Reconciling Top-Down and Bottom-Up Estimates of Ecosystem Respiration in a Mature Eucalypt Forest

Abstract

Ecosystem respiration (R_eco) arises from interacting autotrophic and heterotrophic processes constrained by distinct drivers. Here, we evaluated up-scaling of observed components of R_eco in a mature eucalypt forest in southeast Australia and assessed whether a land surface model adequately represented all the fluxes and their seasonal temperature responses. We measured respiration from soil (R_soil), heterotrophic soil microbes (R_h), roots (R_root), and stems (R_stem) in 2018–2019. R_eco and its components were simulated using the CABLE–POP (Community Atmosphere-Biosphere Land Exchange–Population Orders Physiology) land surface model, constrained by eddy covariance and chamber measurements and enabled with a newly implemented Dual Arrhenius and Michaelis-Menten (DAMM) module for soil organic matter decomposition. Eddy-covariance based R_eco (R_eco.eddy, 1,439 g C m⁻² y⁻¹) was slightly higher than the sum of the respiration components (R_eco.sum, 1,295 g C m⁻² y⁻¹) and simulated R_eco (1,297 g C m⁻² y⁻¹). The largest mean contribution to R_eco was from R_soil (64%) across seasons. The measured contributions of R_h (49%), R_root (15%), and R_stem (22%) to R_eco.sum were very close to model outputs of 46%, 11%, and 22%, respectively. The modeled R_h was highly correlated with measured R_h (R² = 0.66, RMSE = 0.61), empirically validating the DAMM module. The apparent temperature sensitivities (Q₁₀) of R_eco were 2.22 for R_eco.sum, 2.15 for R_eco.eddy, and 1.57 for CABLE-POP. This research demonstrated that bottom-up respiration component measurements can be successfully scaled to eddy covariance-based R_eco and help to better constrain the magnitude of individual respiration components as well as their temperature sensitivities in land surface models.