A complex simulation model, MAESTRO, has been developed and validated against field measurements in plantation in both Scotland and Australia. It has been shown that MAESTRO can reasonably predict the daily course of PAR (photosynetically active radiation) transmittance at points below the canopies of radiata pine and Sitka spruce plantations. 1. Four structural properties of the Sitka spruce tree crown have been identified and evaluation in relation to PAR absorption, photosynthesis and transpiration. They are : the total amount leaves within the tree crown, leaf inclination angle distribution and crown shape. The total area of leaves and their spatial distribution within the tree crown are ranked as the two most significant properties. 2. Among the four different structural properties studied, crown shape is least important for PAR absorption, photosynthesis and transpiration. For the Sitka spruce stand studied, transpiration was most sensitive to the total area of leaves within the tree crown, whereas photosynthesis can be significantly affected by the leaf angle distribution. The influence of crown structure also depends on the beam fraction, radiance of diffuse radiation, incident angle of beam radiation. 3. Application of MAESTRO has shown the importance of the spatial distribution of leaf area density distribution and the age structure of the leaf population within a dense Sitka spruce canopy for the radiation regime and for photosynthesis. It has been found that the non-uniform canopy structure. The actual leaf age structure within a dense Sitka spruce canopy has been compared with the leaf age structure that is optimal for maximising the PAR use efficiency of the canopy and it has been found that PAR use efficiencies of both canopies are very close. 4. The non-uniform structure of a tree crown has been described using a two-dimensional leaf area density distribution function within the tree crown. It was found that the spatial distribution of leaf area density and the age structure of the leaf population within the crowns of radiata pine trees were significantly affected by water stress and nutrient deficiency during the growing season. 5. The radiance distribution of diffuse radiation from the sky and the beam fraction of the incident radiation also affect PAR absorption, photosynthesis and transpiration by altering the uniformity of the radiation regime within the tree crown. The light use efficiency of the canopy decreases with the increase in beam fraction. 6. The stomatal and photosynthetic properties of the shoots of Sitka spruce were measure in an open path gas exchange system to parameterize a biochemical photosynthesis model. It was found that co-limitation of photosynthesis plays an important role in the PAR responses of both an isolated shoot and of a canopy. It was shown that co-limitation of canopy photosynthesis enables the canopy to use a high flux density of incident PAR efficiently. 7. The dry matter production of the above-ground biomass of a Sitka spruce stand was studied in relation to PAR absorption to test the Monteith hypothesis. It was found the relationship between the above-ground dry matter production and PAR absorption is approximately linear. The slope of this linear relationship is affected by nitrogen fertilization, but not by thinning for the Sitka spruce stands studied. Crown structure plays an important role in the processes or radiation transfer, photosynthesis and transpiration within the tree crown. Detailed descriptions of the important structural properties of the tree crown are necessary for adequate understanding of canopy processes, such as photosynthesis. Co-limitation of crown photosynthesis, determined by the structure of all the tree crowns in the stand, results in an efficient use of the incident solar radiation by the trees in the stand. Photosynthesis of the forest canopy is roughly proportional to the incident PAR. PAR absorption is the primary determinant of dry matter production of the above-ground biomass for the Sitka spruce stand studied.