MODELING THE SOIL-PLANT-ATMOSPHERE CONTINUUM IN A QUERCUS-ACER STAND AT HARVARD FOREST - THE REGULATION OF STOMATAL CONDUCTANCE BY LIGHT, NITROGEN AND SOIL PLANT HYDRAULIC-PROPERTIES/

Our objective is to describe a multi-layer model of C-3-canopy process es that effectively simulates hourly CO2 and latent energy (LE) fluxes in a mixed deciduous Quercus-Acer (oak-maple) stand in central Massac husetts, USA, The key hypothesis governing the biological component of the model is that stomatal conductance (g(s)) is varied so that daily carbon uptake per unit of foliar nitrogen is maximized within the lim itations of canopy water availability, The hydraulic system is modelle d as an analogue to simple electrical circuits in parallel, including a separate soil hydraulic resistance, plant resistance and plant capac itance for each canopy layer, Stomatal opening is initially controlled to conserve plant water stores and delay the onset of water stress, S tomatal closure at a threshold minimum leaf water potential prevents x ylem cavitation and controls the maximum rate of water flux through th e hydraulic system, We show a strong correlation between predicted hou rly CO2 exchange rate (r(2) = 0 . 86) and LE (r(2) = 0 . 87) with inde pendent whole-forest measurements made by the eddy correlation method during the summer of 1992, Our theoretical derivation shows that obser ved relationships between CO2 assimilation and LE flux can be explaine d on the basis of stomatal behaviour optimizing carbon gain, and provi des an explicit link between canopy structure, soil properties, atmosp heric conditions and stomatal conductance.