How an iconic desert tree survives extreme heat—and the unique risk it’s facing now

New research has found that the punishing summer temperatures and persistent drought conditions in much of Arizona and the Southwest are dealing a double whammy to trees attempting to regulate their own temperature, putting a critical part of the desert ecosystem at risk.

The study, published in October in Proceedings of the National Academy of Sciences, looked at the iconic Fremont cottonwood, which has great ecological importance because of its capacity to support other biota. It is also resilient; the trees can thrive in both the Phoenix summers and the Flagstaff winters.

As heat waves increase, however, a key aspect of the trees’ survivability—its mechanism to cool itself through its leaves—is increasingly at risk, said co-author Alexandra Schuessler, a two-time NAU alumna who is now a lab manager at the Desert Botanical Garden in Phoenix. It’s not just the heat causing stress on the trees; the other factor is the trees need sufficient water to cool their leaves, and between climate change and dams changing natural watersheds, water is harder to come by.

“Human-caused water limitation coupled with the increase in heat and drought throughout the Southwest is going to be very hard on these trees, which will in turn create habitat loss and stress on other species that depend on cottonwoods for shade; food, such as beavers; and nesting sites,” she said. “This paper and others like it continue to build a case that climate impacts and water resource management are going to drastically change our world unless we make some major changes.”

What the researchers found

The researchers used potted trees in the Desert Botanical Garden, which gave them the ability to control environmental factors, to study how plants coped with increasingly severe heat waves. They selected four populations of Fremont cottonwoods from different environmental conditions. During the month of July 2023, the hottest month ever recorded globally at that point, they attached sensors to the leaves to measure the trees’ temperatures.

They found that the trees still managed to cool their leaves below lethal thresholds using evaporative cooling, similar to how humans sweat to cool their skin on a hot day. Even in extreme heat waves, the Fremont cottonwoods survived. That’s great news, but there was one major caveat—the trees need access to adequate water supply, which is becoming harder to guarantee as the Southwest gets warmer, drier and more populous.

“Even when temperatures reached 118°F, which stressed both the trees and the research team, the trees were able to prevent overheating, especially the trees from the hottest locations from the lower Colorado River,” Schuessler said. “However, experimentally limiting water leveled the playing field, and trees from all four populations suffered from a combination of heat stress and drought.”

What does this mean?

For a water-loving tree like the Fremont cottonwood, these findings are concerning. The trees thrive when their roots are in water, which means we’ll lose them as both aboveground and underground streams dry up.

Schuessler pointed to Lake Havasu, which used to have a vibrant population of Fremont cottonwoods that have since died from limited water availability and extreme heat. The demise of that desert forest eliminated critical habitats for birds and other animals that rely on the trees for refuge from extreme heat. Even if the water returns, it’s unlikely that the trees will. In fact, the range of the Fremont cottonwood, which has been quite broad and diverse, likely will narrow in the coming years.

Human populations in Arizona and the Southwest are likely to see adverse effects from the loss of these desert forests as well, Schuessler said. Healthy trees cool streams, increase oxygen, provide shelter from the sun while hiking, rafting or otherwise doing outdoor recreation, help control erosion along waterways and increase biodiversity in our public lands.

“The average person, especially the average person in the desert, is often drawn to water and to sit beneath the shade of a tree watching the water flow,” she said. “Cottonwoods are an important part of the so-called ‘ribbon of green’ along our desert rivers that provide us with that shade.”

For the researchers, their next step is to plant trees from hotter conditions back into degraded areas to help restore those ecosystems. This research also laid the groundwork for future restoration and protection of these important ecosystems, Schuessler said.