How these desert shrubs became more efficient amid the megadrought

An undated photo of a hillside of brittlebush in flower during a relatively wet year for the Mojave Desert. A new study co-led by Utah researchers found the plant has adapted to the West's "megadrought" by conserving water.

An undated photo of a hillside of brittlebush in flower during a relatively wet year for the Mojave Desert. A new study co-led by Utah researchers found the plant has adapted to the West's "megadrought" by conserving water. (Avery Driscoll via University of Utah )



Estimated read time: 4-5 minutes

SALT LAKE CITY — Gov. Spencer Cox said last week Utahns cut back on "billions and billions" of gallons of water this year, which helped the state end the year with its collective reservoirs closer to half-full rather than a grim projection of less than 40% capacity.

It turns out that humans haven't been the only ones conserving water amid severe drought conditions not just in Utah but across the West. So did desert shrubs in the American Southwest, according to a new study co-led by Utah researchers.

The report found that various desert shrub species adjusted their method of collecting water and carbon dioxide for photosynthesis at a rate similar to the level deserts have dried up, suggesting that the shrubs are more sensitive to changing Southwest climates than other plant species.

But even with increased efficiency, the researchers add that the climate adaptation may not be enough for the plants to save themselves if the West continues to dry up. Their findings were published Monday in Proceedings of the National Academy of Sciences.

"The Southwest is in a really dry period and this is further evidence of how much the 'megadrought' has impacted plant functioning and how anomalous this period is," said Steven Kannenberg, a postdoctoral research associate within the University of Utah's School of Biological Sciences, and the study's co-lead author, in a news release.

The study focused on brittlebush, button brittlebush and burrobush collected at Arizona and California sites within the Mojave Desert, according to Kannenberg. The species are native plants to the ecosystem that are also beneficial to animals species in the area.

The researchers specifically studied the intrinsic water-use efficiency of the brush. It's a measure that refers to the amount of photosynthesis in relation to the opening of the plant's stomata — the part of the plant that collects water and carbon dioxide for photosynthesis.

This level has historically risen "dramatically" over the past century due to a spike in carbon dioxide in the atmosphere, according to the study. But they also explained that plants restrict their stomatal opening during times of higher temperatures and less water availability, so the plant doesn't lose too much water.

What the researchers wanted to know is how a Western megadrought — a series of droughts over the past two decades, considered possibly the region's worst in 1,200 years — impacted this relationship between the stomata and photosynthesis.

While there isn't a long-term database for intrinsic water-use efficiency, or iWUE, in Southwest brush, University of Utah researchers had started collecting the data in the Mojave Desert every March since the 1980s. This allowed researchers to at least look at intrinsic water-use efficiency levels between 1989 and 2019, which includes times before and after the megadrought began.

An undated photo of researchers conducting a field survey at a site in the Mojave Desert.
An undated photo of researchers conducting a field survey at a site in the Mojave Desert. (Photo: Steven Kannenberg via the University of Utah)

Samples from ponderosa pines in Utah, Arizona, Colorado and New Mexico were also collected as a comparison tool between shrubs and trees in the region.

They found that annual precipitation didn't change much over the past 30 years but air at nine of the 17 sites had become drier, almost seven times drier just since 2010 — an extremely fast rate. The rate increased at 14 sites all together.

When this happened, the shrubs adjusted their intrinsic water-use efficiency levels to match, almost seven to 10 times faster in the Mojave Desert than elsewhere. It was much faster than how the trees reacted to changing climate and faster than almost anything ever recorded on the subject.

The findings were a surprise for researchers like Kannenberg.

"Given the rapid uptick in aridity across the region in recent decades, this has resulted in shrub iWUE increasing at — as far we can find — the second-fastest rate ever documented in any plant species," he said.

Funding for the project came from the National Science Foundation, the U.S. Department of Energy, the U.S. Department of Agriculture and the David and Lucille Packard Foundation.

The science isn't over, though. Researchers wrote that since water scarcity in the American Southwest is expected to worsen in the future, the relationship between plant water-use efficiency and the climate "have important implications for plant health and carbon and water cycling."

For instance, Kannenberg said it can hinder plant growth — if it continues to get drier, the plants may not have enough to survive. The researchers also wrote the drying air has already impacted the region in dire ways, such as more frequent and larger wildfires.

"While there are ongoing concerns that many dryland ecosystems may be nearing a tipping point, the rapid increases in iWUE that we observed have the potential to partially mitigate the impacts of aridification through water savings," the researchers wrote. "The fate of these ecosystems will thus likely rest on the limits of plant acclimation and the future trajectory of this long-term drought event."

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