Goldblum, D. and M. C. Kwit (Department of Geography, Northern Illinois University, DeKalb, IL 60115). The relative photosynthetic contribution of old and new fronds of the wintergreen fern Dryopteris carthusiana, Ontario, Canada. J. Torrey Bot. Soc. 139: 270–282. 2012.—A number of understory species in temperate and boreal forests are characterized by the wintergreen habit which entails retaining leaves for a portion of a second growing season. Several hypotheses have been offered to account for this trait, most commonly: old leaves serve as storage organs for nutrients required for spring growth or old leaves contribute to the overall carbon gain of the plant given their presence in the understory during the period of high radiation prior to canopy leafout. In this study, using field-derived light response curves, plant demographic data, leaf phenological data, and twice-hourly understory light levels, we model the relative contributions of old and new fronds to overall seasonal net carbon gain of a common fern, Dryopteris carthusiana, in a boreal and deciduous forest in Ontario, Canada. Approximately 43% of the total fern carbon gain in the deciduous forest occurs in the 40 days before canopy closure, compared to 46% in the boreal forest during the same period. In the deciduous forest, approximately 30% of the total fern net carbon assimilation occurs at the end of the growing season following overstory leaf fall, which was not the case in the boreal forest. Old fronds contribute 29% of overall carbon gain in the deciduous forest, but substantially more (63%) in the boreal forest. We estimate the total net carbon assimilation by D. carthusiana ferns in the deciduous forest at 90.7 kg C ha−1 for the growing season and 45.5 kg C ha−1 in the boreal forest. Our findings quantify the importance of the wintergreen habit for overall fern seasonal carbon gain, as well as the importance of early season high light conditions for understory plants in deciduous forests.