Patterns of mass, carbon and nitrogen in coarse woody debris in five natural forests in southern China

Coarse woody debris (CWD, a parts per thousand yen10 cm in diameter) is an important structural and functional component of forests. There are few studies that have estimated the mass and carbon (C) and nitrogen (N) stocks of CWD in subtropical forests. Evergreen broad-leaved forests are distributed widely in subtropical zones in China. This study aimed to evaluate the pools of mass, C and N in CWD in five natural forests of Altingia gracilipes Hemsl., Tsoongiodendron odorum Chun, Castanopsis carlesii (Hemsl.) Hayata, Cinnamomum chekiangense Nakai and Castanopsis fabri Hance in southern China. The mass of CWD was determined using the fixed-area plot method. All types of CWD (logs, snags, stumps and large branches) within the plot were measured. The species, length, diameter and decay class of each piece of CWD were recorded. The C and N pools of CWD were calculated by multiplying the concentrations of C and N by the estimated mass in each forest and decay category. Total mass of CWD varied from 16.75 Mg ha(-1) in the C. fabri forest to 40.60 Mg ha(-1) in the A. gracilipes forest; of this CWD, the log contribution ranged from 54.75 to 94.86 %. The largest CWD (a parts per thousand yen60 cm diameter) was found only in the A. gracilipes forest. CWD in the 40-60 cm size class represented above 65 % of total mass, while most of CWD accumulations in the C. carlesii, C. chekiangense and C. fabri forests were composed of pieces with diameter less than 40 cm. The A. gracilipes, T. odorum, C. carlesii and C. chekiangense forests contained the full decay classes (from 1 to 5 classes) of CWD. In the C. fabri forest, the CWD in decay classes 2-3 accounted for about 90 % of the total CWD mass. Increasing N concentrations and decreasing densities, C concentrations, and C:N ratios were found with stage of decay. Linear regression showed a strong correlation between the density and C:N ratio (R (2) = 0.821). CWD C-stock ranged from 7.62 to 17.74 Mg ha(-1), while the N stock varied from 85.05 to 204.49 kg ha(-1). The highest overall pools of C and N in CWD were noted in the A. gracilipes forest. Differences among five forests can be attributed mainly to characteristics of the tree species. It is very important to preserve the current natural evergreen broad-leaved forest and maintain the structural and functional integrity of CWD.