Forest gaps are one of the most common phenomena in forest communities that provide a variety of microhabitats. The formation of forest gaps can change the environmental microsite conditions of forested land such as affecting the availability of light, heat, water and influencing soil fertility. Gaps can increase environmental heterogeneity and the diversity of microhabitats present in forested ecosystems, thus becoming an important driving force for the long-term maintenance of forest ecosystems while aiding in cycling and regeneration. For the past few years, with the goal of improving the operational efficiency and management of plantations, researchers have simulated the function of gaps in natural forest during forest regeneration, and have gradually developed a mass transformation practice of artificial pure forest. However, the question remains; what size of forest gap patches is most conducive to the growth of mixed species forests? Additional work is needed to study the impact of forest gap gradients on the growth characteristics of mixed species forests. Pinus massoniana is an important afforestation pioneer species and the main fiber-producing species in China, especially in the Yangtze River basin and in southern China. The planting of large areas of single-species Masson pine forest has created many problems related to forest ecology, such as reduced biodiversity, soil acidification and poor soil fertility, along with an increased risk of forest diseases and insect pests, low forest productivity and so on. This study was designed to help land managers understand the impacts of forest gaps in Masson pine plantations on associated tree species in light of the extensive use of such plantations. The goal was to provide a scientific basis for the use of various sized gaps in Masson pine plantations and to analyze the effects of gaps on other forest species. The effects of gaps on mixed species seedlings were studied by comparing the changes of leaf morphology and photosynthetic physiological characteristics of an important indigenous broad-leaved tree species, Cinnamomum longepaniculatum, in forest gaps of different sizes. The main results follow. First, when the size of forest gaps was less than 20 m × 20 m, the maximum net photosynthetic rate of the leaves of C. longepaniculatum seedlings was significantly lower than that in open land. Second, with an increase of forest gap size, leaf mass per unit area increased significantly. Compared with the control, although the leaf nitrogen content of C. longepaniculatum seedlings decreased significantly in small forest gaps (e.g., 10 m × 10 m), it increased significantly in large gaps (e.g., 20 m × 20 m). Nevertheless, no significant difference was observed in the phosphorus content of leaves of seedlings grown in either forest gaps or in open land. Third, with an increase in forest gap size, total partition coefficients of leaf nitrogen in the photosynthetic apparatus increased, especially the partition coefficients of leaf nitrogen in carboxylation components. Nevertheless, the coefficient of leaf nitrogen partitioning into the light harvesting apparatus decreased significantly with an increase in the size of forest gaps (10 m × 10 m-20 m × 20 m). The results demonstrated that if the gap size is less than 20 m × 20 m forest gaps have marked effects on the photosynthetic capacity of C. longepaniculatum seedlings. Seedlings may adapt to changes in the forest gap environment by adjusting a number of morphological and physiological characteristics, e.g., specific leaf area, leaf nitrogen content and the distribution of leaf nitrogen in the photosynthetic apparatus.