Community structure features of the larch-birch secondary forest on the northeastern slope of Changbai Mountain, Northeast China.

The larch-birch forest is a secondary forest formed after the spruce-fir forest is destroyed. In order to understand its species composition and community structure, we established a 1-ha (100 m×100 m) permanent plot in the northeastern slope of the Changbai Mountains according to CTFS in August, 2015. All trees with diameter at breast height (DBH) ≥1 cm were tagged, investigated and mapped. And then we applied the pair-correlation g(r) function to determine the spatial distribution of two dominant species with different rank-classes. The results showed that there were 1 499 individuals belonging to 9 families, 16 genera and 22 species in this plot, mostly consisting of the north temperate flora species. Larix olgensis and Betula platyphylla were the major dominant species whose importance values were 36.08% and 24.75%, respectively. The Margalef species richness index (R), Shannon-Wiener index (H′), Simpson index (D) and Pielou index (Jsw) of this plot were 2.87, 1.71, 0.62 and 0.56, respectively. This forest was in good condition of regeneration and showed a reversed J-shape form in tree size distribution of all individuals. The size structure of eight major species was different. L. olgensis showed an inverted J-shape form, B. platyphylla and Populus davidiana showed an approximately normal distribution, while the others including Sorbus pohuashanensis, Crataegus maximowiczii, Pinus koraiensis, Prunus padus and Abies nephrolepis all showed L-shape form. The spatial distribution pattern of all individuals was aggregated, and the eight major species also showed aggregation patterns to different extent in this plot. The result of g(r) function analysis showed that the two dominant species with varying rank-classes and distances showed different distribution patterns. The aggregation intensity decreased with the increasing rank class of L. olgensis, while the medium trees of B. platyphylla had more aggregation intensity than small trees and large trees.