Plant water, nutrient, and carbon relations were evaluated in the co-occurring evergreen and deciduous species Quercus agrifolia and Quercus lobata, to determine how differences in lifeform affect resource utilization. Although standing leaf biomass was more than twice as high in Q. agrifolia as Q. lobata, the total quantity of nitrogen and phosphorus cycled through the litter and throughfall annually was similar. During this study, populations of the specialist herbivore Phryganidia californica increased dramatically, leading to complete defoliation of both oak species and profound changes in nutrient cycling. Q. agrifolia was relatively more severely impacted by this herbivore than Q. lobata because of differences in the timing of the defoliation and a higher leaf area index in the evergreen. Leaf conductances were generally higher in Q. lobata than Q. agrifolia, however diurnal xylem water potentials were similar. Differences in the stomatal response to an increased leaf-air vapor concentration gradient in the evergreen Q. agrifolia lead to a more conservative use of water than in Q. lobata. Leaf photosynthesis was measured in the field with an environmentally controlled cuvette system to determine carbon gain as a function of the incident photosynthetically active photon flux, leaf temperature, leaf-air vapor concentration gradient and leaf nitrogen content. These data were combined with field measurements of the daily and seasonal course of temperature, light, humidity and other environmental variables in a biochemically-based leaf and canopy photosynthesis model. The model suggests that differences in the canopy characteristics compensate for inherent leaf level differences in photosynthesis in these species, and shows that when water or nitrogen are limiting, a more productive canopy can be maintained by the evergreen than the deciduous species.