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  • The following description is taken from this source.SPA: Soil-Plant-Atmosphere canopy model. Amulti-layer simulator of carbon and water fluxes in forest stands. The model links leaf-level photosynthesis and transpiration with a model of stomatal conductance (gs) that optimizes daily C gain per unit leaf N, within the limitations of canopy water storage and soil-to-canopy water transport. The model assumes that maximum carboxylation capacity and maximum electron transport rate are proportional to foliar N concentration, and that foliar respiration is also a function of N concentration and leaf temperature. The model does not treat each species in the ecosystem individually, but instead treats the system as a single functional type, described by vertical variations in light absorbing area (LAI), photosynthetic capacity (foliar N), and plant hydraulic properties.The model has been modified to introduce a revised radiation subroutine that calculates sun-lit and shaded fractions of the foliage in each canopy layer. Sun-lit fractions are lit by both incident and diffuse radiation, while the shaded fractions receive only diffuse radiation.