Modeling the Exchange of Energy and Matter for a Forest-Clearing Transition Area with Taking Particular Account of Coherent Structures

Simulating the exchange of energy and matter of horizontally heterogeneous terrain with forest-clearing transition utilizing a one-dimensional SVAT model (Soil-Vegetation-Atmosphere Transfer) represents a multi-faceted challenge. On the one hand, turbulent exchange processes of forest are characterized by counter-gradient fluxes caused by coherent structures. On the other hand, the edge of a forest represents heterogeneity, inducing secondary structures and leading to a possible increase of turbulent exchange. Additionally, clearings are characterized by alternating vegetation, which complicates adequate description by a one-dimensional model.

The present study utilizes the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model to simulate the exchange processes of a spruce forest and a clearing. ACASA is an advanced SVAT model with an implemented third-order closure, capable of resolving the coherent flux contribution. This represents an advantage over K-theory and first-order closure most commonly utilized by models. By means of extensive micrometeorological measurements, the third intensive observation period of the EGER project (ExchanGE processes in mountainous Regions) in summer 2011 offers the database for the model input files as well as comparative values for the simulation results. Additionally, plant physiological measurements for spruce forest and clearing were used to adapt photosynthetic processes in the ACASA model to the Waldstein-Weidenbrunnen (FLUXNET site, DE-Bay) measuring area.

The turbulent exchange of the clearing is realized by a tile-approach considering the footprint on the existing measuring point. Sensible and latent heat fluxes as well as net ecosystem exchange were simulated with ACASA. The model results of turbulent exchange for spruce forest and clearing were not solely evaluated by measured turbulent fluxes. Detection software, based on wavelet analysis, has determined the contribution of coherent structures to the turbulent flux as well their vertical coupling inside the forest and at its edge. Consequently, model deviations were checked for their correlation to specific coupling conditions and shortcomings of ACASA by simulating coherent exchange were evaluated.