By changing the climate, humans have doubled the magnitude of drought’s impact on the availability of vegetation for herbivores, including livestock, to eat in the greater Four Corners region, according to a study published this summer in the journal Earth’s Future.

This is because increasing air temperatures and increasing levels of evaporative demand—or more water being soaked up into the atmosphere—stresses the grasses and shrubs that livestock and many other herbivores rely upon.

Emily Williams, who is now a postdoctoral scholar at the Sierra Nevada Research Institute at the University of California Merced, was the lead author of the study. At the time, she was a doctoral student at the University of California Santa Barbara.

She has a personal connection to the Southwest, as her grandparents once lived in Arizona. 

“As a kid, I would go to the Southwest quite a bit and really fell in love with the desert landscape,” she said. 

But that wasn’t the only reason Williams chose to look into how climate change could be impacting the vegetation in the greater Four Corners region.

“We’re really interested in understanding better how rising temperatures contribute to, exacerbate and play with the natural variability leading to drought,” she said. “And the southwestern US is really this beautiful, interesting, dramatic landscape. It’s a semi-arid region. You have vegetation that’s so well adapted to pretty dry conditions and can deal with droughts. But with climate change droughts are hotter, droughts are more intense, and these higher temperatures are really exacerbating droughts and impacting vegetation.”

Williams said she and the other researchers wanted to understand that process in part because of the climate similarities to other regions of the world, including parts of the Horn of Africa where the Climate Hazards Center at UCSB is working to understand the impacts of drought and it’s relationship to food security. 

They set out to answer a two-part question: “How has human-induced climate change affected air temperatures and atmospheric evaporative demand in the greater Four Corners region of the Southwest and what have the corresponding impacts been to vegetative productivity?”

In other words, how are humans causing the Four Corners region to become hotter and drier and what impacts does that have on the vegetation? 

One unique approach that the researchers took was evaluating the vapor pressure deficit. This refers to the difference between how much moisture is in the air and how much moisture the air can hold when it is fully saturated. 

Casey Spackman, a range management  specialist with the New Mexico State University extension office, said the vapor pressure deficit is not something that he has seen implemented into the applications used to help ranchers determine how much forage may be available for their livestock. This caught his attention, but he said more research is needed to determine to what extent it could be worked into the models that ranchers use.

“Time will tell on if vapor pressure (deficit) can be used to better predict forage production, which, from the study, it’s a great hypothesis,” he said. 

Spackman, who was not involved in the study, said the researchers were pretty straightforward in what they were trying to accomplish and that eventually the models could be implemented into some of the applications for predicting stocking rates and long-term environmental impacts for rangelands.

“It has its strengths, but with every model, we have to understand both the strengths and weaknesses to make any sort of inference into how to use it,” he said. 

Through the modeling exercise, Williams and her team concluded that humans have worsened the periods of time when there is not enough forage. This is especially apparent in New Mexico, according to the study. 

“Our findings indicate that increased temperatures from human-caused climate change have had and will have an increasingly damaging effect on rangeland vegetation, with large implications for both local ecosystems and communities that depend on southwestern rangeland resources,” the researchers wrote in the study.  

The research brought some results that Williams had not expected. 

“Deserts are not rainy places. They’re places that don’t get a lot of precipitation, you expect hot temperatures,” she said. “And so what we were expecting is that precipitation would be sort of the biggest constraining factor for vegetation. So, hot temperatures, yes, important, but if you don’t get enough rainfall, grasses and shrubs just aren’t going to grow.”

But Williams found she had underestimated the influence that increasing temperatures have on plants. The research indicated that during times of high temperatures, the heat had as much impact on the plants as the precipitation—or lack of precipitation—did.

“The temperatures are doing enough to reduce soil moisture and constrain plant growth,” she said.

This did not surprise Spackman, who said ranchers are aware of the impacts heat has on vegetation.

When it gets hot, plants close their stomata and just stop photosynthesis. Essentially, they stop growing.

“We, as scientists, try to quantify (producers’) knowledge. They know. They just don’t have the scientific information to back what they already know…It’s pretty common sense to say when it gets hot, what do we as people do?” he said. “We shut down. We go into hiding. We don’t want to be out in that heat. The plants do the same thing. They have a coping strategy of shutting down their respiration and trying to conserve as much water as possible to survive. So it’s going to have an impact. It’s just how much of an impact the higher temperatures will have on these plants. Will their coping mechanism of shutting down or limiting respiration affect their survival?”

Williams focused on the impacts to plants that are important to livestock in part because she and her team felt that the impacts of climate change on the ranching sector have not been broadly studied and because ranching is such an integral part of many people’s lives in the Southwest. 

While doing the research, she said she spoke with various groups and people who live in the greater Four Corners region. Those people include members of Zuni Pueblo who told her that their animals were not finding sufficient forage and that they had resorted to purchasing feed to supplement what was naturally available.

These are concerns that Spackman hears as well. In his role as a range management specialist, he travels the state and speaks to ranchers and other stakeholders. The three main concerns people ask about are drought, stocking rates and forage quality. All three of those subjects can be interlinked. 

When it comes to purchasing supplemental feed, Spackman said that is never a good idea unless the ranchers are buying forage. 

Oftentimes, ranchers choose to cope with drought by selling off some of their animals to reduce the size of their herds. 

Spackman said this is a difficult decision because it is hard for ranchers to recover after selling off animals.

“It’s not as simple as going out and buying new animals,” he said, explaining that the ranchers build the genetic composition of herds to produce the best quality product for consumers. 

“Even if we have a few good years, there’s a little bit of lag in rebuilding those herd sizes,” he said. 

There’s still more work to be done investigating how climate change is impacting rangelands health and what that could mean for ranchers. 

Williams explained that the recent paper was an attribution study that looked at the observed impacts of climate change. She said it could easily be extended into projections that could give people a sense of what conditions may look like in the coming decades. 

In the meantime, her focus has shifted away from the rangelands. She is now looking at how rising temperatures impact how much water is available for irrigation. 

While her attention may have turned to irrigation water, she said she would like to keep exploring projections for how climate change is impacting and will impact rangeland.