California’s desert trees can’t take the heat: study

Some of the Southwest’s most iconic desert trees are running for their lives in what could be a grim harbinger for more temperate ecosystems across the West.

A study in Functional Ecology offers evidence that desert ecosystems, long perceived as the most resilient to climate change, may be hitting their limits.

Researchers at the University of California Riverside found that rising temperatures and protracted drought have driven piñon pines and juniper trees to seek refuge at higher elevations in the deserts north of Palm Springs.

In the place of these slow-growing, iconic forests is rising an empire of weeds. 

That is part of a wholesale transition in arid landscapes caused by the burning of fossil fuels, the scientists said.

“This is the hottest, driest desert in North America — you would think that surely it would be resilient to higher temperatures,” Tesa Madsen-Hepp, a University of California Riverside botanist who was first author on the paper, told The Hill.

Even the limited amount of planetary heating experienced since the 1970s has reshaped these ecosystems — and emissions continue to rise.

The study offers a vision of where other ecosystems “with cooler, wetter conditions are potentially headed,” Madsen-Hepp added.

Broad trends based on the study of temperate ecosystems led researchers to expect that as temperatures increased and moisture in the ecosystem fell, all plants would migrate upwards in search of cooler temperatures.

What they found instead was more complicated: a wholesale shift towards “weed” species, which had moved down from high elevations to colonize former forests left open by the mass dying-off of trees.

While the piñon pines and junipers are often seen as hardier, they nonetheless depend on ready access to underground water. That’s in ever-shorter supply thanks to the West’s long drought, though this year’s record rainfall has provided a brief respite.

As the land dried up in past decades, those trees had thinned out at lower elevations  — to be replaced by fast-growing, quick-dying shrubs like ocotillo and brittlebush, species that can quickly root, flower and die back to take advantage of flash floods. 

The result of this change, Madsen-Hepp said, was a “leakier” ecosystem — one far less able to capture and retain carbon than the woodlands. 

It is also a change for which it is uniquely easy to point the finger at climate change, and the fossil fuel emissions that are its overwhelming cause.

Attributing the responsibility of fossil fuel emissions to any particular ecosystem change is generally very difficult. Most landscapes are impacted by a wide variety of both human-induced and natural factors.

But this study — which took place on reserve lands owned by the University of California system — took place in what amounts to a natural laboratory for studying the impacts of climate change on the fragile ecosystems of the desert.

Unlike the irrigated golf courses and farmlands that spread across the deserts to the research areas south, this area is pristine — untouched by either fire or human development.

Even in that area, “we’ve pushed the system past their threshold,” study co-author Marko Spasojevic told The Hill.

That makes the impacts on the UC research sites a troubling indicator for the vast majority of dryland ecosystems, which must weather both climate stress and human impacts.

By midcentury, an area of Western forests more than three times the size of Yellowstone National Park is projected to become too hot for young conifer trees to grow, as The Hill reported.

The fate of these landscapes is politically fraught right now as the Western states battle over future allotments of the shrinking Colorado River and turn to irreplaceable groundwater to replace it.

One of the sites originally sampled in the 1970s is now lush and green, Spasojevic said — because it’s been turned into one of the golf courses that now surround the resort towns of Palm Springs and Palm Desert.

“It’s a massive green lawn in the desert,” he said. “It’s maybe not the best use of water.”

The broader trajectory was clear, he added.

“We often think of the tundra as the bellwether for climate change. Arctic and alpine ecosystems are very sensitive. We’re seeing here that this ecosystem is just as sensitive if not more so,” Spasojevic said.

“And we already know the answer to easing the stress on it. It’s very simple,” he added. “Cut fossil fuel emissions.”