Alnus maritima and Alnus serrulata are riparian shrubs that occur in similar habitats in the southern and eastern United States.Alnus serrulata is abundant throughout this range, but A. maritima is rare, occurring only in small populations in Oklahoma and Georgia and on the Delmarva Peninsula. Alnus maritima is more resistant than A. serrulata to water and temperature stresses, but the degree to which insolation influences the restricted distribution of A. maritima is unknown. Our goals were to characterize the shade tolerance of A. maritima and A. serrulata and determine whether differences in shade tolerance could help explain the differing ecological success of the two species. Measurements in nature showed that leaves of A. serrulatahave greater concentrations of chlorophyll than do leaves of A. maritima, and a greater percentage of A. serrulata inhabit shaded sites. Two experiments evaluating the resistance of seedlings to light‐deficit stress revealed that A. maritima had a greater photosynthetic capacity and grew more quickly than A. serrulata in full sunlight. In shade, survival of seedlings was lower and reductions in photosynthesis and growth were greater for A. maritima than for A. serrulata. We conclude that A. serrulata is tolerant and A. maritima is intolerant of shade. Moreover, we conclude that shade intolerance strongly restricts the potential niches of A. maritima within the region where the shade‐tolerant A. serrulata is comparatively abundant.