Abstract: Sexual expression in Acer pensylvanicum is environmentally determined and changes between sexes can occur annually.  Light competition is an essential factor for growth in this understory tree and since sex changes are thought to be size-dependent, then light competition should influence the sexual expression. I surveyed the vegetation and the physical environment surrounding 50 male and female paired A. pensylvanicum to explore whether the physical environment, interspecifc competition, or intraspecific competition influence the sexual expression of the striped maple.  The results showed that the physical characteristics of the striped maple along with light levels are significantly related to sex expression. The species interactions, however, were not significantly associated with sex.  Therefore, it was concluded that the factors influencing sexual expression in A. pensylvanicum were shown to be the physical size of the tree and the amount of light it receives, rather than the composition of the surrounding understory and canopy layers.