$Disporum$ $smilacinum$ A. Gray is a typical monocarpic pseudo-annual plant which is distributed under forest canopy in the East Asia. $D.$ $smilacinum$ reproduces both vegetatively and sexually although its population size is mainly increased by rhizomes. We investigated the morphological characteristics and reproductive strategy of $D.$ $smilacinum$ in gap and under closed canopy sites to evaluate the effects of changes in light condition on the population dynamics of shade-adapted $D.$ $smilacinum$. Shoot densities, number of leaves, number of flowering shoots, shoot length, rhizome length and biomass of individual organs of $D.$ $smilacinum$ were compared between gap and closed canopy sites which were located adjacent to each other. Whole plant biomass, rhizome biomass and the number of flowering shoots of $D.$ $smilacinum$ in gap were significantly lower than those under closed canopy ($P$ < 0.05), while root/shoot ratio in gap sites was higher than that under closed canopy, indicating that canopy gap decreased growth and sexual reproduction of $D.$ $smilacinum$. Spatial autocorrelation using spatial distribution of morphological characteristics showed that the size of aggregates of individuals with similar morphological characteristics was larger under closed canopy than in gap sites. Increased light by gap brings about stressful condition to shade-adapted $D.$ $smilacinum$, reducing growth, population productivity and sexual reproduction.