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 det ermined either by the amount of intercepted radiation (by means of the light utilization coefficient, epsilon) Or by the maximum rate of wat er extraction by roots (a function of root biomass and soil water cont ent). In either case, transpiration and growth are related by the wate r-use efficiency (q), which is inversely proportional to the daily mea n 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 m easured soil water contents were in close agreement (r(2) > 0.9) over a 4-year period involving several wet-dry cycles. The parameter combin ation epsilon/qD was estimated to be 0.28 and 0.26 kg H2O (MJ total)(- 1) kPa(-1) for the C and F stands, respectively. Because of the close physiological link between water use and CO2 uptake, the results sugge st that tree growth may be realistically simulated by simple models ba sed on conservative values for epsilon and qD.