Slow green waves: desert vegetation flows uphill
Mathematics conference hears some dry climate ecosystems behave like oceans. Richard A Lovett reports.
Anyone who has ever hiked through the desert knows that in dry climates vegetation grows in isolated patches, separated by bare ground. Understanding why, however, is a matter not of ecology, but mathematics.
Often, such patches are simply scattered clumps. But in some places, such as parts of Africa and Western Australia, they take the form of long arcs of dense vegetation that from space look like fingerprint whorls or waves on the ocean.
The waves can be more than 10 kilometres long and 30 metres wide, separated by as much as 100 metres of bare ground, says Mary Silber, an applied mathematician from the University of Chicago in the US, who has been studying them for several years.
And, like ocean waves, she said recently at a meeting of the Society for Industrial and Applied Mathematics (SIAM) in Portland, Oregon, these vegetation waves are also in motion, albeit very slowly.
The waves only occur, she says, on near-flat terrain, with a barely visible slope of between 0.3 and 0.8%, meaning the elevation increases by between three and eight metres every kilometre. The bands lie perpendicular to the slope, almost like contour lines.
Plants, of course, can’t move — they are literally rooted in place. But each new generation of plants slowly “colonises” bare ground on one side of the band, while older plants die off on the other. The result, says Silber, is that the bands slowly move upslope.
“[It’s] about a band width in 50 years,” she says, noting that the only way it’s been detected is by comparing modern satellite photos with aerial photos taken many decades earlier.
“It is remarkable that you can go to a paper from 1950 and go to Google Maps and find exactly the same thing, still there,” she says. “I find that incredible.”
What’s happening, her models indicate, is that the rare desert rains run off the bare patches between the bands – but not, on such gentle slopes, in gully-washing torrents. Instead, the water comes off in sheets that also remove fine material from the surface, along with light-weight litter, dead grass, and even animal faeces.
When the water hits the next downstream vegetation band, its flow is arrested and all of these nutrients, including the water itself, are caught. The effect is compounded by the fact that plants and their roots break up the soil, helping moisture to percolate into it.
One result is that the vegetation bands trap water and nutrients flowing off from upstream, giving them a significantly larger supply than the average rainfall would predict. But also, some of that moisture accumulates on the uphill side of the bands, encouraging plants to slowly grow in that direction.
Hans Kaper, an applied mathematician at Georgetown University, Washington DC, sees this as important research, with possible relevance to global climate change.
“We want to figure out the warning signals that predict a tipping point [in climate],” he says.
When climate dries and deserts spread, he adds, plants have to adapt: “They have to share what little water there is, so they try to position themselves optimally to get the maximum water. That means they have to keep a certain distance from each other. That leads to pattern formation.”
Not that Silber’s study of vegetation bands has progressed to the point of making predictions of how, or where, deserts are spreading. At the moment, it’s confined to understanding how such vegetation bands are formed.
But figuring out the behavior of drylands ecosystems may help us understand their health and how they are changing, she and others say.
Silber’s research is available on the preprint server Arxiv.