For cases in which total tree height is unavailable, we developed a diameter-height allometric equation of the form of Eq. (3). Using forward selection (see Methods), we found that among-site variation in diameter-height allometries was best explained by TS (temperature seasonality), (climatic water deficit), and (precipitation seasonality), CWD PS with only small further declines in model RSE with the sequential inclusion of additional bioclimatic variables (Fig. S5). We thus propose the following pantropical diameter-height allometric model (6a) (AIC=47, RSE=0.243, df=3998), where is defined as E (6b) Formula (6b) has an intuitive interpretation. is a measure of environmental stress. Indeed, E E increases with temperature seasonality, which relates to the amount of time a plant is exposed to stressful temperature, and CWD (a negative quantity) increases in magnitude with increasing annualized water stress. The dependence of on PS (precipitation seasonality) is E less obvious but appears to be mostly driven by monsoon-dominated rainfall regime (Nemani 2003). The dependence of diameter-height allometry on is illustrated in Fig. 4. A et al. E global gridded layer of at 2.5 arc-second resolution is available at E Equation (6a) predicts that the log-transform of tree height for a given diameter declines with both water and temperature stress. We also verified that the functional form of Eq. (3) is biologically consistent: for all the quadratic function in ln(D) E, reaches a maximum at a value that far exceeds biologically realistic values for Combining D. Eqs. (4) and (6a) directly, AGB can be inferred in the absence of height measurements (RSE=0.431, df=3999). Here, we prefer to fit the data directly, yielding the following model: (7)
For cases in which total tree height is unavailable, we developed a diameter-height allometric equation of the form of Eq. (3). Using forward selection (see Methods), we found that among-site variation in diameter-height allometries was best explained by TS (temperature seasonality), CWD (climatic water deficit), and PS (precipitation seasonality), with only small further declines in model RSE with the sequential inclusion of additional bioclimatic variables (Fig. S5). We thus propose the following pantropical diameter-height allometric model (6a) (AIC=47, RSE=0.243, df=3998), where E is defined as (6b) Formula (6b) has an intuitive interpretation. E is a measure of environmental stress. Indeed, E increases with temperature seasonality, which relates to the amount of time a plant is exposed to stressful temperature, and CWD (a negative quantity) increases in magnitude with increasing annualized water stress. The dependence of E on PS (precipitation seasonality) is less obvious but appears to be mostly driven by monsoon-dominated rainfall regime (Nemani et al. 2003). The dependence of diameter-height allometry on E is illustrated in Fig. 4. A global gridded layer of E at 2.5 arc-second resolution is available at Equation (6a) predicts that the log-transform of tree height for a given diameter declines with both water and temperature stress. We also verified that the functional form of Eq. (3) is biologically consistent: for all E, the quadratic function in ln(D) reaches a maximum at a value that far exceeds biologically realistic values for D. Combining Eqs. (4) and (6a) directly, AGB can be inferred in the absence of height measurements (RSE=0.431, df=3999). Here, we prefer to fit the data directly, yielding the following model: (7)
Chave_GCB13_final_main_1.docx