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Jornada winds: Can they help reestablish the grasslands?

Spring is the windy season in southern New Mexico. The high winds and accompanying dust contribute to traffic accidents, aggravate allergies and pulmonary health problems, and fan the flames of wildfires, just to name a few negative effects.

Roxanne Chepsongol and Jameson Brennan, Department of Biology research assistants, conduct dust collections on vegetation removal plots in a study of how wind transports soil and nutrients. The study is part of the NSF-funded Jornada Long Term Ecological Research project, which primarily involves researchers from New Mexico State University and the USDA Agricultural Research Service. (Photo by Andrea Campanella)

Greg Okin, a UCLA geomorphologist and co-principal investigator in the Jornada Long Term Ecological Research project near Las Cruces, studies the effects of wind on shrub encroachment in the Chihuahuan Desert. The NSF-funded LTER project primarily involves researchers from New Mexico State University and the USDA Agricultural Research Service. (NMSU photo by Jay A. Rodman)

One group of people less likely than most to dread the spring winds, however, are some of the researchers in the Jornada Basin Long Term Ecological Research project who study how wind impacts the ongoing transition from grasslands to shrublands in the Chihuahuan Desert near Las Cruces. The long-term data they are collecting and analyzing is increasingly being seen as crucial to understanding this desertification process.

The Jornada LTER is a National Science Foundation-funded project primarily involving researchers from New Mexico State University and the USDA Agricultural Research Service. The primary area of the Chihuahuan Desert included in the project is NMSU's Chihuahuan Desert Rangeland Research Center and the USDA's Jornada Experimental Range. Together, the two ranches comprise more than 250,000 acres north and northeast of Las Cruces.

Rangeland research at the JER has been ongoing since 1912, with a focus on the increasing dominance of perennial shrubs and the resultant decrease in grazing capacity.

Greg Okin is a co-principal investigator in the Jornada LTER project. A UCLA geomorphologist who specializes in wind and wind-erosion processes, he is especially interested in how wind affects the distribution of what he calls "the important elements for life," including nitrogen, carbon, sulfur and phosphorus, all important components of fertile soil.

Okin has been involved in the Jornada LTER since 1996. His role in the project is two-fold: to supply wind expertise as needed in LTER projects directed by others, and to head up his own research projects with a primary focus on the role of wind in the area's desert ecosystems.

The transition from perennial grasses to perennial shrubs in the area was obvious to local ranchers and NMSU researchers in the late 1800s, and the shift was considered to be a problem because grasslands support more cattle than shrublands do. The primary questions that rangeland researchers tried to answer centered on how many head of cattle a piece of arid grassland could sustain without undergoing irreversible degradation and whether there were economically viable ways to make the rangeland more productive. The extensive research did not really explain why shrubs expanded into areas previously covered in grasses.

More recent research seemed to suggest that shrubs were favored over grasses in a post-grazing situation because shrubs were more effective in capturing water run-off and the nutrients the water carried.

"Prior to my research, water was considered to be the major non-biological mechanism that was actually driving the movement of soil and nutrients and that this was a key component in promoting this conversion of grasslands to shrublands," Okin said.

Okin agrees that this is a reasonable explanation for this transition process in hilly areas, but is insufficient to explain what happens in the flat areas of the Jornada and the Rio Grande Valley.
"My research here has been to show that wind can also move materials like water does and, in fact, plays this role more than water in many places where you have wind-erodible soils."

Evidence for this conclusion comes, in part, from a project Okin began in 2004. At sites where water erosion was not really an issue, he oversaw the selective removal of vegetation at various Jornada sites and the subsequent monitoring of the effect that wind erosion had on the growing capacity of the cleared sites. "That dust is important from a plant or ecological perspective, because it is rich in nutrients. So if a landscape loses the dust, it's losing nutrients, ability to hold water, losing carbon, it's basically losing its ability to sustain life quite as well," he said.

One question this experiment was designed to answer was how much vegetation needs to be removed in order to reach a point of no return where the ecosystem was unlikely to return to its previous state. The result of this research on such thresholds was surprising, Okin said.

"We've discovered that you don't actually have to remove all of the vegetation," he said. "In fact, you only have to remove about half in the experiments we've done in order to disrupt the soil nutrient cycle and the cycling of nutrients between plants and the soil. And when you do that, you start this slow, downward spiral of the system from being a grass-dominated system to becoming a shrub-dominated system."

But the flip side of soil erosion is that, as he puts it, "What goes up has to come down." So he and his team also monitored the effects of plant removal and dust production on plots downwind of the vegetation removals. What they found was that shrubs were also favored downwind. "As those sediments move, they redistribute nutrients, and they tend to do so under the canopies of plants, and in doing so, promote the growth of shrubs," he said. "We're also starting to see grasses being buried by encroaching sand on these plots."

An additional question Okin is very excited about answering more fully in the next LTER grant period is to what extent the knowledge they have gained about wind and ecosystem changes might be used to actually reverse rangeland degradation. His emerging strategy has a certain martial arts element to it.

"We've invented methods where we can essentially harness the power of the wind in order to try to do that," he said. "Wind redistributes soil litter, it redistributes seeds, it redistributes nutrients. If we can figure out ways to trap those nutrients, litter, and seeds in places where it will allow plants to grow when it rains, and therefore reestablish grasses, then wind has a role in remediation as well as degradation."