1. In temperate deciduous forests, understory light environments vary dramatically throughout spring, summer, and autumn due to tree canopy leaf display. This variability in light level is a physiological challenge for understory species that produce sun-adapted leaves in the spring before being shaded by the tree canopy. Similarly, some understory species display leaves late into autumn after the tree canopy senesces. 2. Many species in North American deciduous forests with extended leaf display are not native to North America. Since many non-native species have been shown to have greater plasticity than natives, we hypothesized that leaves of non-native species may be more plastic with respect to seasonal light changes than natives, and that this plasticity may allow them to assimilate more carbon in the same environment. 3. We measured leaf traits and photosynthetic capacity of 17 native and 13 non-native understory shrub species in a common garden in Syracuse, New York, during spring, summer, and autumn. We tested for the contribution of seasonal mean and variance (plasticity) of leaf traits to a species’ average photosynthetic rate and total leaf production. We also analyzed the extent to which leaf adjustments depended on whether plants continued to produce new leaves over the growing season. 4. Leaf traits of both native and non-native species varied seasonally, but plasticity varied in extent and contribution to overall carbon gain. Non-native species had the highest seasonal plasticity, but, contrary to our hypothesis, such plasticity did not contribute to their overall carbon gain. However, leaf trait plasticity was adaptive for native species that continued to produce leaves throughout the year, primarily due to increases in quantum efficiency and electron transport rate in leaves produced mid-year compared to leaves produced early in the year. 5. Synthesis. Despite large adjustments in leaf traits across seasonal light environments for both native species and non-native species, we found little evidence that leaf-level plasticity drives non-native invasion or contributes strongly to annual carbon gain or productivity in understory species. Instead, differences in mean leaf traits across seasons are sufficient to explain carbon gain advantages of non-native woody species in deciduous forests.