Abstract
ABSTRACTOur objective is to describe a multi‐layer model of C3‐canopy processes that effectively simulates hourly CO2 and latent energy (LE) fluxes in a mixed deciduous Quercus‐Acer (oak–maple) stand in central Massachusetts, USA. The key hypothesis governing the biological component of the model is that stomatal conductance (gs) is varied so that daily carbon uptake per unit of foliar nitrogen is maximized within the limitations of canopy water availability. The hydraulic system is modelled as an analogue to simple electrical circuits in parallel, including a separate soil hydraulic resistance, plant resistance and plant capacitance for each canopy layer. Stomatal opening is initially controlled to conserve plant water stores and delay the onset of water stress. Stomatal closure at a threshold minimum leaf water potential prevents xylem cavitation and controls the maximum rate of water flux through the hydraulic system. We show a strong correlation between predicted hourly CO2 exchange rate (r2= 0.86) and LE (r2= 0.87) with independent whole‐forest measurements made by the eddy correlation method during the summer of 1992. Our theoretical derivation shows that observed relationships between CO2 assimilation and LE flux can be explained on the basis of stomatal behaviour optimizing carbon gain, and provides an explicit link between canopy structure, soil properties, atmospheric conditions and stomatal conductance.
Highlights
Eco-physiological processe:-. such as photosynthesis and lc;if energy balance. arc relatively well understood e Ecm..~1·sre111.1· renlt'I', Marine /Jiological La/111ra111ry
Whal n:mains elusive is a sound understanding or how these processes integrate over space and time. and interact within a community of plams. lmcre:-i in scaling. canopy processes derives in pan from question~ raised about the global carbon (C) cycle. and the nature ol the missing. terrestrial C si nk (Schimel 1995)
The model we present is based in part on a hypothesized spatial independence o f stomata! control, and precludes an aggregated approach
Summary
Eco-physiological processe:-. such as photosynthesis and lc;if energy balance. arc relatively well understood e Ecm..~1·sre111.1· renlt'I', Marine /Jiological La/111ra111ry. One approach to this problem uses models that allow scaling of processes at the leaf level to whole canopies Our goal in this paper is to describe
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