ObjectiveThe study selected Chinese fir plantations in Hunan Province, Anhui Province and Jiangxi Province of southern China as the research object to establish estimation models between the investigation factors of the tree-shrub layer and its biomass.The study tried to build a more reliable and accurate models for biomass estimation of shrub layer and provided reference for improving the accuracy of models for biomass estimation of shrub layer under Chinese fir plantation.

MethodA typical sample survey was conducted in the study area and the canopy density C_s, stand density D_s(plant/ha), average height H(m), average diameter D(cm), cover degree C, fresh and dry mass (kg) of shrub branches, stems, leaves, roots were determined in this study.Through the calculation, the stock volume of tree layer V (m³/ha)and the biomass of shrub layer (t/ha) were obtained.The effects of shrub layer structure and arbor layer investigation factors on shrub biomass were analyzed by Pearson's correlation, and the optimal shrub layer structure parameters were selected as model parameters to establish the models for estimation of branch, stem, aboveground and underground biomass.The arbor layer surveying factors were added into the model as an independent variable to compare and analyze the changes of R² after arbor layer investigation factors were added. The models were tested with extra-sample data to construct a more accurate model for estimating shrub biomass.

ResultThe results showed that the biomass model of shrub layer was dominated by power function, and the under-ground biomass of each forest age shrub layer was obtained, R² was 0.516-0.955 with independent variable D²H. The rest biomass got a good fitting model fitted with CH as the independent variable, R² was 0.516-0.718. Compared with the models for estimating biomass of shrub layer using structural factors of irrigation alone as independent variables, except underground biomass, the adding of average diameter D_m of young-middle age forest tree layer, as a variable, made the accuracy of models for other component increased significantly, R² was 0.718-0.990;the adding of canopy density C_s of mature age tree layer, as a variable made the accuracy of models for other component increased significantly, R² was 0.817-0.886.

ConclusionTherefore, if we evaluate and analyze the shrub biomass in the subtropical shrub layer, not only the relationship between the structural biomass of the shrub layer itself should be considered, but also the influence of the relevant factors in the arbor layer should be taken into account to establish a biological structure more consistent with the shrub biology and ecology.This study may provide a reference for the estimation of shrub biomass in the subtropical area of Chinese fir plantation.