Asimina triloba is replacing shade-tolerant Acer saccharum as the most abundant seedling in many Ac. saccharum-dominated forests in the midwestern USA. Selective browsing of Ac. saccharum by Odocoileus virginianus and avoidance of As. triloba contributes to this increased As. triloba abundance. However, shade tolerance may also be a factor contributing to increasing As. triloba. We tested our hypothesis that As. triloba is more shade tolerant than Ac. saccharum. Fraxinus quadrangulata is less shade tolerant than As. triloba and Ac. saccharum; however, other than As. triloba, it was the only species that increased stems per hectare in the seedling stratum from 2003 to 2008. We evaluated leaf photosynthetic traits to assess shade tolerance of these three species. Light response curves (LRCs) of As. triloba, Ac. saccharum, and F. quadrangulata saplings were measured under a dense Ac. saccharum canopy in situ using a portable infrared gas analyzer and were tested for fit to rectangular hyperbola, nonrectangular hyperbola, and exponential LRC models. Multivariate analysis of covariance, with estimated ambient irradiance and leaf temperature as covariates, tested interspecific differences in gas exchange properties: (R_d [dark respiration], α [LRC initial slope], and A_MAX [maximum photosynthetic rate]). Multivariate contrasts indicated that F. quadrangulata had significantly higher rates of photosynthesis than the two other species, indicating lesser shade tolerance, whereas As. triloba had greater initial light response (α values) than Ac. saccharum, which confers an advantage to As. triloba under low light conditions. These results implicate greater shade tolerance by As. triloba as another contributor to its increasing dominance in the understory.