MODELLING OF AIR CO 2 CONCENTRATION AND FLUX IN/ABOVE A BOREAL BLACK SPRUCE STAND

A complete multilayer model of vegetation-atmosphere exc hange has been developed for a boreal black spruce stand, taking into account re sponse of sunlit and shaded leaves to ambient conditions respectively, leaf clumping and nitrogen level in the leaves at different layers. The established model consists of one dimensional air diffuse meteorological equation for transfer of water, heat, CO 2 within and above the forest canopy, a two leaf submodel for leaf air exchange, and an exponential function for distribution of nitrogen level in leaves with height of canopy. Estimation of sunlit and shaded leaf area index was borrowed from Chen′s method, modified for layer-by -layer by the authors of this paper. Photosynthesis at leaf level was computed by combining Farquhar' s equations and CO 2 conductance equation. Soil temperature, moisture, and thermal and water fluxes between adjacent soil layers, obtained from ESES, were used for calculating exchanges of radiation, latent and sensible heat between soil surface and its immediately overlying canopy air. Data for model validation came from a southern study area (53°59′N, 105°07′W) of BOREAS (Boreal Ecosy stem -Atmosphere Study) located in Saskatchewan, Canada and beloging to temperate cl imatic zone. The involved data, just covering main growing season in 1994, were sensible heat flux, latent heat flux, CO2 flux which were all measured by eddy correlation method, and soil heat flux measured by thermal flow plate, CO 2 concen tration profile by LI 6262 IRGA, net radiation and other regular meteorological elements such as air temperature, humidity, wind speed, and air pressure. Measur ements of leaf photosynthesis, transpiration and stomata conductivity to CO2 were made by LI6200 portable photosynthesis system. The validation for physiological progresses of the leaf stomata conductance, photosynthesis and transpiration shows that there were good agreements and no marked systematic discrepancies bet ween the measurements and the estimates. The flux modeling indicates that sensible heat fluxes were slightly overestimated in comparison with observed ones, and between them a squared correlation coefficient (R 2) amounted to 0.71 bu t the difference fluctuated from time to time. In contrast to the sensible heat flux,estimates of latent thermal flux were a little lower than observed ones and R 2 r eached 0.78. CO2 flux simulation had the lowest R 2 of 0.65 and slightly underest imated when it goes over 7 μmol/(m 2?s). CO 2 concentration was of the biggest R2 of 0.90 although there were larger errors in the canopy air close to the forest floor, rising from omitting moss respiration. Analysis of these flux simulations leads to conclude that the complete multilayer model of vegetation-atmosphere resented achieves good results and is helpful to better investigat ion into vegetation-atmosphere exchange process because of its well dealing wit h the leaf radiance absorption, nitrogen level and clumping.