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Climate change threatens iconic cottonwood trees in Arizona, research shows

Cottonwood trees
Getty Images
Cottonwood trees in Arizona.

Fremont cottonwoods are found along many rivers and streams across Arizona. But new research shows the iconic trees are struggling to survive as the climate warms.

In a paper published in October in the journal Proceedings of the National Academy of Sciences, a team of researchers from the Desert Botanical Garden, Northern Arizona University, and other universities said the changing climate poses a number of risks to the trees. That’s concerning because cottonwoods are so critical to ecosystems of the Southwest.

“They attract a lot of biodiversity,” said Gery Allan, professor of biology at NAU, who was among the paper’s authors. “Everything from microbes in the soil, to insects that colonize the trees, to birds that nest in the trees, to beavers that cut them down and use them to build dams.”

Researchers observed cottonwoods taken from different areas of the state – some samples came from higher elevations and cooler climates, and some came from lower, warmer areas.

They found cottonwoods are capable of surviving extreme heat. In fact, trees that had come from warmer parts of the state were more successful at keeping cool in hot weather, suggesting they had already begun adapting to rising temperatures.

“We’re hoping that the strategies that they’re using currently to adapt to very hot temperatures will allow these trees to persist in as many areas as possible, but we’ve also seen significant die-off in a lot of areas,” Allan said.

Allan said the trees’ ability to use evaporative cooling to keep their leaves cool in hot weather is highly dependent on water. The paper said even modest reductions in soil moisture resulted in rapid damage to leaves and stems. The risk is significant because heat and drought often coincide.

Allan said if cottonwoods begin to die off, the impacts to Arizona will be broad.

“If we start to lose these trees then entire riparian ecosystems will begin to fail and all of the associated species will begin to disappear along with those ecosystems,” Allan said.

Katherine Davis-Young is a senior field correspondent reporting on a variety of issues, including public health and climate change.