Two photosynthesis models are used, depending on the photosynthetic pathway of the vegetation: a model for C3 plants by Farquhar et al. [1980], and an adaptation of this model for C4 plants developed by Collatz et al. [1992]. In both cases, the net canopy leaf assimilation rate, and the canopy-integrated rate of leaf or "dark" respiration, RdiC, are calculated, with dark respiration assumed to be proportional to the maximum carboxylation rate (with a factor of 0.011 or 0.042 at 25°C, for C3 or C4 plants, respectively), a limiting rate set by the amount of primary fixating enzymes present in the leaf. This maximum carboxylation rate and other parameters determining vegetation productivity are prescribed by fixed values for each vegetation type [Beerling and Quick, 1995]. PAR absorption by leaves is calculated with the two-stream approximation [Sellers, 1985] for three layers, using direct and diffuse PAR computed as in Weiss and Norman [1985]. The time step of this model part is one hour.

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