Focus: Patterns of Green in the Desert Brown
Most of us picture the desert as brown with a few scattered, scraggly green plants. But dry regions are far more complicated, showing complex patterns of green growth amidst the brown. A model in the 5 November print issue of PRL reproduces these stripes, spots, and other vegetation patterns based on equations that use only two variables. The authors expect their model to improve environmental scientists’ understanding of areas on the verge of becoming deserts. It may even suggest ways of reversing the “desertification” process, which is often caused by human activity.
Before addressing human influences on dry environments, researchers need to better understand the ways in which plants compete for water as conditions change, says Jost von Hardenberg of Ben Gurion University in Israel. So he and his colleagues built what they call “a toy model of an ecosystem.” Their equations contain just two variables: biomass (the total amount of vegetation) and water (including rainfall and water in the soil). The model includes processes such as evaporation, plant growth, and the spread of plants by seed dispersal. It also includes the interplay between biomass and water supply–shade from plants reduces evaporation, for example.
To start off, the team filled a grid of points–their virtual desert–with a homogeneous plant coverage corresponding to a certain level of precipitation. Next, they perturbed this uniformity by slightly increasing or decreasing the vegetation at each point in a random way. From this unstable, “noisy” situation they let the equations do their work, with plant growth at each grid point slowly evolving toward an equilibrium. Different vegetation patterns emerged, depending on the level of rainfall the researchers assumed. All of the patterns they saw exist in arid regions around the globe: from scattered patches in the driest climates, to intricate labyrinths in slightly wetter ones, to green-covered areas with some barren holes in semi-arid lands.
The team was surprised to find that for a given level of precipitation, different vegetation patterns could coexist. This effect suggests that when a prolonged drought kills all the plants, a region may not recover, even when rainfall comes back to its original level. Instead, it must get even wetter than it had been before in order for the plants to return. Von Hardenberg explains that the vegetation patterns may also have more wide-reaching ecological consequences, as they influence the interactions between plants and small animals. He and his colleagues are now refining their model to improve its accuracy, with modifications such as separating the flow of water above and below the soil.
“You can quibble about the exact form of a few of the terms, or how you would measure them in the field,” says Princeton University ecologist Chris Klausmeier, “but that doesn’t take away anything of the model’s general validity.” He is impressed that a single model captures the wide variety of vegetation patterns nature has to offer. Given the complexity of these ecosystems, Klausmeier says, the level of agreement with field data is probably “as good as it’s gonna get.”
–Rob van den Berg
Rob van den Berg is a physicist and freelance science writer in Oegstgeest, the Netherlands.