Understanding the effect of disturbance from selective felling on the carbon dynamics of a managed woodland by combining observations with model predictions

The response of forests and terrestrial ecosystems to disturbance is an important process in the global carbon cycle in the context of a changing climate. This study focuses on the effect of selective felling (thinning) at a managed forest site. Previous statistical analyses of eddy covariance data at the study site had found that disturbance from thinning resulted in no significant change to net ecosystem carbon uptake. In order to better understand the effect of thinning on carbon fluxes, we use the mathematical technique of four-dimensional variational data assimilation. Data assimilation provides a compelling alternative to more common statistical analyses of flux data as it allows for the combination of many different sources of data, with the physical constraints of a dynamical model, to find an improved estimate of the state of a system. We develop new observation operators to assimilate daytime and nighttime net ecosystem exchange observations with a daily time step model, increasing observations available by a factor of 4.25. Our results support previous analyses, with a predicted net ecosystem carbon uptake for the year 2015 of 426 ± 116 gCm−2 for the unthinned forest and 420 ± 78 gCm−2 for the thinned forest despite a model-predicted reduction in gross primary productivity of 337 g Cm−2. We show that this is likely due to reduced ecosystem respiration postdisturbance compensating for a reduction in gross primary productivity. This supports the theory of an upper limit of forest net carbon uptake due to the magnitude of ecosystem respiration scaling with gross primary productivity. Plain Language Summary Forests play an important role in the global carbon cycle, removing large amounts of CO2 from the atmosphere and thus helping to mitigate the effect of human-induced climate change. The response of forest ecosystem carbon dynamics to disturbance events (e.g., fire, felling, and insect outbreak) is not well understood. Many observations are currently made that give us insight into the processes occurring in forests; however, there are many processes that we cannot observe. Therefore, we must use mathematical models of forest carbon balance to better understand underlying behaviors; however, these models are often subject to large errors. In this study we use advanced mathematical techniques to combine information from both observations and model predictions of forest carbon balance, to improve current estimates and better understand the effect of disturbance from selective felling at a managed woodland. We find no significant change to the total forest carbon uptake after approximately 46% of trees are removed from the woodland. Our best modeled estimates suggest that this is due to photosynthesis being the main driver for ecosystem respiration, so that reducing the number of trees has reduced carbon uptake from photosynthesis but concurrently reduced respiration, meaning total carbon uptake is not significantly affected.