Abstract: The change pattern of tree species diversity in gaps of different sizes and developmental stages in the lower subtropical evergreen broadleaved forest was analyzed in this paper. The species diversity index(Shannon-Wienner) did not change in the gaps with a size of<400m²,reached the highest in those with a size of 400～500m² and the lowest in the gaps of 500～600m² size, and increased in those with a size of >600m².The index was the highest in the middle period of 20～50 years, secondly high within 20 years, and lowest after 50 years of gap formation.The species diversity in regeneration layer(H<1.5m) reached a peak in the gaps with a size of 500～600m²,and the lowest values occurred at the gaps with a size of >600m² and 200～300m².Species diversity index in the regeneration layer reached the highest in the first 10 years after gap formation,then revealed a general trend of decreasing, but two relative peaks were formed in the gap ages of about 30～40 years and 50～60 years,respectively. The general trends of species richness were correspondent with those of species diversity index.Gaps of different sizes and ages had a strong influence on the tree species regeneration through changes of ecological factors in gaps, thus,tree species diversity was different in gaps of different sizes and ages. Gap is one of the most important mechanisms for the maintenance of tree species diversity in the lower subtropical evergreen broadleaved forest.

Key words: Gap dynamics, Tree species diversity, Lower subtropical evergreen broadleaved forest, sap flux, Quercus liaotungensis, sapwood area, loess hilly region, soil moisture