Light and hydrology are believed to have important effects on the population vigor of the globally rare and declining plant Trollius laxus (Ranunculaceae), which occurs in both open and forested geogenous, calcareous wetlands (e.g., rich fens, northern white-cedar swamps) in the northeastern United States. In a forested peatland that supports the largest population of T. laxus worldwide, we explored the relationships between T. laxus vigor and both light (photosynthetic photon flux density) and hydrology at 30 subpopulations. Subpopulation vigor was quantified as abundance (plant and seedling counts), size (stem counts), and reproductive output (flower and follicle counts), and these 5 vigor variables were regressed on 14 light environment and 6 hydrology predictor variables. The two measures of reproductive output were related to a number of the predictor variables, but subpopulation size and abundance were not. Although T. laxus emerges and blooms early in the spring, subpopulation vigor showed similar positive relationships between the spring and summer light environment variables. Vigor had stronger relationships with diffuse light than with direct light. Higher spring groundwater level, the only hydrology variable related to any component of subpopulation vigor, was related to greater follicle production. Because canopy gaps can cause small-scale changes in both light transmittance and hydrology in forested peatlands, our results indicate that gaps may be key to the persistence of T. laxus and other rich fen species during secondary succession.