Ecosystem allometry: The scaling of aboveground biomass and carbon flux through canopy height across plant communities

Background/Question/Methods

Based on individual level allometric scaling relations, Enquist et al. (2009) developed a quantitative model for the structure and dynamics of forests at demographic and resource steady state by linking ecosystem level stand biomass and metabolism (or ecosystem carbon flux) to maximum stem size within a given community. This model is promising; however, it hasn’t been well tested. Here, we use data from 64 tower sites from the AmeriFlux network, which includes average canopy height, aboveground biomass, and level 4 carbon flux products, in order to test Enquist et al.’s prediction.

Results/Conclusions

Our results show 1) when forest stand age is above 50, the scaling exponent between ecosystem level total carbon flux (Gross Primary Productivity, GPP, and Ecosystem Respiration, ER) and total aboveground biomass is 3/5, as predicted by Enquist et al. (2009); 2) other scaling relations between aboveground biomass, ecosystem carbon flux (GPP and ER) and canopy height also support model expectations. However, the scaling exponents are significantly different from the prediction of Enquist et al. (2009). One possible explanation that might contribute to such inconsistency is caused by the difference between average canopy height used in this study and maximum canopy height used in theoretical deduction; 3) Net ecosystem exchange (NEE) cannot scale with both canopy height and aboveground biomass when forest is at resource steady state (forest stand age >50); while NEE can weakly scale with both canopy height and aboveground biomass when forest stand age is below 50. This finding is consistent with many previous studies (Odum 1969; Law et al. 2001; Goulden et al. 2006; Turner 2010). However, other challenges remain, such as mechanistically resolving the inconsistency between the scaling relations derived from observation from the Ameriflux network and that of model expectation, as well as how aboveground biomass can scale with canopy height regardless of resource steady state, while ecosystem carbon flux (GPP and ER) cannot.