Wood microclimate as a predictor of carbon dioxide fluxes from deadwood in tropical Australia

Abstract. Deadwood is an important yet understudied carbon pool in tropical ecosystems. Wood microclimate, as defined by wood moisture content and temperature, drives decomposer (microbial, termite) activities and deadwood degradation to CO₂. Microclimate is strongly influenced by local climate, and thus, climate data could be used to predict CO₂ fluxes from decaying wood. Given the increasing availability of gridded climate data, this link would allow the rapid estimation of deadwood-related CO₂ fluxes from tropical ecosystems worldwide. In this study, we adapted a mechanistic fuel moisture model that uses weather variables (e.g. air temperature, precipitation, solar radiation) to characterize wood microclimate along a rainfall gradient in Queensland, Australia. We then developed a Bayesian statistical relationship between microclimate and CO₂ flux from pine (Pinus radiata) blocks deployed at sites and combined this relationship with our microclimate simulations to predict CO₂ fluxes from deadwood at 1-hour temporal resolution. We compared our pine-based simulations to moisture-CO₂ relationships from stems of native tree species deployed at the wettest and driest sites. Finally, we integrated fluxes over time to estimate the amount of carbon entering the atmosphere and compared these estimates to measured mass loss in pines and native stems. Our statistical model showed a positive relationship between CO₂ fluxes and wood microclimate variables. Comparing cumulative CO₂ with wood mass loss, we observed that carbon from deadwood decomposition is mainly released as CO₂ regardless of the precipitation regime. At the dry savanna, only about 19 % of the wood mass loss was released to CO₂ within 48 months, compared to 86 % at the wet rainforest, suggesting longer residence times of deadwood compared to wetter sites. However, the amount of carbon released in-situ as CO₂ is lower when wood blocks are attacked by termites, especially at drier sites. These results highlight the important but understudied role of termites in the breakdown of deadwood in dry climates. Additionally, mass loss-flux relationships of decaying native stems deviated from that of pine blocks. Our results indicate that wood microclimate variables are important in predicting CO₂ fluxes from deadwood degradation, but are not sufficient, as other factors such as wood traits (wood quality, chemical composition, and stoichiometry) and biotic processes should be considered in future modeling efforts.