Monoterpene fluxes estimated using canopy models for a Pacific northwest old growth forest

Scaling monoterpene emissions from leaves to canopies depends on an array of environmental variables, not to mention the dynamics of the forest canopy, the species composition and the forest biomass. Over the past 20 years, field studies to measure biogenic emissions have increased, providing a large dataset of both leaf-level and canopy scale emission data. Efforts to accurately scale-up leaf-level emission data have been focused with improved leaf-level emission data and more detailed canopy scale models. The efforts associated with more detailed canopy scale models are measured in this paper with a comparison between a simple forest canopy model (Biogenic Emission Inventory System, BEIS) and a more advanced vegetation/atmosphere exchange model (UCD Advanced Canopy-Atmosphere-Surface-Algorithm model, ACASA). The BEIS canopy model is a first order approach to address the effects of the canopy upon the leaf environment. Simple scaling functions based upon canopy height and vertical biomass distribution are utilized in order to relate the vertical profiles of light, temperature, relative humidity, and wind speed in the canopy to above-canopy conditions. ACASA on the other hand is a multi-layer canopy-surface-layer model that solves the steady-state Reynolds-averaged fluid flow equations to the third order.

The models are tested using the Wind River Canopy Crane Research Facility (WRCCRF) site, which is an old-growth Douglas fir/western hemlock forest located in the Pacific Northwest. During the summer of 1998, branch level monoterpene emissions were measured, and eddy-covariance derived CO2 and H2O fluxes were recorded. This dataset provides a standard monoterpene emission rate for the two predominant species and half-hour averaged environmental parameters, including CO2 and H2O canopy fluxes. A comparison of the environmental parameters (in particular leaf temperature) for each model with observational data will provide insight into the reliability of the canopy scale monoterpene emissions.