By David Greenland and John Anderson
from "A Climatic Analysis Of Long-Term Ecological Research Sites"
Field research at the Jornada LTER is conducted in various habitat types found within New Mexico State University's Chihuahuan Desert Rangeland Research Center (25,900 ha) and the adjacent lands of the USDA Jornada Experimental Range (78,266 ha). These lands, which form the Jornada del Muerto Basin in southern New Mexico, are found at the northern end of the Chihuahuan desert (MAP- 47Kb), which extends from southcentral New Mexico,USA to the state of Zacatec as, Mexico, comprising 36% of North American Desert land (MacMahon and Wagner 1985).
Vegetation varies along the north-south axis of the Chihuahuan desert, and the habitat types studied at the Jornada are most representative of the northern Trans-Pecos subdivision of this region. The Jornada LTER focuses on five habitat types: black grama grassland (Bouteloua eriopoda), creosotebush shrubland (Larrea tridentata), mesquite duneland (Prosopis glandulosa), tarbush shrubland (Flourensia cernua) and playa . The playas, dominated by a variety of grasses, are found in low- lying, periodically flooded areas that receive drainage waters from the various upslope communities.
The relatively low latitude of this site brings it generally under high surface atmospheric pressure. It also finds itself under the influence of easterly winds during most months with surface level airstreams having passed over the Gulf of Mexico. However, the site is in the rain shadow of both the San Andres mountains to the east and, for westerly flows, the Black Range and other ranges of the southern part of the western cordillera. Despite this rain shadow effect, in summer the Gulf air can provide moisture for intense convectional thunderstorm activity. This is especially the case when moist Gulf air meets dry air from the Arizona desert. During winter a southerly Pacific airflow can penetrate to Jornada but it is generally limited to the area west of the southern Rockies. Also, although frontal and cyclonic activity is not frequent, it is possible in winter for the area to come under the influence of cold air masses from the north.
The climate of the northern Chihuahuan desert is characterized by high amounts of solar radiation, wide diurnal ranges of temperature, low relative humidity, extremely variable precipitation, and high potential rates of evaporation. The average maximum temperature of 36 C is usually recorded in June; during January the average maximum temperature is l3 C. Precipitation averages 23 cm annually, with 52% typically occurring in brief, local, but intense, convective thundershowers during July to September. Winter precipitation during synoptic weather patterns that derive from the Pacific Ocean is more variable than summer precipitation, but it is more effective in wetting the soil profile.
Despite the fact that there is a summer maximum of precipitation, all of this precipitation is consumed in actual evapotranspiration. The latter is therefore restrained by the low values of the former. These monthly computations mask the fact that in the summer following convection storms there can be adequate soil moisture that might last for several days.
Climatic Factors Affecting Flora and Fauna
The Jornada lies within the Basin and Range physiographic province, in which parallel north-south mountain ranges are separated by broad valleys filled with alluvial materials. This Basin and Range topography extends westward through Arizona and Nevada to the Mojave Desert of California.Throughout this region, soil development is strongly determined by topographic position, parent material, and climatic fluctuations during the Quaternary (Gile et al. 1981). Pleistocene-age alluvial materials form Aridisols with highly developed calcic/petrocalcic horizons, known as caliche, while Holocene alluvium is often poorly differentiated.
Extremes of moisture conditions affect the flora. The general dryness of the climate causes the xerophytic vegetation to adopt numerous strategies for water conservation. These strategies include long root systems, and waxy, impermeable skin surfaces. The existence of a caliche layer in the soil acts as a barrier to moisture loss, giving rise to long term moisture availability to plants during dry seasons (Conley and Conley, 1984). Water conservation methods by the flora are important in light of the five severe droughts that have occurred at the site in the last 100 years (Van Cleve and Martin, 1991). At the other extreme, occasionally a series of convectional storms can leave surface water in the playa. When this happens a number of species, not normally active, can take advantage of the moisture conditions and flourish for a short time. The high diurnal temperature range and the high radiation loads during the day cause many of the fauna to be nocturnal in their feeding habits.
Topographic position, soil development, and human impact interact to determine vegetation dynamics in the northern Chihuahuan desert, where dramatic changes in vegetation have been observed during the last 100 years (Buffington and Herbel 1965). Large areas of former black grama grassland have been replaced by shrubland communities dominated by creosotebush, mesquite and tarbush. This has led to changes in soil resources which have important consequences for ecosystem function, linking the ecosystem processes in deserts to changes in the global environment (Schlesinger et al.1990). Similar changes in vegetation and soils have occurred over large areas of the Chihuahuan desert and in other areas of the world, where semiarid grasslands have been replaced by shrubland vegetation. It is unclear how over-grazing, climatic change, fire suppression, or rising concentrations of atmospheric CO2 have acted solely or in concert to lead to these changes in vegetation. Although the shrubland communities show lower species diversity than the original grasslands, studies at the Jornada LTER show little change in the absolute level of net primary production as a result of these changes in vegetation.
Buffington, L.C . and C.H. Herbel. 1985. Vegetation changes on a semidesert grassland range from 1858 to 1963. Ecological Monographs 35: 139-164.
Conley, M.R. and Conley, W.C. 1984. New Mexico State University College Ranch and Jornada Experimental Range: A summary of Research, 1900 - 1983. Dept. of Fishery and Wildlife Sciences. New Mexico State University. Las Cruces, NM. 83 pp.
Gile, L.H., J.W. Hawley, and R.B. Grossman. 1981. Soils and geomorphology in the Basin and Range area of southern New Mexico--Guidebook to the Desert Project. Memoir 36, N.M. Bureau of Mines and Mineral Resources, Socorro.
MacMahon, J.A. and F.H. Wagner. 1985. The Mojave, Sonoran and Chihuahuan deserts of North America. pp. 105-202. In M. Evenari et al., (eds.). Hot Deserts and Arid Shrublands. Elsevier Scientific Publishers, Amsterdam.
Schlesinger, W.H., J.F. Reynolds, C.L. Cunningham, L.F. Huenneke, W.M.Jarrell, R.A. Virginia and W.G. Whitford. 1990. Biological feedbacks in global desertification. Science 247: 1043-1048.
Van Cleve, K. and Martin, S. 1991. Long Term Ecological Research in the United States: A Network of Research Sites. LTER Network, University of Washington, College of Forest Resources, AR-10, Seattle, WA 98195. 178 pp.