Growth rates of Thuja occidentalis in an ecosystem altered by the double-crested cormorant, Phalacrocorax auritus.

Abstract

Double-crested cormorants, Phalacrocorax auritus, have been shown to have drastic effects on the vegetation in their nesting and roosting sites. In this study I examined how large depositions of cormorant guano affected the growth rates of Northern white-cedar trees, Thuja occidentalis, on Epoufette Island, MI. I also attempted to determine the pattern of change in growth rates over time. I examined three measures of growth rates--radial growth, basal area growth, and percent basal area growth. I investigated overall growth patterns by looking at trends over two five-year periods--the last five years of growth, termed Y-5 trends, and the five years previous to that, termed Y-10 trends. Although there appeared to be a trend toward increasing growth rates the last five years of growth for the trees affected by the presence of cormorants, lack of ability to assign times of death for the trees prevented solid analysis of this trend. Additional research is suggested to further explore this possible pattern of changes in growth rates as caused by nesting double-crested cormorants.