Vertical differentiation in canopy environmental variables and exchanges of energy and mass in the soil-vegetation-atmosphere continuum are physically and physiologically interrelated processes. While the description of vertical differentiation in the light environment has become a routine part of canopy process modeling, temperature, water vapor partial pressure and carbon dioxide concentration are often assumed to be constant over the canopy depth. Yet these variables exhibit vertical variations in most canopies. Since plant physiological activities are sensitive to these factors, their vertical variations could have important impact on energy and mass exchanges between vegetation and the atmosphere. In this study, a multilayer canopy process model is developed. The model is designed to be applied to both one-story and two-story canopies. It first predicts profiles of temperature, water vapor and carbon dioxide partial pressures in plant canopies by using the localized near-field (LNF) theory. Then from these predicted profiles, exchanges of sensible heat, water vapor and carbon dioxide in each layer of the canopy are computed. Finally, canopy-level fluxes are obtained by integrating these exchanges over the canopy depth.
The model was tested against measurements from a two-story boreal aspen forest. The tests included diurnal cycles of canopy net radiation, sensible heat flux, water vapor flux, carbon dioxide flux, friction velocity, and profiles and diurnal cycles of air temperature, water vapor partial pressure and carbon dioxide concentration. The flux partitioning results showed that exchanges in this two-story aspen forest were largely controlled by the upperstory even through its leaf area index was smaller than that of the understory. However, the degree of dominance by the upperstory depended on what type of flux was considered. For example, the canopy sensible heat flux was overwhelmingly contributed by the upperstory while its role in determining canopy-level water vapor and carbon dioxide fluxes was significantly reduced due to much increased contributions from the understory. The functions of other components of the forest can not be ignored. For example, a significant amount of canopy carbon dioxide assimilation was balanced by soil and stem respiration during the daytime while during the nighttime, soil and stem respiration was much larger than the foliage respiration.