Recent studies have documented local-scale associations between tree species and habitat factors in forests worldwide. A 30 hm² plot (500 m×600 m) was established in the Jiaohe Forestry Experimental Zone Bureau of Jilin province to study the long term dynamics of a broad-leaved Korean pine mixed forest. Within the plot, all free-standing trees and shrubs with a DBH of at least 1cm were identified, tagged and mapped, and their geographic coordinates recorded following a standard field protocol. One objective was to study mechanisms of the formation of spatial distribution patterns of understory tree species and to explore how environmental gradients influence these patterns in a broad-leaved Korean pine mixed forest. Based on the data from the 30 hm² plot, all trees with a DBH not greater than 5cm were defined as under storey trees. Only species represented by at least 300 individuals were chosen. 14 tree species representing 94.32% of all understorey trees satisfied this condition. First, the spatial patterns of the 14 selected species were analysed at scale of 20m×20m cells, using Morisita's index(I_δ). The results indicate that all 14 species show a clumped spatial distribution. Second, the spatial autocorrelations of the 14 species were studied at 15 distance classes using the spatial autocorrelation coefficient Moran's I. The results show a significant positive correlation at small distance classes and significant negative correlations at large spatial distance classes in most species were. Third, the partial Mantel test was used to find the relationship between the abundance of the 14 tree species and certain habitat factors. The results show a significant correlation with spatial distances for 6 species which reprtesent 42.86% of the tested species. 7 species (50%) were significantly correlated with habitat factors. 4 species showed neither a significant correlation with spatial distance nor with habitat factors. 3 species showed a significant correlation with both distance and at least one habitat factor. 5 of 7 dominant species with more than 1000 individuals had significant correlation with spatial distances. 5 species had significant correlation with habitat. The partial Mantel tests suggested that the mass effect from neighboring quadrats and habitat association could explain most of the tree species abundance distribution in a quadrat in the broad-leaved Korean pine forest community. At last, the Pearson correlation coefficient between species abundance in a quadrat and microtopographic attributes of the quadrat was calculated to test the effects of microtopography (elevation, slope, aspect, convexity) on species distribution. The results showed that Acer mono, Carpinus cordata preferred lower elevations and southern aspects. Acer barbinerve, Acer ukurunduense and Corylus mandshurica preferred higher elevations, steeper slopes and north-facing aspects. This result shows obvious habitat niche specialization and confirms niche theory. It is concluded that habitat niche specialization and mass effect are important factors in structuring distributions of understorey tree species in a broad-leaved Korean pine forest community. Many tree species prefer similar habitats which indicates similar resource usage and niche overlapping. Other tree species prefer specific habitat indicating repellency between different species, which may be the result of long term adaptation to different habitats, different resource usage, and which may explain the niche separation.