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Natural forests comprise trees of different species and sizes, constituting a “biotic framework”. Although examinations of diversity patterns at various spatial scales are frequently conducted, life stages are rarely accounted for. Pine-oak mixedwood forest is widely distributed around the world and constitutes main forest type in the Nanpan River Basin in southwest China. We established a fixed plot with an area of 100 m × 100 m in an undisturbed forest stand and classified trees according to five life stages based on their diameter at breast height (DBH) and height. Then, we calculated eight traditional diversity indices for each life stage. We found that species richness (R), abundance (N), and three diversity indices first increased and then decreased with increasing life stage. As sampling area increased, R, the Shannon-Wiener index (H') and Simpson’s diversity index (D) first increased quickly, followed by a reduced rate of increase, whereas N showed a linear increase and three evenness indices showed gradual decreases. Global Moran’s I values for each diversity index were small, indicating weak spatial autocorrelation. Both R and N of shrubs and saplings decreased sharply with life stage, with only large trees comprising the later life stages. Our results indicate that species diversity patterns in pine-oak forests, particularly in early successional stages, differ among life stages. The changes contribute to the understanding and conservation of forest biodiversity.

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