In order to understand vegetational gradients which develop in response to environmental gradients, the physiological capabilities of each species must be examined in relation to the observed environmental gradient. The distributions of three temperate zone shrub species which occupy different positions on the spur ridges of Brush Mountain may be influenced by their tolerances to light and moisture. Greenhouse studies indicate that <i>R. maximum</i> has a 60 % reduction in photosynthesis at water potentials below -1.0 MPa. <i>R. nudiflorum</i> has only a 30 % reduction and <i>K. latifolia</i> has less than 10 % reduction. Seasonal pressure volume curve determinations, conductance measurements, and water potential measurements of plants in the natural environment indicated that photosynthesis is not affected by water potential in <i>K. latifolia</i>. <i>R. maximum</i> has a significant reduction in conductance during drought which may limit photosynthesis. R. nudiflorum appears to begin senescence prior to severe drought in this area. Greenhouse experiments of adaptability to increasing light intensity show that <i>K. latifolia</i> and <i>R. nudiflorum</i> can significantly increase their light saturation point and maximum rate of photosynthesis in high light environments. <i>R. maximum</i> does not increase its photosynthesis rate in high light environments and appears to undergo chloroplast degradation when grown in high irradiance. Gradient analysis of vegetation on Brush Mountain shows that <i>R. maximum</i> reaches its highest importance in low light, high moisture sites. <i>K. latifolia</i> is most important in low moisture, high irradiance sites. <i>R. nudiflorum</i> is infrequent in the study area but occurs mostly in moderate moisture and irradiance sites. The distributions of <i>R. maximum</i> and <i>K. latifolia</i> appear to match their experimentally determined physiological tolerances. The results for <i>R. nudiflorum</i> are inconclusive. / Master of Science