Moving Sand - sand dunes - Brief Article

by Dale S. Turner

Life in the Limbo Zone Between Soil and Dunes of Granular

Walking through Mexico's Gran Desierto dunes, I suddenly dropped six inches down into the "earth," neck cracking, teeth clacking, knees popping, forcefully reminded that life abounds inside stable sand dunes. It was probably the burrow of a desert kangaroo rat. Closer inspection (easy when you've just been brought to your knees) revealed abundant life in this arid sea of sand.

Julia, my geologist wife, informs me that the sand of most dunes doesn't, in the technical sense, qualify as soil. It lacks structure, layers, clumpiness, and organic material that evolve with time, climate, and soil-forming processes. But a farmer's definition of soil is simply earth material which will support rooted plants; and sand can do that admirably, though sparsely. Indeed, for arid regions which get only brief but intense rainfall, sand may be the ideal material for growing plants. Water penetrates rapidly and percolates deeply through sand's large pore space. Deep percolation protects it against evaporation from the sun's relentless heat. The "protected" water is readily available for plant life, if not too deep below the root zone. Plants on dunes are limited primarily by the effects of moving sand--most plants don't do well when their leaves are buried or their roots exposed.

And therein lies an interesting dynamic in dunes. The photogenic sand dunes by which so many people--drawn to the images of Hollywood's Sahara--are smitten, are actually the least interesting biologically. Those smooth, bare, muscular curves are formed by a fairly sterile pile of mobile sand grains. The dunes that are truly alive are those low, stable mounds covered with plants; lively dunes sit still. On them, perennial plants form islands of life, feeding and shading the whole community. Their roots bind the sand, making the dune a more agreeable place for less-adapted plant species, and help stabilize the tunnels for my friends the kangaroo rats. Their tunnels, in turn, provide shelter for snakes that prey on the rats, lizards hiding from the mid-day heat, beetles to clean up after everything else. The stabilized surface nurtures a crust of nitrogen-fixing bacteria, providing an otherwise-scarce nutrient.

The distinction between active and stable dunes is, of course, just an arbitrary and temporary point in a continuum between the totally barren and the completely vegetated. Different parts of a dune field will often be more or less vegetated, and an area covered with wildflowers in a wet spring may be bare sand the next dry year.

So what happens when we get a string of dry years? Aerial photographs taken of the Great Plains during the drought of the 1930s show active dune crests appearing in what had previously been grass- or farmland. Large portions of the Great Plains sit uneasily on top of dormant dunes which last were active when the climate was slightly hotter and drier. A touch of global warming, and the 20,000-square-mile Nebraska Sand Hills region might reclaim its place as the largest active dune field in the Western Hemisphere (it's nearly ten times larger than the Gran Desierto of the lower Colorado, current champion in these parts).

For partially stabilized dunes in an arid climate, it doesn't take a climatic shift to get the sand moving again. Weekend warriors in off-road vehicles can do the job: smashed plants, torn roots, crushed burrows, shattered crust, and there's nothing left to hold the sand. California Highway 78, where it crosses the Algodones Dunes, provides an amazing example of this. North of the road, in an area closed to vehicles for many years, assorted shrubs are scattered across the dunes, interspersed with living annual plants and the brittle debris of dead plants. It looks kind of messy, like someone's living room. Tracks of fringe-toed lizards and shovel-nosed snakes, kangaroo rats and kit foxes, testify to the chaotic business of life in the dunes. South of the road, it's a motorized playground. Tires leave the only tracks, and the sand lies bare in huge, smooth, gleaming heaps of granular quartz. The sand is active, but the dune is dead.

So dunes teach an essential lesson: soil needs an architecture, even if frail, shaky, and weak. As its first task, a living terrestrial community organizes itself to be more cohesive, interlocked, and resistant to deformation or blowing away. With even the most transient stability, life thrives.

Dale Turner just received his MA in biology at the University of Arizona (Tucson), studying lizards in the Gran Desierto. He's been a committed advocate for the sky-island ecosystems (isolated mountain ranges) that extend from Arizona to the Sierra Madre of Mexico. He has joined my spectacularly unofficial non-organization of maniacal naturalists, open only to those who survive by feasting daily on the detailed natural beauty of local ecosystems. --PW