Eastern hemlock (Tsuga canadensis [L.] Carr.) is a foundation species that occupies a unique niche in forest ecosystems and which often forms pure stands throughout the eastern United States. Throughout the last half of a century, widespread mortality of T. canadensis had been occurring with the introduction of the invasive pest, hemlock woolly adelgid (Adelges tsugae Annand) (HWA). HWA now threatens to destroy millions of hectares of hemlock dominated forests and to disrupt its associated ecosystems. In order to determine how HWA impacts hemlock physiology, three sites with various degrees of infestation were chosen and half of the trees at each site were treated with imidacloprid (Merit® 2 F, Bayer, Kansas City, MO) while the rest were left untreated. Needle gas exchange was assessed monthly using a LI-COR 6400 portable open path gas exchange system (LI-COR Inc, Lincoln NE). Chlorophyll fluorescence and bud break were also characterized for all trees at two of the three sites. After one complete growing season, we found a slight increase in photosynthetic rates (4.98 %), increased bud break at Fishburn (562 %) and Mountain Lake (25.6 %) sites, and no change in chlorophyll fluorescence for imidacloprid treated trees. These results suggest that HWA is causing tree mortality largely through a reduction of leaf area and not a reduction in leaf level photosynthetic capacity. By quantifying the physiological response of T. canadensis to HWA, better insights can be made into understanding tree decline as it relates to HWA.