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

Ecosystem respiration (Reco) arises from interacting autotrophic and heterotrophic processes constrained by distinct drivers. Here, we evaluated up‐scaling of observed components of Reco 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 (Rsoil), heterotrophic soil microbes (Rh), roots (Rroot), and stems (Rstem) in 2018–2019. Reco 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 Reco (Reco.eddy, 1,439 g C m−2 y−1) was slightly higher than the sum of the respiration components (Reco.sum, 1,295 g C m−2 y−1) and simulated Reco (1,297 g C m−2 y−1). The largest mean contribution to Reco was from Rsoil (64%) across seasons. The measured contributions of Rh (49%), Rroot (15%), and Rstem (22%) to Reco.sum were very close to model outputs of 46%, 11%, and 22%, respectively. The modeled Rh was highly correlated with measured Rh (R2 = 0.66, RMSE = 0.61), empirically validating the DAMM module. The apparent temperature sensitivities (Q10) of Reco were 2.22 for Reco.sum, 2.15 for Reco.eddy, and 1.57 for CABLE‐POP. This research demonstrated that bottom‐up respiration component measurements can be successfully scaled to eddy covariance‐based Reco and help to better constrain the magnitude of individual respiration components as well as their temperature sensitivities in land surface models. Plain Language Summary: Ecosystem respiration (Reco) represents losses of carbon from the land to the atmosphere and consists of aboveground plant respiration and belowground root and microbial respiration. Because respiration processes increase exponentially with temperature, understanding their contributions to Reco is critical to predicting carbon cycle responses to warming. We used field observations to test and improve the modeling of respiration components of an evergreen eucalypt forest in Australia. Field measurements indicated that the model adequately captured the quantitative contributions of respiration components to Reco. In particular, the improved microbial version of the model was in good agreement with measurements. However, improvements are needed for modeling and measuring the autotrophic components from roots, stems, and forest canopy. This study highlights that scaling up individual respiratory sources and their temperature responses provides insights to understanding ecosystem scale carbon cycle‐climate feedbacks. Key Points: Concurrent scaled chamber measurements matched flux tower observations to within 10%Implementing a substrate function into a land surface model improved representation of heterotrophic respirationDiscrepancies between observations and simulations were largest for temperature sensitivity of canopy and root respiration [ABSTRACT FROM AUTHOR]