Representing Sub-Plot Canopy Heterogeneity Improves Model Prediction of Net Ecosystem Exchange in a Mixed-Deciduous Forest

Renato P M Frasson, Ohio State University Main Campus, Civil, Environmental & Geodetic Engineering, Columbus, OH, United States, Gil Bohrer, The Ohio State University, Department of Civil, Environmental and Geodetic Engineering, Columbus, United States, David Medvigy, University of Notre Dame, Biological Sciences, Notre Dame, IN, United States, Christoph S Vogel, University of Michigan Biological Station, Ann Arbor, United States, Christopher Michael Gough, Virginia Commonwealth University, Biology, Richmond, VA, United States and Peter Curtis, Ohio State University Main Campus, Columbus, OH, United States
Abstract:
Canopy density and composition may vary within an eddy covariance tower’s footprint in response to small-scale topographic features, biotic interactions such as herbivory, local disturbances, etc. We are investigating how different representations of canopy heterogeneity influence predictions of net ecosystem CO2 exchange in a mixed-deciduous forest by an age/plant functional type structured ecosystem model.

Our study area is located at the University of Michigan Biological Station (UMBS) where two eddy covariance towers and periodic tree censuses provide a rich long-term record of ecosystem structure, weather, and carbon uptake. Meteorological measurements collected at the US-UMB AmeriFlux tower served to force, optimize, and evaluate the Ecosystem Demography model version 2 (ED2), while tree census information was used to initialize ED2.

To test the influence that representing canopy heterogeneity has on model-tower agreement, we ran a set of ED2 site-level simulations with an increasing number of sub-grid patches. The first simulation, which we call ‘aggregated’, had one large patch explicitly containing all trees. The aggregated canopy represents a case where different size cohorts of each plant functional type are distributed homogeneously throughout the plot with uniform stem density. Six other simulations represented patch-level canopies with varying degrees of heterogeneity, ranging from 5 to 64 sub-plot patches; each patch represented from one to several of the 0.1 ha tree census plots.

A preliminary comparison of the aggregated and the 20-plot heterogeneous simulations showed that including patch-level heterogeneity in the canopy description improved model prediction quality. For example, compared to the single-patch, aggregated simulation, including 20 sub-plot patches improved model bias in the estimated accumulated 5-year net ecosystem exchange from 17% to 5%, which is smaller than our tower observation uncertainty.

As a result of this study, we will identify methodologies that lead to better simulation designs by allowing the user to employ sub-plot census information to evaluate how much detail regarding canopy heterogeneity is needed to most accurately describe the total carbon flux observed in a forest.

Back to: Biosphere-Atmosphere Greenhouse Gas Fluxes in Terrestrial Ecosystem III