You are here
Harvard Forest REU Symposium Abstract 2012
- Title: Effects of hemlock woolly adelgid and elongate hemlock scale interactions on eastern hemlock growth and foliar, fine branch and litter chemistry
- Author: Julia N Brokaw (Cornell University)
- Abstract:
Heavy infestations of herbivorous insects can contribute to rapid changes in forest ecosystems. Currently in the northeastern U.S., two co-occurring invasive pests, the hemlock woolly adelgid, Adelges tsugae, and the elongate hemlock scale, Fiorinia externa both feed on the eastern hemlock, Tsuga canadensis. While they share a native host plant, their feeding mechanisms differ-A. tsugae feed on the ray parenchyma cells on the undersides of twigs and F. externa feed on the mesophyll cells on the undersides of the needles. Fiorinia externa is not considered to be as destructive as A. tsugae, however, understanding individual tree responses to the combined impacts of these insects is especially important as these insects may interact in unpredictable ways. In order to document the impact of these pests on hemlock health, new branch growth was measured and the percentages of branches producing new shoots were documented in twelve hemlock stands differing in insect densities across southern New England. Four treatments were examined in these stands: high A. tsugae, high F. externa, both A. tsugae and F. externa, and no infestation. Foliar, twig, and fallen litter chemistry was also examined. Additionally, we analyzed the chemistry of scale covers and adelgid wool. Our results showed that the percentage of branches with new growth was significantly reduced in stands infested with A. tsugae or both insects (40-50%) compared to F. externa-infested and uninfested trees (75% and 60%, respectively). The length of new growth was significantly lower on trees with both A. tsugae and F. externa (1.5 cm) compared to uninfested branches (> 3 cm). New shoot growth declined significantly with increasing A. tsugae density but not with increasing F. externa density. We also found that these insects influence foliar and fine twig chemistry, as foliar nitrogen content (%N) increased significantly in infested versus uninfested needles, but did not differ among insect treatments. Fine twig %N was significantly higher in sites with high A. tsugae and the presence of both insects (~1-1.2%N). The %N in hemlock litter collected from the forest floor under each tree was not significantly different across stand/insect site types but contained more N (1.11-2.78%) than living needles. The percent carbon (%C) did not differ across treatments in any parameter. The scale covers had a high nitrogen content (5.75%-9.16%) and adelgid wool had lower values (0.78%-2.57%). T. canadensis are not resistant to A. tsugae or F. externa, however the higher %N in fine twigs suggests a defensive response to A. tsugae infestation. The feeding of both insects led to significant changes in growth and chemical changes within needles and twigs compared to uninfested forests, but did not differ substantially from impacts by A. tsugae alone. Our findings of pest-induced changes in twig and foliar chemistry and their by-products will likely affect subsequent ecosystem processes like decomposition, and should be examined further.
- Research Category: Invasive Plants, Pests & Pathogens
- Figures: Brokaw_Hemlock Symposium Photo.jpg
