Abstract A simulation model combining photosynthesis and transpiration for coniferous trees, DAYTRANS/PSN, was used to explore CO 2 and water exchange efficiencies under different seasonal climatic conditions. The simulation model first evaluated an eight-month photosynthetic season using measured meteorological data from a western Montana site. Elevation, slope, aspect, and region-related variations in precipitation, incoming shortwave radiation, and air temperature were then programmed into the meteorological data set and the model re-run a total of 42 times. In spite of the cool climate in western Montana, a three degree reduction in seasonal air temperature increased photosynthesis 15%, because less tree water stress developed. When seasonal precipitation was set to zero only an 11% reduction in photosynthesis occurred, illustrating that snowpack is the primary source of water for tree growth in Montana. Simulated annual photosynthesis accurately predicted some observed patterns of forest productivity in the Northern Rocky Mountains of the United States. In water-limited Montana, low elevation south slopes produced 14% less seasonal photosynthesis than corresponding north slopes. However, with precipitation equivalent to the Pacific Coast, lower south slopes produced 23% higher photosynthesis than north slopes, as energy, rather than water, became the factor limiting physiological activity.