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Climate means and variability are shifting rapidly, leading to mismatches between climate and locally adapted plant traits. Phenotypic plasticity, the ability of a plant to respond to environmental conditions within a lifetime, may provide a buffer for plants to persist under increasing temperature and water stress. We used two reciprocal common gardens across a steep temperature gradient to investigate plasticity in six populations of Fremont cottonwood, an important foundation tree species in arid riparian ecosystems. We investigated two components of leaf hydraulic architecture: leaf venation and stomatal morphology, both of which regulate leaf water potential and photosynthesis. These traits will likely affect plant performance under climate stressors, but it is unclear whether they are controlled by genetic or environmental factors, and whether they respond to the environment in parallel or independent directions. We found that: 1) Populations had divergent responses to a hotter growing environment, increasing or decreasing vein density. 2) Populations showed surprisingly independent responses of venation vs. stomatal traits. 3) As a result of these different responses, plasticity in hydraulic architecture traits was not predictable from historic climate conditions at population source locations, and often varied substantially within populations. 4) Hydraulic architecture was clearly linked to growth, with higher vein and stomatal density predicting greater tree growth in the hottest growing environment. However, higher plasticity in these traits did not increase average growth across multiple environments. Thus, P. fremontii populations and genotypes vary in their capacity to adjust their leaf hydraulic architecture and support growth in contrasting environments, but that this plasticity is not clearly predictable or beneficial. Identifying genotypes suitable for future conditions will depend on the relative importance of multiple traits, and on both evolutionary and ecological responses to changing temperature and water availability.