We investigated whether dynamic photosynthesis of understory Quercus rubra L. (Fagaceae) seedlings can acclimate to the altered pattern of sunflecks in forest patches with Rhododendron maximum L. (Ericaceae), an understory evergreen shrub. Maximum photosynthesis (A) and total CO(2) accumulated during lightflecks was greatest for 400-s lightflecks, intermediate for 150-s lightflecks, and lowest for 50- and 75-s lightflecks. For the 400-s lightflecks only, maximum A and total CO(2) accumulated were significantly lower for seedlings in forest patches with shrubs present (SF) than for seedlings in forest patches without shrubs (F). These effects were found only when A was calculated on a leaf-area basis because the specific leaf area of seedlings in F patches was 16% lower than it was for seedlings in SF patches. Photosynthesis reached 50% induction in 159 s for seedlings in F patches compared with 226 s for seedlings in SF patches. The faster induction of A for seedlings in F patches resulted in a significantly higher lightfleck use efficiency than for seedlings in SF patches. The inefficient use of lightflecks by Q. rubra seedlings in SF patches may be a primary mechanism by which Q. rubra seedlings are inhibited by subcanopy thickets of R. maximum.