The Chihuahuan Desert, similar to many arid and semiarid ecosystems of the world, has experienced dramatic changes in vegetation structure and ecosystem processes over the past several centuries (Fig. 1). The reasons for the expansion of woody plants and decrease in perennial grasses are numerous and controversial, including livestock grazing, drought, climate change, reduction in fire frequency, and change in small animal populations (Humphrey 1958, Allred 1996, Van Auken 2000, Havstad et al. 2003). The problem is further complicated by the existence of interactions among these factors that feature positive feedbacks and that create threshold behavior and nonlinearity in ecosystem responses (Archer 1989; 1994, Archer et al. 1995, Schlesinger et al. 1990, Rietkirk and van de Koppel 1997). A general consensus does not exist regarding the key factors that control the “desertification” process or the conditions that explain varying patterns of shrub invasion or grass persistence under similar conditions (Yao et al.submitted). It is also unclear why many attempts to remediate shrublands back to grasslands have failed whereas some methods have worked well, but with long time lags (> 50y; Rango et al. 2002). At the Jornada LTER, we are investigating the spatial and temporal variation in these desertification processes and patterns by examining variation in biotic and edaphic factors and their drivers.
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 5 habitat types: black grama grassland (Bouteloua eriopoda), creosotebush scrub (Larrea tridentata), mesquite duneland (Prosopis glandulosa), tarbush shrublands (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 climate of the northern Chihuahuan desert is characterized by abundant sunshine, wide diurnal ranges of temperature, low relative humidity, and extremely variable precipitation. 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.
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. l98l). Pleistocene-age alluvial materials form Aridisols with highly developed calcic/petrocalcic horizons, known as caliche, while Holocene alluvium is often poorly differentiated.
Topographic position, soil development, human impact, and climatic variation (e.g., drought) interact to determine vegetation dynamics in the northern Chihuahuan desert, where dramatic changes in vegetation have been observed during the last l00 years (Buffington and Herbel l965). Large areas of former black grama grassland have been replaced by shrubland communities dominated by creosotebush, mesquite and tarbush. Our goal is to determine 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. We are also interested in identifying the key processes limiting grass recovery and remediation of these systems and the most sensitive sites for remediation potential.