A simple model of light and water use evaluated for Pinus radiata.

An existing model of light and water use by crops (RESCAP) was adapted and evaluated for trees. In the model, growth on any given day is determined either by the amount of intercepted radiation (by means of the light utilization coefficient, epsilon) or by the maximum rate of water extraction by roots (a function of root biomass and soil water content). In either case, transpiration and growth are related by the water-use efficiency (q), which is inversely proportional to the daily mean saturation vapor pressure deficit (D). The model was applied to two Pinus radiata (D. Don) stands (control (C) and fertilized (F)) growing near Canberra, Australia, using data collected during the Biology of Forest Growth experiment (1983-1988). For both stands, predicted and measured soil water contents were in close agreement (r(2) > 0.9) over a 4-year period involving several wet-dry cycles. The parameter combination epsilon/qD was estimated to be 0.28 and 0.26 kg H(2)O (MJ total)(-1) kPa(-1) for the C and F stands, respectively. Because of the close physiological link between water use and CO(2) uptake, the results suggest that tree growth may be realistically simulated by simple models based on conservative values for epsilon and qD.