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 the USDA Jornada Experimental Range (78,266 ha) and the adjacent lands of New Mexico State University’s Chihuahuan Desert Rangeland Research Center (25,900 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) . It extends from southcentral New Mexico, USA to the state of Zacatecas, Mexico (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, co-dominated by tobosa grass (Pleuraphis mutica), alkali sacaton (Sporobulus airoides), and vine-mesquite grass (Hopia obtusa), are found in low- lying, periodically flooded areas that receive drainage waters from the various upslope communities.
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; minimum temperatures occur in January when the average maximum temperature is l3 C. Precipitation averages 23 cm annually, with 52% typically occurring in brief, local, but intense, convective thundershowers from 1 July to 1 October. Winter precipitation during synoptic weather patterns that derive from the Pacific Ocean is more temporally variable than summer precipitation, but it is more effective in wetting the soil profile because it typically occurs as gentle rain showers.
Despite the fact that there is a summer maximum of precipitation, all of this precipitation is consumed in actual evapotranspiration under high daytime temperatures and low relative humidity. In the summer following convective storms, there can be adequate soil moisture that might last for several days. Runoff is produced during large winter storms and convective events during the North American monsoon, leading to streamflow in channels and ponding of playas in the basin floor.
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 deep root systems (Gibbens and Lenz 2001), 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 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 (Gibbens et al. 2005). 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 (Peters et al. 2012).
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