The steady decline of plants in Southern California’s portion of the Sonoran Desert — which includes Anza-Borrego Desert State Park — is caused by climate change-driven heat increases, according to a new UC Irvine study.
That area grew hotter by 3 degrees over the study period, 1984 to 2017, with vegetation decreasing an average of about 1% a year in the desert portions of the study area. While fluctuations in rainfall accounted for some of the year-to-year variation, the broader trend resulted overall decrease of 35% of vegetation in desert ecosystems and a 13% decline in the adjacent mountains.
The findings, based on 34 years of NASA satellite images of 5,000 square miles of desert, add to a small but growing body of evidence that manmade climate change is reducing the amount of vegetation in drylands — primarily desert areas — worldwide, with trickle-down effects on animals and, in some cases, humans. About 41% of the Earth’s land mass is drylands, according to the UCI report.
“These results suggest that recent climate change has already had a significant impact on these (Sonoran) drylands and highlight the potential for future warming to increase risks for dryland ecosystems in other regions,” according to the study, published in the Journal of Geophysical Research in May.
While desert vegetation has a reputation of being hardy and adaptable, the UCI researchers found such resilience only goes so far.
“These (plants) have been able to eek out an existence on the edge of what they can physiologically tolerate. But any additional stress, such as climate change, can exceed their limits,” said James Randerson, an earth science professor and one of five UCI authors of the report. “Our study clearly shows that there’s a toll of climate change.”
That toll affects not only plants — in the Sonoran desert, that includes creosote bush, yucca, ocotillo and mesquite — but also animal life that depends on that vegetation.
“There are cascading effects,” Randerson said.
Local, global trends
From the 1980s to the 2000s, “desertification hotspots” — areas with declining vegetation — expanded globally to include about 9% of drylands, which are defined as “arid, semi-arid, and dry sub-humid areas,” according to a 2019 report by the United Nations’ Intergovernmental Panel on Climate Change.
The study found that climate change was reducing biological diversity in drylands around the world, and noted consequences for humans. Some 500 million people live in those hotspots, with the most profound effects on agricultural productivity and “human well being” hitting the driest parts of Asia, North Africa and the Middle East.
The study also noted that the negative effects of climate change in some areas was exacerbated by the expansion of croplands and unsustainable land management practices.
Closer to home, a 2019 report by the state Office of Environmental Health Hazard Assessment looked at plants in Riverside County’s Deep Canyon in the Santa Rosa Mountains over a 30-year period. It found that as annual temperatures warmed and humidity trended downward, some species began migrating to higher elevations where it was cooler and wetter.
“Understanding local changes happening in this area may help to predict how vegetation throughout the state may respond to a warmer and drier climate in the future,” the report said.
Because the UCI study relied on Landsat satellite images, it couldn’t identify which species were disappearing from the Sonoran Desert. On a broader regional basis, Joshua trees are expected to be among the casualties.
A 2011 study by the United States Geological Survey used climate modeling to analyze the long-term prospects for Joshua trees. It determined that “temperature increases resulting from climate change in the Southwest will likely eliminate Joshua trees from 90% of their current range “in 60 to 90 years.”
Another casualty of climate change could be the ocotillo, and the hummingbirds that feed on them. Biologist Jim Cornett recently told the Los Angeles Times that ocotillo nectar was critical for hummingbirds that migrate from Mexico and that with the demise of the ocotillo, some hummingbird species could cease migrating or become extinct, he said.
Randerson placed the new UCI report in context with these other findings.
“There are relatively few studies that have looked at long-term trends,” he said. “But what they’ve found seems to corroborate what we did.”
