Absorbed photosynthetically active radiation (APAR or
Fp.a.) is estimated from global solar radiation, assuming that
PAR 50% of solar radiation, and L; the utilized fraction
(Fp.a.u.) is calculated by reducing Fp.a. by an amount
determined by a series of modifiers that take values between
0 (system ‘shutdown’) and 1 (no constraint) to limit
photosynthesis. Constraints imposed on Fp.a. depend on the
degree to which stomatal conductance is limited by average
day-time D, subfreezing conditions (if daily temperatures are
below 0 8C it is assumed that there is no growth, i.e.,
Fp.a. = 0) and available soil water (ASW). The soil water
balance in any month depends on the ratio of precipitation
and stored water to transpiration, estimated with the Penman–
Monteith equation. The amount of water that can be stored in
the soil (soil water holding capacity) is a function of soil
texture and stand rooting depth.
Gross primary production (PG) is calculated by multiplying
Fp.a.u. by a canopy quantum efficiency coefficient (a), with a
maximum value (ac) set by the soil fertility rating (FR). It was
assumed in this analysis that ac increases linearly with FR from
a minimum of 0.38 to a maximum of 3.84 g C MJ1 Fp.a..ac is reduced when mean monthly temperatures are sub-optimal
for a given species.