The Fremont cottonwood (Populus Fremontii) is a keystone species, but the number of Fremont cottonwoods is decreasing rapidly primarily due to the grazing of domestic cattle. Planting Fremont cottonwoods is an important part of projects to restore riparian environments in the American southwest. Dozens of Fremont cottonwoods with different physiological properties were grown in gardens across the state of Arizona at various elevation and latitudes. Over the course of two years, leaves were collected from the Fremont cottonwoods in the common gardens. These leaves were dried, ground, and tested with mass spectroscopy to determine their carbon-12 to carbon-13 ratios. The carbon-12 to carbon-13 ratio of the leaves of a tree is a robust indicator of that tree’s water-status, how dehydrated that tree is. This is true because trees intake carbon dioxide for photosynthesis, but they prefer to use carbon-13. Photosynthesis also requires water. When trees are willing to expend water to perform photosynthesis with carbon-12, it means that water is abundant, and when trees are not willing, it means that water is scarce. Considering the carbon-12 to carbon-13 ratio of these trees and climatic data over the course of two years will provide a detailed picture of which physiological traits are most advantageous in different climates. This information will make environmental restorations more effective, which will help alleviate the ecological and economic harms of raising cattle like larger wildfires and the endangerment of native species.