Matching scales of eddy covariance measurements and process-based modeling – Assessing spatiotemporal dynamics of carbon and water fluxes in a mixed forest in Southern Germany

Abstract. Eddy covariance (EC) measurements are a backbone of ecological research and have provided valuable insights into the variability of carbon and water fluxes in different ecosystems and under varying environmental conditions. Since these measurements are integrative and weighted over changing areas (footprint), species-specific information cannot be easily derived except for extended monocultures. However, EC sites are increasingly established in mixed forest stands which are considered to be more resilient under changing environmental conditions. This imposes the question of how species-specific responses can be derived, and how the magnitude of fluxes originating from temporally varying flux footprints predictions (FFPs) might provide insights into species-specific responses.

At a site in the Black Forest (southwestern Germany), which mainly consists of a mix of mature beech and Douglas fir trees, we investigate how EC flux measurements depend on different FFP areas and how species-specific contributions to gas exchange can be disentangled. We applied an ecosystem model that has been calibrated from EC measurements at various sites with beech- and Douglas fir monocultures, and evaluated it with data of soil water content and soil respiration taken at homogeneous parts of the investigated mixed forest site. Then we compared hourly aggregated measurements of net carbon exchange (NEE) and evapotranspiration (ET) with model simulations under four configurations: (i) pure beech, (ii) pure Douglas fir, (iii) a static weighted average of both species, and (iv) a dynamic weighted average based on footprint variations.

The results show that weighted combinations of the two species generally provide a better match with hourly EC measurements than single-species simulations, while differences between static and dynamic weighting approaches remain relatively small. Nevertheless, specific-species responses to the environment can be significantly different during transitional periods such as autumn and spring when physiological differences between Douglas fir and beeches are most pronounced. We demonstrate that considering these differences is particularly important for gap-filling EC measurements and thus for determining annual carbon and water budgets. We herewith demonstrate that EC measurements over mixed forests provide important model evaluation information and that species-specific modeling is essential for untangling and distributing the underlying species-specific ecosystem dynamics.