Even desert plants are not immune to climate stress

In the heart of North America’s most arid and scorching desert, climate change is driving the decline of plant species once believed to be nearly indestructible. In their place, shorter shrubs are emerging, capitalizing on sporadic rainfall and warmer temperatures. 

Although this phenomenon has been observed in temperate mountain regions, a new study from the University of California, Riverside (UCR) reveals unexpected patterns of plant redistribution in the Sonoran Desert.

Study first author Tesa Madsen-Hepp pointed out that the findings challenge conventional expectations: “The plants are shifting, but in weird ways. We thought most of them would move to higher elevations with cooler temperatures. But while some lower-elevation trees are declining and shifting upwards, we’re also seeing some other species moving down, toward hotter parts of the desert.”

The research, published in the journal Functional Ecology, not only documents these surprising shifts in elevation but also investigates the physical characteristics of the plants in question to uncover the reasons behind this change.

The research team conducted their study in 2019 at the Boyd Deep Canyon Desert Research Center, located just south of Palm Desert. This area, spanning an 8,000-foot desert-to-mountaintop range, had been previously examined by ecologists in 1977 and 2008, providing valuable context for the recent findings.

Madsen-Hepp notes that some traditionally stress-tolerant species, such as California juniper and pinyon pine, are declining or shifting upwards, yet they are not thriving in their new locations. By contrast, plant species with shallower root systems, like brittlebush, burrow bush, and ocotillo, are taking over their former low-elevation spots. These shorter plants are not only less reliant on deep soil water, but they can also grow faster and invest fewer resources in their leaves.

Study senior author Professor Marko Spasojevic describes these species as “weedier.” He says that these plants have “cheaper” leaves in terms of carbon cost to produce them, and they are drought deciduous, meaning they can shed leaves during stressful conditions and wait for the drought to subside.

Plants that retain their leaves year-round typically invest in thicker leaves with higher carbon content, putting them at a disadvantage compared to those that can easily shed leaves. When plants drop their leaves, the atmosphere can no longer draw water from them, reducing the burden on their roots to replenish the lost water.

According to Madsen-Hepp, the “live-slow-die-old” strategy that used to work for plants in this extreme environment is no longer effective due to increased climate stress. Once these plants reach their physiological thresholds, she warns, there is little that can be done to revive them.

The researchers also discovered that, contrary to more temperate ecosystems, the lower desert elevations are warming faster than the higher ones. The shrubs and bushes taking over are not necessarily originating from the highest points in the desert, but rather are lower-elevation plants expanding their range.

The observed upward range shifts of approximately 29 meters per decade are consistent with the higher end of global rates for plant movement in response to climate warming. On average, plants in temperate regions have shown range shift rates between 5 and 30 meters a decade.

Professor Spasojevic emphasizes the vulnerability of desert ecosystems to climate change, stating that they may be even more sensitive than Arctic and alpine ecosystems, which are typically seen as bellwethers for climate change. He urges the reduction of fossil fuel emissions as the key to alleviating stress on these fragile environments.

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