Jiangshan Lai

The Allometry of Coarse Root Biomass: Log-Transformed Linear Regression or Nonlinear Regression?

Precise estimation of root biomass is important for understanding carbon stocks and dynamics in forests. Traditionally, biomass estimates are based on allometric scaling relationships between stem diameter and coarse root biomass calculated using linear regression (LR) on log-transformed data. Recently, it has been suggested that nonlinear regression (NLR) is a preferable fitting method for scaling relationships. But while this claim has been contested on both theoretical and empirical grounds, and statistical methods have been developed to aid in choosing between the two methods in particular cases, few studies have examined the ramifications of erroneously applying NLR. Here, we use direct measurements of 159 trees belonging to three locally dominant species in east China to compare the LR and NLR models of diameter-root biomass allometry. We then contrast model predictions by estimating stand coarse root biomass based on census data from the nearby 24-ha Gutianshan forest plot and by testing the ability of the models to predict known root biomass values measured on multiple tropical species at the Pasoh Forest Reserve in Malaysia. Based on likelihood estimates for model error distributions, as well as the accuracy of extrapolative predictions, we find that LR on log-transformed data is superior to NLR for fitting diameter-root biomass scaling models. More importantly, inappropriately using NLR leads to grossly inaccurate stand biomass estimates, especially for stands dominated by smaller trees.

Topographic variation in aboveground biomass in a subtropical evergreen broad-leaved forest in China.

The subtropical forest biome occupies about 25% of China, with species diversity only next to tropical forests. Despite the recognized importance of subtropical forest in regional carbon storage and cycling, uncertainties remain regarding the carbon storage of subtropical forests, and few studies have quantified within-site variation of biomass, making it difficult to evaluate the role of these forests in the global and regional carbon cycles. Using data for a 24-ha census plot in east China, we quantify aboveground biomass, characterize its spatial variation among different habitats, and analyse species relative contribution to the total aboveground biomass of different habitats. The average aboveground biomass was 223.0 Mg ha−1 (bootstrapped 95% confidence intervals [217.6, 228.5]) and varied substantially among four topographically defined habitats, from 180.6 Mg ha−1 (bootstrapped 95% CI [167.1, 195.0]) in the upper ridge to 245.9 Mg ha−1 (bootstrapped 95% CI [238.3, 253.8]) in the lower ridge, with upper and lower valley intermediate. In consistent with our expectation, individual species contributed differently to the total aboveground biomass of different habitats, reflecting significant species habitat associations. Different species show differently in habitat preference in terms of biomass contribution. These patterns may be the consequences of ecological strategies difference among different species. Results from this study enhance our ability to evaluate the role of subtropical forests in the regional carbon cycle and provide valuable information to guide the protection and management of subtropical broad-leaved forest for carbon sequestration and carbon storage.

The coarse root biomass of eight common tree species in subtropical evergreen forest

Though coarse root biomass is an important part in biomass of forest ecosystems, few studies were found in subtropical area. In this study, whole digging method was used to determine the biomass of coarse root (diameter>2 mm) of 175 trees of 8 species in subtropical evergreen forests. The 8 species includes Castanopsis fargesii , Castanopis sclerophylla , Schima superba , Pinus massoniana , Cunninghamia lanceolata , Liquidambar formosana , Sassafras tzumu , Alniphyllum fortunei . Also, the lost roots during excavation were corrected by developing scaling relationship between biomass of root and diameter. Then, four kinds of allometric models for each species and combined trees were fitted to link stem diameter at breast height (dbh) and height ( H ) to coarse root biomass. The results showed that all the scaling relationship between biomass of root and diameter of roots were significant ( P P R 2 values than the height-coarse root biomass. The results also indicate that dbh is an adequate estimator for coarse root biomass of subtropical evergreen trees. The results in this study provide important information to estimate the coarse root biomass in subtropical evergreen forests.