The effects of growth light environment on the seasonal acclimation of photosynthetic capacity as well as rates of photosynthesis in the field were investigated in the angiosperm Mitchella repens L. (partridgeberry) and the gymnosperm Tsuga canadensis (L.) Carr. (eastern hemlock). Irrespective of growth light environment, photosynthetic capacity decreased in winter in partridgeberry but exhibited no seasonal change in eastern hemlock. These acclimatory patterns contrast with previously published results that report strong effects of growth light intensity on seasonal acclimation in another understory angiosperm, Mahonia repens (Lindley) Don., and strong winter-time decreases in photosynthetic capacity in a number of gymnosperms. With the exception of eastern hemlock in the exposed site, actual rates of photosynthesis were similar on summer days and on mild days in winter, whereas rates of photosynthesis measured on extremely cold days were much lower. Thus, even within the winter season, daily fluctuations in temperature profoundly affect photosynthesis. The carotenoid pigments that make up the xanthophyll cycle acclimated to winter in a manner that is consistent with their role in the protective process of energy dissipation.