Ecological Characteristics of Otero Mesa and Impacts of Gas Development
Walter G. Whitford, Ph.D.
Ecological Setting: The landscapes of the Otero Mesa in south-central New Mexico are gently undulating at elevations ranging from 4000 feet to 5600 feet. The dominant vegetation on the mesa is classified as Chihuahuan Desert grassland. The dominant grass species are black grama ( Bouteloua eriopoda ), hairy grama ( B. hirsuta ) on the crests and slopes and tobosa grass ( Pleuraphis mutica ) in the low valleys and swales. The grasslands are essentially shrub free. There are stands of soaptree yucca ( Yucca elata ) and infrequent all-thorn ( Koeberlinia spinosa ) that provide some vertical structure to this uniform height grassland. The soils supporting the Otero Mesa grasslands are extremely shallow ( < 50 cm (approximately 20”)deep) in most places. The shallow sandy soils sit on a cemented calcrete layer that is relatively impermeable to water and plant roots. Because the soils are so shallow, burrowing animals such as pocket gophers, prairie dogs, and banner-tail kangaroo rats are limited to small patches of deeper soils on the watersheds.
Status of Chihuahuan Desert Grasslands: Chihuahuan Desert grasslands are the most endangered ecosystem or plant community type in North America. Once widespread in south-west Texas, southern New Mexico, Arizona and the state of Chihuahua in Mexico, almost all of the Chihuahuan Desert grasslands have been converted to desert scrub, or grassland with high cover of shrubs such as mesquite and creosotebush (McClaran, 1995). The history of degradation of Chihuahuan Desert grassland has been documented in a number of recent publications (see citations in Whitford, 1995, Whitford, 2003(pages 275-304). Chihuahuan Desert grasslands that covered millions of acres in the 1880's are now reduced to small patches surrounded by a sea of desert scrub. The Otero Mesa grasslands are a unique remnant of the formerly widespread Chihuahuan Desert grassland with black-grama grass as a dominant species. The unique features of Otero Mesa grasslands are (1) thousands of acres of unfragmented grassland, shallow, fine sandy soils, desert scrub limited to the piedmont slopes of the Cornudas mountains, remnant populations of prairie dogs, and a large number of pronghorn antelope.
Fragility of the Otero Mesa Chihuahuan Desert Grasslands: The Otero Mesa grasslands represent a fragile ecosystem type. The primary reasons for this fragility are the shallow soils and the physiological and life history characteristics of the dominant plants. The light sandy soils are very susceptible to wind and/or water erosion if there is a significant reduction in plant cover and root systems that hold the soil in place. Black-grama grass ( B. eriopoda ) is a long-lived (living for several decades), stoloniferous grass. Black-grama responds to drought by death of tillers at the outer edges of a tussock. During wet growing seasons, the tussocks expand by adding tillers to the outer edges of a tussock. The dense root mass of a black-gram tussock allows the plant to take advantage of soil water. The sandy nature of the Otero Mesa soils result in high infiltration of rainfall, even in intense rains, and the grasses protect the surface from splash erosion. Thus even low depth rainfalls (i.e. <0.1 inch) will provide useable moisture for the grama grasses. Evidence for the high infiltration rate character of the soils is the paucity of rills (shallow channels) that are produced by rain- water run-off.
The life history, morphological and physiological characteristics of the grama grasses combined with the soils characteristic of the Otero Mesa grasslands have provided the resistance and resilience of this vegetation to the stress of drought and grazing. Other mixes of desert grass species growing on sandy loam soils do not have these resistance-resilience characteristics. Dropseeds ( Sporobolus spp.) and three awns ( Aristida spp.) in livestock impacted grasslands exhibit high mortality rates even during short drought periods (Whitford et. al., 1999). When short-lived species die, land areas with sandy soils lose the root systems and obstruction provided by the above-ground grass stems and leaves, and therefore are subject to wind erosion and/or water erosion. The persistence of Otero Mesa grasslands with intensive livestock grazing is a function of the species composition of the grassland. Any activity that changes the species composition will therefore eliminate the resistant/resilience characteristics of the current grassland ecosystems.
Roads: Most of the roads in the Otero Mesa grasslands have been graded to the caliche (calcrete) layer. Roads that parallel the slope of the terrain have become water conduits. In order to manage the run-off water transmitted down the roadways, deep cuts have been made on the edges of the road bed. The deep cuts serve as channels for the run-off water. Sediments transported downslope in the cut-channels are deposited in the low areas in the shallow valleys. Roads parallel with slopes have been engineered with diversion sluices that carry water and sediments from the road-bed to the deposition areas in the low swales. There is also evidence of road surface materials that have been transported by wind and deposited around emergent plants in areas adjacent to the roads. This results in increased deposition of sediments that potentially bury and kill vegetation in the run-on areas. Oil and gas development will require construction of many additional miles of roads that will increase both wind and water erosion and potentially affect the survival of the local vegetation.
Impacts of Well Pads: The test well pads on Otero Mesa are approximately five acres in size. The soils have been removed from the test well pads and left in long piles at the edges of the pads. The soil piles exhibit signs of soil loss by wind erosion. The well pads are exposed caliche hardpan and are approximately two feet lower than the surrounding terrain. The well pads act as extremely long fetches for wind. Wind moving across long bare patches has maximum erosion power. The well pads are acting as erosion cells. Soils downwind from the bare well pads are exposed to winds that exceed the threshold velocity for entrainment of sand particles. Once sand particles begin to move, they dislodge other sand grains by impact of bouncing sand grains (salutatory action of mobile sand grains) producing a cascading effect of moving sand. This mobile sand buries vegetation at some distance downwind from the well pad.
Impacts of Roads and Pipelines: Construction of additional roads to develop and service well pads will add to the risk of soil erosion and water erosion depending upon the orientation of the roads. A greater risk of degradation and fragmentation of Otero Mesa grasslands will result from construction of pipelines to transport gas and/or oil. Pipelines will have to be buried at least 1 m to 2 m below the surface. Because of the extremely shallow soils of Otero Mesa, trenching for pipelines will, of necessity, break up the water impermeable caliche (cemented calcium carbonate or calcrete). After pipe is laid and the trenches re-filled with soil, the trench lines will serve as a new habitat for plants. Because the caliche layer is breeched, deep-rooted shrubs such as creosotebush and mesquite will be able to establish along pipelines. Expansion of woody shrubs in this shrub-free grassland will fragment the grassland and may compromise the grassland as habitat for Pronghorn Antelope.
Restoring Vegetation on Cleared Well Pads, pipelines and roads: Cleared well pads should be considered as irreparable clearings within the grasslands. There are a number of problems that will be encountered in an attempts to restore a grassland equivalent to that destroyed in the construction of the well pads. When the pads are cleared, the stockpiled soil removed from the pads, is subject to wind and water erosion. Therefore the volume of stockpiled soil remaining to re-cover the pad will be insufficient to return the area to the appropriate depth. Stockpiled soil does not maintain the soil biota that is necessary for a functional soil (Reith and Potter, 1986; Whitford and Elkins, 1986 ). The material returned to the surface of the well pad will not have the rhizosphere (root-zone) bacteria and fungi that are necessary to support nutrient cycling processes nor the protozoans, nematodes and microarthropods that are essential for nutrient cycling processes in arid land soils (Whitford, 1996; Whitford and Parker, 1989; Whitford, 1988; Whitford, 1996; Liu et al. 2000). The inadequacy of the stockpiled soil for support of grasses will limit the efficacy of seeding any type of grass. The reconstruction of the biodiversity of arid rangeland soils requires considerable investment of time and resources and success is not guaranteed.
Because of the unique species composition of the Otero Mesa grasslands, it is essential to consider all of the implications of allowing the use of “commercially available seed mixes” in revegetation efforts. “A prudent approach is to begin by considering native species for most wildlands. They (native species) have proven long-term climatic adaptations and their coexistence with other species suggests compatability” (Whisenant, 1999). When seeking sources of seeds for restoration experiments, I was able to obtain only a few grassland species, such as dropseeds (Sporobolus spp.) and three awns ( Aristida spp.). These grass genera are absent in most of the Otero Mesa grasslands and thus should be considered exotic species. Placing these kinds of grasses on well pads will fragment the native grassland and potentially introduce competitors with the native species.
An additional concern that should not be overlooked is the opinion that seed from distant sources are dangerous to use in reclamation because they are poorly adaptated and contaminate the local genetic material, thereby reducing the vigor and competitive ability of the local plant community (Knapp & Rice, 1994). It has been suggested that native seed should come from local populations and that collections of seeds from areas more distant than 1 Km from the restoration site is dangerous because it risks ‘genetic pollution ( Linhart, 1993). The potential genetic problems with planting grasses on Otero Mesa is exacerbated by the life history characteristics of the dominant grasses. Long-lived species such as black-grama are genetically more variable than short-lived species such as dropseeds and three awns (Linhart, 1993). “When locating plants for harsh environments, it is best to collect seed from similar sites. Many seedings in arid and semiarid ecosystems fail because seeds (or transplanted seedlings) are poorly adapted. Commercial seed sources seldom provide enough seed source information” (pages132-133 in Whisenant, 1999).
The problem soils, commercial sources of seeds, and the potential genetic problems with reseeded rangelands combine to make the risk of inability to restore the Chihuahuan Desert grasslands of Otero Mesa extremely high. On the Jornada Experimental Range, efforts to restore Chihuahuan Desert grassland communities have had limited success. In most cases, the only grasses that re-establish are the short-lived tussock grasses. Even when seeding with black-grama grass was attempted, establishment and survival of this species was so spatially sparse that the area could not be considered a grassland. Suitable climatic conditions for germination, establishment and survival of black-grama from seeds occur at low frequency. Since established black-grama plants survive for several decades, this is not a problem in undisturbed grassland. However, because suitable rainfall amounts and seasonal timing for germination may occur on a decadal time scale, the probability of successful establishment of black-grama grasses from seeding remains very low. Hence even if black-grama seeds were available, it is very unlikely that road cuts, pipleline corridors and well pads could be restored to a black-grama – hairy grama grassland.
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