Seasonal characteristics and determinants of tree growth in a Chinese subtropical forest

Aims To quantify the seasonal differences in effects of leaf habit, species identity, initial diameter, neighborhood interaction and stand environment on tree absolute diameter growth rates in a subtropical forest in China. Methods We used man-made dendrometer bands to record radial increments of all trees with diameter at breast height (DBH) >= 5 cm and height >= 3 m within 25 comparative study plots (30 x 30 m for each) of the `Biodiversity- Ecosystem Functioning Experiment China' (BEF-China) in the Gutianshan National Nature Reserve, Zhejiang Province, China. We measured stem circumferences twice a year from 2011 to 2014 to calculate absolute diameter growth rate of a warm and wet season (WWS, April to September) and a dry and cold season (DCS, October to the next March) for each individual tree: annual growth (GR(year)), growth during the WWS (GR(WWS)) and growth during the DCS (GR(DCS)). We firstly tested the differences in growth rates between different seasons using paired t-tests with Bonferroni correction. Then we applied linear mixed models to explore the effects of leaf habit, species identity, initial diameter, neighborhood interaction (indicated by richness, density and total basal area of all neighboring trees within a radius of 5 m around target trees), stand age and topography (elevation, slope and aspect) on tree growth rates of the two different seasons in three deciduous and 14 evergreen species. Important Findings GR(year), GR(WWS) and GR(DCS) varied between 0.04-0.50 cm year(-1) (mean = 0.21), 0.03 0.46cm season-(1) (mean = 0.18) and 0.01 0.05 cm season(-1) (mean = 0.03) across the 17 species, respectively. GR(WWS) was significantly higher than GR(DCS) for all species. Growth rates of faster growing species tended to have larger absolute differences between the WWS and DCS. Tree growth rates of both seasons and of the year (GR(year), GR(WWS) and GR(DCS)) varied significantly among leaf habit and species, and increased allometrically with initial diameter, decreased with stand age, but were not significantly related to topography and neighborhood richness or density. GRWWS decreased with neighborhood total basal area, while GRDCS did not. In conclusion, species might the temporally complementary, contributing to plot growth at different times of the year.