Vegetation cover in California's Sonoran Desert has diminished over the past three decades as a result of droughts and warming temperatures, highlighting dryland ecosystems' vulnerability to climate change.

Scientists examined satellite imagery in lowland desert and nearby mountainous areas of Southern California, and found that drier regions suffered the strongest decline in vegetation. The team reported the findings May 6 in JGR: Biogeosciences.

"The desert might be more fragile than people assume," said Stijn Hantson, a project scientist in the department of Earth system science at the University of California, Irvine and first author of the paper. "Under global change, we can expect pretty large changes across these warm deserts that we have been studying."

Dryland ecosystems, which include arid and semi-arid habitats such as deserts and shrublands, cover more than 40% of Earth's terrestrial surface and support about one-third of the world's human population. The plants inhabiting these environments are adapted to withstand harsh conditions, such as prolonged droughts and intense heat. But doing so pushes many of these species close to their physiological limits, and it's unknown how resilient these plants will be in the face of global warming.

Scientists have documented a number of die-offs of perennial plants in the Sonoran Desert over the past several decades. However, the full extent and the reasons behind these events aren't well understood, Hantson and his colleagues noted in the paper.

To investigate these questions, the team focused on a swath of Southern California that covered 1.27 million hectares (4,903.5 square miles) and encompassed the Anza-Borrego State Park. The region included parts of the Sonoran Desert and the Laguna, Palomar and Santa Rosa mountains. 

Within this area, the high-elevation spaces were covered by woodlands and chaparral. They had an average annual temperature of 10.7 degrees Celsius (51.3 degrees Fahrenheit) and experienced an average of 770 millimeters (30.3 inches) of precipitation annually. 

By contrast, the lowland desert areas hosted a variety of vegetation that included creosote bush, cactuses and mesquite. The average annual temperature was 23.9 degrees Celsius (75 degrees Fahrenheit), and the average annual precipitation was 73 millimeters (2.9 inches).

The researchers used satellite images captured from 1984 to 2017 to analyze how plant cover changed over time, based on how sunlight was reflected by leaves. They found widespread declines in vegetation density, particularly during the summer months, affecting 87.1% of the study area. Vegetation loss was greatest in lowland desert areas, but changes in rainfall did not fully account for these observations. 

"If we want to explain this trend, we also need to include the change in temperature," Hantson said. "While the droughts have been important, the recent warming has also been hitting the plants." 

Statistical models that combined summer temperatures and precipitation better predicted the vegetation loss observed in the satellite images than models that included only one of these conditions. 

Additionally, the researchers found that after the onset of drought conditions in 1999, which coincided with rising temperatures, desert areas failed to produce as much vegetation per given level of precipitation as they had previously. The results suggest that new temperature extremes have forced these lowland desert areas to cross an "ecological threshold," the researchers wrote, leading to lasting changes in the ecosystem.  

"It really seems like the vegetation is going to take a long time to recover, if it ever recovers, under the new climate conditions," Hantson said.

The researchers examined satellite images with a resolution of 30 meters rather than visiting the plants in person. Additionally, Hantson says, the reflectance measurements didn't represent all plant species equally well. 

"A given species that has more leaves reflects more, so it has a bigger impact on the signal," he said.

To create a more detailed picture of how the region is responding to climate change, the team will have to visit desert plants in the park over time and monitor how different species are reacting to changes in rainfall and temperature. 

Droughts are expected to become more frequent in many dryland areas, including Southern California.

"However, we found that the desert perennial vegetation is also sensitive to temperature change," the researchers concluded. "While we have quantified the decrease in desert vegetation that already has taken place, our findings indicate further, potentially important restructuring of desert vegetation with future warming."

The study, "Warming as a driver of vegetation loss in the Sonoran Desert of California," published May 6 in JGR: Biogeosciences, was authored by Stijn Hantson, Travis E. Huxman, Sarah Kimball, James T. Randerson and Michael L. Goulden, University of California, Irvine.