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

Abstract

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 g C m−2 for the unthinned forest and 420 ± 78 g C m−2 for the thinned forest despite a model-predicted reduction in gross primary productivity of 337 g C m−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.