This thesis investigates biotic and abiotic regulation of photosynthesis and transpiration at the leaf and canopy scales in a dry-land Pinus radiata D. Don forest by combining gas exchange measurements with biophysical process-based models of photosynthesis, radiation transfer, and soil water balance. Responses of photosynthesis to leaf intercellular CO₂ concentration in two-year old P. radiata seedlings were measured at a range of temperatures and leaf nitrogen concentrations in order to quantify parameters describing photosynthetic capacity and temperature response in a biophysical model of C₃ photosynthesis. Increasing leaf temperature from 8 °C to 30 °C caused a four-fold increase in Vcₘₐₓ the maximum rate of carboxylation (10.7 to 43.3 μmol m⁻² s⁻¹), and a three-fold increase in Jₘₐₓ, the maximum electron transport rate (20.5 to 60.2 μmol m⁻² s⁻¹). Foliar nitrogen concentration (N) varied between 0.36 mmol g⁻¹ and 1.27 mmol g⁻¹, and there were linear relationships between N and both Vcₘₐₓ and Jₘₐₓ. Measurements made throughout the crown of a forest tree, where N varied from 0.83 mmol g⁻¹ near the base to 1.54 mmol g⁻¹ near the leader, yielded similar relationships. The leaf-level photosynthesis model was combined with a water balance model to successfully explain a seasonal pattern in stable carbon isotope composition (δ¹³C) measured within annual rings of P. radiata from two sites which differed markedly in annual water balance. Over two growing seasons there was good agreement between mean canopy-level cᵢ derived from the tree-ring δ¹³C data and modelled leaf-level cᵢ levels. The amplitudes of seasonal δ¹³C variation at the wet and dry sites were 1-2 %₀ and 4 %₀ respectively, and mean δ¹³C values from the wet site were 3 %₀ more ¹³C depleted than those from the dry site implying lower water-use efficiency (carbon assimilation per unit transpiration). Seasonal variation in carbon isotope discrimination of leaves in the canopy is therefore reflected directly in the δ¹³C of stem wood. A canopy photosynthesis model was developed by combining the leaf-level model with a model of canopy radiation transfer, and used as a framework to analyse a field experiment designed to quantify the response of photosynthesis and tree growth to a long-term reduction in illuminated leaf area. Shading the lower crown of two young forest trees reduced absorbed radiation and canopy photosynthesis by 11 and 9 % respectively in the first year. Nitrogen was translocated from current year foliage below the shade cloth to that above, and carbon partitioning to the branches increased at the expense of stem growth. In the second year, the effect of the shading on absorbed radiation, canopy photosynthesis and tree growth was less due to a reduction in shaded foliage proportional to total leaf area. Additionally, a prolonged period of soil water deficit during the summer of year 2 reduced photosynthesis, stomatal conductance and growth similarly in both shaded and control trees. Models which scale up processes from the leaf-level to the canopy can provide a framework to analyse and interpret field experiments.