Modeling the effects of increasingly heterogeneous canopy environments on flux dynamics using high-resolution, forest-resolving large-eddy simulations

Turbulent eddies control the flux of carbon, water and other gases between forested environments and the atmosphere. Inside the canopy, eddy correlation length is very small and surface heterogeneity due to tree-crown structures occurs at these scales. Computer simulations, particularly Large-Eddy Simulations (LES), provide the foundation to test the sensitivity of flux exchange and turbulent mixing to small scale processes, such as successional- or disturbance-driven changes to canopy structure. At the Forest Accelerated Succession ExperimenT (FASET), we disturbed 39 ha of forest by girdling all canopy-dominant early-successional aspen and birch trees, leading to a large mortality event, followed by a shift in forest structure that is typical of a more mature successional stage. We use the Regional Atmospheric Modeling System (RAMS)-based Forest Large-Eddy Simulation (RAFLES), and the more dynamic RAFLES- Ecosystem Dynamcs (ED2) model, to investigate the consequences of increasingly heterogeneous forest environments to canopy-atmosphere exchange. RAFLES-ED2 resolves multi-layered light attenuation and vegetation and surface heat, vapor and CO2 fluxes and includes a multi-layered soil column under each atmosphere-vegetation column, as opposed to the single-layered soil-vegetation model in RAFLES. The model environment was determined by remote sensing of the actual forested area of interest using airborne Light Detection and Ranging (LiDAR) measurements and eddy-flux gas exchange measurements at two neighboring AmeriFlux eddy-flux towers, the manipulated site (US-UMd) and its undisturbed control (US-UMB) both at the University of Michigan Biological Station (UMBS) cluster site. We find changes to observed domain-averaged flux dynamics with increasingly heterogeneous canopy environments and a more dynamic atmosphere-vegetation model, as well as changes to simulated small-scale fluxes, which cannot be directly measured from flux towers.