Oak leaf morphology is highly variable. Leaf shape in oaks has been shown to covary with climate, and leaf dissection is negatively correlated with hydraulic resistance. Such patterns may reflect local adaptation of genotypes or plastic responses to local environments. We tested two hypotheses regarding variation in leaf shape in Quercus lobata Née, a widely distributed oak tree endemic to California, using subsamples of trees from a large provenance trial involving source populations throughout the species range. First, we tested the hypothesis that variation in leaf dissection (perimeter · area^–1) and specific leaf area could be explained by adaptation to climate. If so, we predicted that the progeny of trees originating from contrasting climates would also contrast in leaf dissection or specific leaf area when grown in a common environment. Second, we tested the hypothesis that variation in leaf dissection is associated with photosynthetic rate. Because dissection affects hydraulic resistance and heat transfer, more dissected leaves are thought to experience lower water stress. If so, we predicted that individuals with more dissected leaves would maintain higher rates of photosynthesis than those with less dissected leaves, especially during hot and dry conditions. We found no association of leaf traits with maternal climate, but we observed a positive relationship between leaf dissection and photosynthetic rates. This result suggests variation in leaf shape may have functional consequences and influence how valley oaks cope with environmental stress in California's Mediterranean climate.