Even the Sonoran Desert is threatened by climate change

The same climate changes known to be reshaping mountain ecosystems in places like the Alps and Yosemite also are driving alarming new patterns in the Sonoran Desert near Palm Springs, according to the latest findings from a long-running study by UC Riverside.

If temperatures continue to rise and droughts continue to become more severe, the study suggests that portions of the Sonoran and similar deserts someday could become barren, with little plant or animal life.

“These ecosystems are incredibly fragile, actually,” said Tesa Madsen-Hepp, an ecology doctoral candidate at UCR and first author of the study. “They’re not super resilient, and they are reaching their limits.”

The findings, which track changes measured over several decades, are surprising to some scientists who had assumed that deserts and other dryland ecosystems would be resilient to more extreme heat and prolonged drought. Instead, Madsen-Hepp said that unless we get greenhouse gas emissions under control, and stop or reverse the current global warming trajectory, the planet is on track to create stretches of desert that are “completely collapsed.” Such a change, she added, would affect humanity “in a lot of different ways.”

“Not just in terms of losing species that we love. But by changing how nutrients and water are being cycled and filtered through those ecosystems.”

Locally, such trends may be hitting the Sonoran Desert first, since it is one of the hottest and driest stretches of North America. But if warming trends continue, Madsen-Hepp said there’s no reason to suspect the pattern won’t also play out in parts of the neighboring Mojave Desert and in drylands globally, which make up some 45% of land on Earth.

“Other dryland ecosystems are likely headed in this direction if we continue to see rising temperatures and greater atmospheric demand on the plants,” she said. “The timing of this ecosystem shift change will probably vary. But I think that we could expect to see similar things in general as these ecosystems reach similar levels.”

There’s solid research to show how climate change is forcing plant populations that grow in temperate climates to adjust in all sorts of ways. Trees around the world are moving further north and west, and they’re growing faster but dying younger. Some plants are moving to lower elevations, where climate change is bringing torrential rains to once-dry valleys, while other plants are creeping up mountain slopes in search of cooler temperatures and steadier moisture.

This global redisruption of species also is expanding the risk of diseases, such as malaria, along with crop-threatening funguses and pests. That, in turn, is leading to more food insecurity, which can turn residents of some communities into climate refugees.

While rising temperatures are expected to make desert climates tougher for humans to tolerate, Madsen-Hepp said desert plant life was assumed to be almost immortal. But after her full-time crew of four spent six months gathering 84 football fields worth of data she no longer believes that to be true.

The newer work was possible thanks to botanist Jan Zabriskie, who established a study area while working for the UC system in 1977 at the Boyd Deep Canyon Desert Research Center. Zabriskie stretched out a tape measure across 400 meters, documenting every plant species that touched the line. That process was repeated 21 times, with new linear transects every 150 meters, starting from the desert scrub ecosystem of the low-lying Coachella Valley, moving up through the pinyon-juniper woodlands into the chaparral ecosystem, and finishing with the coniferous forest some 8,000 feet up.

Another research team repeated those plant surveys in 2008. And Madsen-Hepp’s team did it again in 2019, with their findings comparing the changes since 1977 recently published in the journal “Functional Ecology.”

One surprising finding was that populations of species long thought to be very hardy, such as California juniper and pinyon pines, are shrinking or shifting to higher elevations much like the changes seen in alpine forests. But even higher up, Madsen-Hepp said, these species don’t appear to be thriving. Instead, with climate warming, she said, “they’re basically reaching their physiological thresholds.”

Another surprise was that in the years since the previous reports, some higher-elevation  plant species have moved lower, to hotter parts of the desert. Shorter, shrub-style plants, such as brittlebrush and ocotillo, are replacing those pines and the newer arrivals have shallower root systems, meaning they can grow faster and without a need for deep soil water. Such water increasingly is scarce in the wake of long droughts followed by more volatile rains, the weather pattern that has hit California this winter.

That shift in plant life is bad news for several reasons, Madsen-Hepp said.

First, such shrubs store less carbon than the pines and other plants they’re replacing. In that way, much like the findings of another recent study out of UCR, climate change appears to be fueling itself.

Second, Madsen-Hepp said broader research shows this is likely the last stage before a shift to bare ground. Since the shrubs replacing hardier plants in these areas don’t live as long, or have the same adaptive strategies, she said the climate can become too hot and dry for their long-term survival.

“We are really reaching the edge of a habitable ecosystem for plants to live in,” she said. “So I think that’s pretty alarming.”

Next up, Madsen-Hepp is using the same data to dig deeper into how plant species in this slice of the Sonoran Desert interact with each other, as they cooperate and compete for resources. Those shifting relationships also can affect plant interactions with pollinators, other organisms dependent on the soil and desert wildlife. In the Sonoran, that includes everything from desert tortoises and Gila monsters to coyotes and bighorn sheep.

Madsen-Hepp said she hopes this work helps people see desert environments as vulnerable and vital, and that it motivates them to take steps to protect these places.

“As we continue to dump carbon dioxide and methane into the atmosphere and warm the planet, all of these ecosystems are tipping,” she said. “It’s just a matter of when and how much warming “