Three Nelder plots of 3-year-old red alder (Alnus rubra Bong) were used for this study at the Cascade Head Experimental Forest, Oregon in the growing season of 1988 at an elevation of 330 meters. Each Nelder plot represented a range of densities from 238 to 101,219 trees per hectare. The objectives of this study were to describe the seasonal and diurnal trends in leaf water potential and stomatal conductance of red alder growing at different densities, and how environmental parameters affect these physiological measures of plant water relations. The following results were observed: (1) Predawn leaf water potential and noontime stomatal conductance of red alder growing at high densities were lower than that of red alder growing at lower densities in the late part of the growing season. This reduction became severe as the season progressed. (2) Tree density did not seem to affect leaf water potential of red alder during the daytime. (3) The optimum temperature for red alder stomatal conductance was from 18 to 27°C. Stomata remained widely open when photosynthetically active radiation levels were as low as 100 uE m-2s-1. Stomatal conductance was related to vapor pressure deficit differently at different times during the growing season. Stomatal conductance increased as VPD increased early in the growing season, but decreased as vapor pressure deficit increased from 0.6 to 2.0 KPa later in the growing season. (4) Leaf water potentials did not seem to limit plant stomatal conductance in the early of the growing season. But plants responded to close their stomata further when leaf water potential was lower than -1.2 MPa in the late part of the growing season. (5) Leaf water potential and stomatal conductance could be predicted from each other, and could be predicted from environmental parameters of which vapor pressure deficit was the most effective.