanding portion of the forests of eastern North American. Eastern hemlock (Tsuga canadensis, hemlock) is an often cited example of a foundation species that regulates microclimate as well as energy and nutrient cycles. As these ecosystems are structured by a few strong interactions, ecological theory predicts the loss of foundation species results in a rapid shift to an alternate state. However, a quantified understanding of the influence of foundation species and alternate state transitions that results from their loss are not well understood. We investigated 38 hemlock riparian forests across the central Appalachians in Virginia, West Virginia, and Ohio, representing a chronosequence of HWA invasion. Specifically, we wanted to determine: 1. How does hemlock regulate community composition and function in forests that remain un-invaded in Ohio? 2. Does HWA impact forests differently across physiographic and environmental gradients? 3. How does composition and function shift with hemlock mortality?

Results/Conclusions

Hemlock is the dominant influence on forest diversity, productivity, and resource availability, but this is rapidly changing. Structural equation modeling (SEM) identified hemlock’s strong, negative influence on vegetation diversity in all strata, as well as light availability and leaf litter biomass (productivity). Yet, these connections are re-organizing as hemlock declines. SEM of hemlock decline across the central Appalachians indicate that the impacts of HWA are initially slowed by increased elevation and physiographic positions that receive less incident radiation (northerly aspects and steeper slopes), but the time of HWA presence is the most influential factor in decline. Our analyses indicate no evidence of resistance to HWA and no difference in its impacts on local scales in transects moving from small streams upslope.  Hemlock continues to dominate the over and understory communities in sites impacted for decades, although it is in severe decline. While functional shifts occur rapidly, multiple response permutation procedure (MRPP) did not detect any difference in overstory or community composition in sites impacted by HWA for decades.  Thus, functional processes will continue to fluctuate as forest composition continues to shift in multiple directions across the range, due to environemtnal gradients, as well as the influence of Rhododendron maxima, a native evergreen shrub.