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2009 Harvard Forest REU Student Symposium Abstracts

Tawny Virgilio - Westfield State College

The Use of Mixed-Bead Resins to Determine the Effect of Two Invasive Insects on Throughfall Nitrogen Dynamics Under Eastern Hemlock (Tsuga canadensis L.)


Herbivores are important components of many ecosystems due to their ability to influence nutrient cycling and primary production. There are several mechanisms by which herbivores can influence ecosystem function, such as through the deposition of frass, cadavers, and a change in the quality and quantity of leaf litter. Insect herbivory may increase the rates of nutrient leaching from the damaged leaves and can therefore change throughfall (the nutrient content of precipitation as it passes through plant canopies). This can also happen by the dissolution of frass from foliage. An increase in foliar microbes due to this input of frass, cadavers, and body coverings might too have an impact on throughfall and nutrient cycling.

This study utilized a new technique to examine how two invasive insects, feeding on the same host, influence throughfall nitrogen content. We examined eastern hemlock (Tsuga canadensis L.) saplings infested with hemlock woolly adelgid (HWA; Adelges tsugae), elongate hemlock scale (EHS; Fiorinia externa), both invasive pests, or neither pest in a field experiment at the University of Rhode Island. Mixed-bead resin bags were placed underneath hemlocks with each of these pest treatments for 30 days and examined for ammonium and nitrate content.

Results suggest that NH4 capture under treatment trees did not differ significantly from ambient precipitation levels (~ 88 g NH4/g resin) captured on resin bags suspended above ground at the site, suggesting that insects did not lead to significant inputs of NH4 as predicted. Resin bags under treatment trees did capture significantly higher amounts (P < 0.005) of NO3 (73-88 g NO3/g resin) than ambient levels in precipitation (~8 g NO3/g resin), but there was no significant difference among infestation treatment. Several potential factors could have contributed to the lack of herbivore related inputs observed in this project including: very wet summer months which may have leached out nutrients previous to our study, very low EHS population densities, and resin bag size which may have limited nutrient capture.

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