Abstract: Natural forests in subtropical regions are usually mixed, multilayered, and uneven-aged, with a few stands being vertically stratified. However, few studies have examined the diversity characteristics of different layers in stratified stands. Analyzing species diversity and structural diversity at vertical layers is conductive to understand the causes of stratification and its role in maintaining species coexistence. In this study, we evaluated diversity at different layers of pine-oak mixed forests located in the Nanpan River basin, Southwest China, using 11 traditional indices of species diversity and three structural diversity indices. The results showed that: (1) Species richness (R), Shannon index (H), Simpson index (D), Brillouin index (HB) and Fisher’s alpha diversity index (α) of the understory was higher than that of the canopy (P<0.01). The abundance (N), Heip evenness index (E_h), Pielou evenness index (E_H), Alatalo evenness index (E_a), Sheldon evenness index (E_s) of the understory was higher than that of the canopy on the whole. The Jaccard similarity index between those two layers was 0.29-0.52. (2) For the understory and canopy layers, 56.4%-60.1% and 56.7%-58.9% of individual trees were in a status of random distribution (W=0.50), 19.1%-23.9% and 17.3%-21% in cluster (W=0.75, 1.00), 19.6%-20.8% and 21.5%-26.0% in uniform (W=0.00, 0.25), respectively. (3) The average species mixture of the understory layer (M=0.50-0.72) was higher than that of canopy layer (M=0.31-0.61). The mixture of understory layer gradually increased with increasing grades of mingling (M) in all measured plots that were mainly characterized by high and complete mixture (M=0.75, 1.00) (44.4%-64.4%), and canopy layer had a low-level species mixture (M =0.00, 0.25), accounting for 22.6%-64.5%; (4) The dominance (U) of each layer was about 0.5, while the frequency of each dominance grade was close to 20%. These results suggest that the stratification determines the diverse characteristics of pine-oak mixed stands and their layers, which could be an important mechanism underlying species coexistence and the maintenance of stand physical structure. More attention should be paid to the role of stratification in biodiversity conservation and management of forests.

Key words: mixed forest, diversity, distribution pattern, canopy, understory.