A computer simulation model, DAYTRANS/PSN, has been used to assess microenvironmental control of tree photosynthesis and transpiration as related to topography. Meteorological data were recorded on three paired north-south slopes in western Montana for the growing season of 1983 and used to drive the model. Although south slopes were only 0.7 to 1.7° warmer with 6.1 to 9.7% lower relative humidity than the opposing north slope, seasonal potential evapotranspiration was 17% to 37% higher. Because of earlier snowmelt and higher springtime transpiration, the south slopes consumed their available soil water more quickly and endured almost 1 month more water stress than the north slopes. Consequently, the north slopes had 33 to 45% higher seasonal photosynthesis than the south slopes. This model prediction matches observed patterns of forest productivity in the mountains of western Montana, USA.