Fremont cottonwoods form a ribbon of green along Arizona’s rivers and streams. They have heart-shaped leaves that turn golden in fall and fluffy, cotton-like seeds. This fast-growing species provides critical habitat for birds and other wildlife.
Naturally occurring below 6,600 feet, cottonwoods are also widely cultivated as ornamental trees and can survive desert summers as well as high-country winters.
To cope with heat, their leaves use evaporative cooling, a process similar to how humans sweat to cool their skin on hot days. But this only works when their roots have access to plenty of water.
During the July 2023 heatwave, researchers at the Desert Botanical Garden in Phoenix conducted an experiment with Fremont cottonwoods from four genetically distinct populations. To test their resistance to heat and drought, half the trees were well watered while water was limited for the others.
During day after day of extreme heat — which stressed both the trees and the research team — temperature sensors attached to the leaves were monitored closely.
Trees with a limited water supply saw a significant leaf die-off due to heat in all four genotypes, especially for trees sourced from cooler locations.
But the well-watered trees managed to keep their leaves cool, suffering minimal damage.
The study shows that predicted future increases in heat, combined with persistent drought, are likely to deal a double whammy to cottonwoods across Arizona if aboveground and underground streams dry up.
This Earth Note was written by Diane Hope and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
