Scientists Use Mathematical Modeling to Fight Encroaching Deserts

The Gobi Desert in Asia is the fastest growing desert in the world. Aided by deforestation and overgrazing, the desert devours more than 2,000 square miles of grassland each year. The expansion causes food scarcity, unemployment, migration, and massive dust storms. Wherever the desert spreads, it devastates the local economy, threatens political stability, and endangers public health.

Gobi Desert, Mongolia (15).
Image Credit: Richard Mortel (CC BY 2.0).

Grasslands, savannahs, and other water-scarce regions that constitute drylands are most susceptible to this type of desertification, but they are important ecosystems to maintain. Drylands are home to nearly a third of the world’s population. They are sources of great biodiversity, support half of the world’s livestock, and provide food for much of the world.

But how do you fight the death of a landscape? In new research recently published in the American Physical Society’s journal Physical Review Letters, scientists from Ben-Gurion University of the Negev (BGU) in Israel have shown that we may be able to stop—or even reverse—the process by which drylands collapse into desert, using simple interventions at the desert border.

“One form of collapse is a domino-like process of plant mortality at transition zones between bare-soil and vegetation areas,” says Cristian Fernandez-Oto, a researcher at the University of the Andes in Chile. In this system, the edge of a desert encroaches on vegetated land like a destructive wavefront, pushing its way in and choking out the plants in its path.

While a postdoc at BGU, Fernandez-Oto worked with Prof. Ehud Meron and Omer Tzuk to simulate and study the behavior of such desert fronts as they moved through vegetation areas. In particular, the team wanted to know if there was a way to interrupt or destabilize the progression of a desert front that would otherwise leave behind a desert state.

Like many systems in nature, dryland vegetation grows and changes in a way that can be mathematically modeled. The vegetation pattern depends on positive feedback loops between plant growth and water transport to the growth location, so the team’s BGU team’s model incorporates these feedback features. The team started with an existing model for dryland vegetation, simplifying it a bit to match the system they wanted to study. A desert state in their model corresponds to a bare soil area devoid of vegetation.

A view of Sede-Boqer campus of Ben-Gurion University at Midreshet Ben-Gurion in the northern Negev (a desert in southern Israel), where the research team was based.
Image Credit: Public Domain

Then, the team analyzed how incoming desertification fronts affect drylands, examining boundary zones under a variety of different conditions, such as more/less water content and more/less vegetation. The team specifically looked for situations where small changes in the boundary region caused an incoming desertification front to slow down or stop. And they found them.

In fact, the researchers identified an instability—a turning point—in their analysis that led to ripples in the vegetation patterns in space and time and caused “fingers” of vegetation to grow backwards from the front, into the desert area. This suggests that small structural changes in boundary zones can actually reverse a desertification front. Furthermore, explains Fernandez-Oto, “Once a desertification front has been shifted to a recovery front, a gradual self-recovery process begins with no need for further intervention.”

The researchers suggest possible ways to bring this mathematical instability to life— by introducing a new plant species that changes the water-uptake rate in a boundary zone, for example, or by guided clearcutting to reduce the competition for water. The intervention depends on the situation.

Of course, seeing the results of a mathematical model isn’t the same as seeing real plants sprout up in a once-lifeless area, but this research suggests we can get there and provides some insight on how. More than 40% of the Earth surface land is covered by drylands, and this percentage is likely to increase with climate change, says Fernandez-Oto. Through their work, the research team hopes to help transform fragile ecosystems into places of resilience.

Kendra Redmond

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