The Last Desert Grasslands

How the grasslands of Otero Mesa survived

Wind Mountain and Flat Top Mountain

Wind Mountain and Flat Top Mountain

Several characteristics enabled the grasslands of Otero Mesa to escape the fate of grasslands elsewhere in the region. The first is soil depth. Otero Mesa’s soils are generally shallow, less than two feet deep, below which lies a cement-like layer of calcium carbonate called “caliche,” or calcrete. The depth to this layer is remarkably uniform over large portions of the mesa, varying by only a few inches from location to location. This caliche layer is not unique to Otero Mesa, but in other parts of the Chihuahuan Desert where grasslands have been invaded by shrubs, its depth below the surface can vary by more than four feet from one spot to another.

Otero Mesa’s layer of caliche is also unique in that it is largely unbroken, devoid of the cracks and holes more typical elsewhere. This uniformity helps to prevent mesquites and other shrubs from getting established. The soil is deep enough for grass, but not for shrubs, and there are few openings in the calcrete for shrub roots to penetrate. The warm-season grasses found on Otero Mesa produce a dense mat of fine roots that are concentrated in the upper 12 inches of soil, which allows them to make efficient use of any amount of rain. Mesquite plants, on the other hand, have tap roots to exploit water found deeper in the soil. To compete with grasses for water, mesquite seedlings must extend their roots below the rooting depth of grasses. The fact that much of Otero Mesa is underlain by a solid floor of natural concrete just two feet below the surface effectively protects the grasslands from invasion by mesquite.

The drought resistance of the dominant grass species, black grama, is another reason why the grasslands of Otero Mesa are still intact, despite continuous grazing by livestock for many years. The ability of plants to resist drought varies considerably from species to species. Black grama grass is particularly well-adapted to endure periods of low rainfall because it is long-lived, and because of the way it grows and reproduces. Black grama is classified as a warm-season grass because it does not produce new leaves and stems until nighttime temperatures are in the low sixties (Fahrenheit) and there is sufficient soil moisture—an adaptation to the seasonality of rains in the region. The Chihuahuan Desert receives an average of eight inches of rainfall annually, but the amount can vary greatly from one year to the next. Much of the rain falls during the summer, when the prevailing winds bring moisture laden air up from the Gulf of Mexico and the Sea of Cortez. These summer “monsoons” provide the rainfall essential for the growth and survival of warm-season grasses. When summer rainfall is low, the grasses die back. Black-grama grass grows in clumps (tussocks) that expand from the periphery outwards. Most of the growth of black-grama tussocks is by runners that produce new stems (tillers) and add to the diameter of the tussock. Black-grama grass reproduction is primarily via runners. During drought, the outer tillers die first with the result that the black-grama tussock is reduced in diameter. Vigorous tussocks of black-grama can survive several consecutive years of growing season drought. Individual tussocks can survive for several decades.

Blooming yucca.

Blooming yucca.

Other grass species are much less resistant to drought and have relatively short life spans of a decade or less. For example, the central tillers of dropseeds and three awns die first during growing season drought, and the peripheral tillers that survive drought are susceptible to damage by wind blown sand. When stressed by livestock grazing, these species experience high mortality rates even during short drought periods.

When short-lived species die, areas with sandy soils lose the root systems and above-ground grass stems and leaves that provide protection against wind and water erosion. Oil and gas activities that cause surface disturbance are likely to result in the replacement of the resilient grama grasses with short-lived bunch grasses, robbing the system of its resiliency and eventually resulting in severe soil erosion and loss of biological integrity.

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Coalition for Otero Mesa
Phone: (505) 843-8696 | Fax: (505) 843-8697
Email: oteromesa@yahoo.com
Contact us for more information about the Coalition for Otero Mesa, oil & gas development, or the ongoing effort to protect the last wild lands and open spaces in New Mexico.