Abstract: Fallen wood is an inevitable product during forest succession, which plays an important role in maintaining the stability, balance, and biodiversity of forest ecosystems. Many studies have explored the formation and decomposition process of fallen woods, but few referred to their quantity and structure. In this study, we investigated fallen woods in three plots with an area of 80 m×80 m, 100 m×60 m and 80 m×70 m in a pine-oak mixed forest in the Nanpan River basin, Southwest China,. Fallen woods were divided into five grades (DCs-Ⅰ, DCs-Ⅱ, DCs-Ⅲ, DCs-Ⅳ and DCs-Ⅴ) according to their decay statuses. Species richness, abundance, and volume were estimated and the characteristics of spatial structure were analyzed with the stand spatial structure parameters (i.e.,  uniform angle index, W; mingling, M; Dominance, U). Trees had many species, but with few individuals, which greatly contributed to volume. Shrubs had many individuals belonging to a few species, and accounted for a small part of volume. The abundance and volume of fallen woods increased directly or first increased and then decreased with increasing decay grades. Species markedly differed from each other in decay. Middle and small-sized woods constituted the main body of fallen woods, and large-sized ones accounted for a small amount. On the whole, fallen woods were at a state of mediumlow mixing (M=0.38-0.62) and slight aggregation (W=0.55-0.58), and had a balance of size differentiation (U=0.49-0.50). With the increases of decay grades, the size differentiation of fallen woods became large. These results clearly demonstrated the characteristics of species composition, quantity and spatial structure of fallen woods in a secondary forest in the Nanpan River basin, and indicated that the decay of fallen woods was closely related to species identity, their quantitative and spatial attributes, providing a basis for the scientific management of coarse woody debris.

Key words: fallen wood, decay class, spatial structure, secondary forest, coarse woody debris