Essays

Moseying: Exploring the Natural World

Basic knowledge about the ecology of the sanddune habitat
June 12, 2011

Sanddunes are found between the Pecos River and the Llano Estacado. These sanddunes were formed by the constant southwest wind blowing sand out of the river valley. Most of the sand is stopped by the western edge of the Llano Estacado, the Mescalero Escarpment. There are two different sand dune formations – the older formation has reddish soil due to iron oxide stains on the sand grains. The younger dunes are cream colored, and will form transverse (nearly straight sand ridges at right angles to the prevailing wind) and barchan (crescent shaped) dunes where there are open dune fields.

Some of the sand makes it to the top of the Llano Estacado.  On top of the Llano Estacado are smaller areas of sanddunes, usually north and northeast of the draws (watercourses that only run after a rain.) Some of the sand comes from the Pecos Valley, and some is from erosion along the draws. There are a few small areas of open dunes on top of the Llano Estacado (in Midland and Cochran Counties.)

East of the Llano Estacado are also regions of sandy soil – some of this sand comes from the top of the Llano Estacado, and some comes from the creek and river valleys of the “breaks.” There are no open dune fields in this region.

Most of the sanddunes are vegetated. It is unknown why some dune fields do not become vegetated – the dunes at Monahans State Park have been open sand for over a hundred years.  Shinnery (Quercus havardii) and Sandsage (Artemisia filifolia) are the indicator plants of sanddune soil – if either plant is seen, a person is in sanddune habitat. There are several dozen species of plants that are only found in the sanddune regions. Marbled Whiptail Lizards are an indicator reptile of the habitat. In some areas the rare Sagebrush Lizard is found. Another possible candidate for the Endangered Species list are Lesser Prairie Chickens, also found in some of the sanddune areas.

When the wind is above 30 miles an hour, sand grains will begin to move – bouncing along a few inches at a time. (This is known as saltation.) After three hours of a 40 mile an hour wind, sand can either pile up or erode over 3 inches. In 24  hours as much as three feet of sand can move. There is always space between the sand grains, so rainwater quickly goes into the soil.  The rainwater goes about four feet deep into the sand and then begins to trickle down the slope of the dune. Down at this depth a number of species of algae and microscopic animalcules survive, for there is just enough filtered light penetrating the sand grains to promote photosynthesis for the algae. 

In many places the water will trickle down to the hardpan soils beneath the sand, and go no further. As a result, in a number of areas small pools of water can be found between the dunes. Even if there is no water visible at the surface, its presence is indicated by black willow (Salix nigra) growing in the dunes. Animals will dig where black willow is to be able to get a drink of water.

Dune plants have high rates of photosynthesis, and therefore can grow rapidly. The worst windstorms occur in March and April, and thanks to winter moisture, dune plants are stimulated for growth during the windy season. Many dune plants are unusually large and vigorous -- a condition called gigantism. Saltbush, normally 3-4 feet tall on tighter soils can reach 6-8 feet in the dunes.

Many plants form adventitious roots – if their stems are covered by sand, the stems sprout roots. Roots need oxygen, and deeply buried roots cannot get the oxygen needed. Adventitious roots allow the plant roots to still get oxygen. Black willow, yucca, and mesquite all have such roots. When the sand blows away the plants appear to be on stilts – stuck way up in the air on “long legs.”

Shinoak and mesquite form hummocks – large mounds. The hummocks become cemented together with the organic material (dead leaves and wood).  Mesquite has nitrogen fixing bacteria in its roots, and those bacteria can extract the nutrients from the organic material.

Sand reflects light and heat – so plants have a number of adaptations to combat that foe as well. Many dune plants are either hairy or have silvery leaves. The hair shades the leaf, and the silvery color reflects the light. Often such plants are green in the spring, when growth is of utmost importance, but then they turn gray and much hairier in the long hot summer.

Some plants, especially the annual wildflowers, produce incredible numbers of seed. Instead of one bloomspike, such plants will have dozens of bloom spikes. Some plants do not “release” their seeds – they stay in a capsule attached to the plant. When the plant dies, and is eventually covered over with sand, the seeds will germinate where the parent died.

Other plants produce large seeds – larger than those of related species in different habitats do. Shinoak acorns are over an inch and a half long. Havard Shinoak is endemic to our sanddunes – it grows no other place in the world. Sanddune regions have more species of endemic plants than habitats in other regions due to the constantly changing habitat and the isolation of one dune field to another. The forest of shinoak from Crane to Fort Sumner is the largest oak forest in the world.