Millennial-Scale Climate Variability and Ecosystem Response at the Jornada LTER Site

The Jornada Long-Term Ecological Research (JRN LTER) program consists of studies superimposed on three research entities, the Jornada Experimental Range, the Chihuahuan Desert Rangeland Research Center, and the Desert Soil-Geomorphology project (figure 17.1). The JRN site is in the northern part of the Chihuahuan Desert and represents, for the LTER network, the desert shrubland and desert grassland ecosystems of the southwestern United States. Climate data at the Jornada site and surrounding area span the last 110 years. Ecological data span the last 144 years. Despite having over 100 years of data, researchers at the Jornada LTER have struggled to answer the focal question of this book: How have ecosystems responded to climatic variability? This is because, simultaneous with climate, another important factor has had a major impact on ecosystems—human land use. Cattle grazing, brush control, and habitat fractionation have merged with climate to produce external pressures on Jornada ecosystems (Schlesinger et al. 1990; Havstad et al. 2000). Even more uncertain is the cause-and-effect relationship between climate and ecosystems in prehistoric times. Here evidence is limited to indicators, such as former lake shorelines, plant fossils in packrat middens, fossil pollen, 13C/12C ratios in paleosols, and erosion rates. When some indicators are used by themselves, circularity arises if a conclusion about ecosystem response to climate change is based on an inference about climate change, which is based, in turn, on ecosystem change. For example, grasslands increased at the end of the middle Holocene as the result of increased rainfall, where the interpretation of increased rainfall is based on increased grass pollen in the middle Holocene sediments (Freeman 1972). Although focusing on millennial-scale climate and ecosystem variability, this chapter briefly discusses historic variability for comparison and as a means for describing the setting. The historic-prehistoric boundary for the Jornada area has been set at A.D. 1850 (table 17.1). Located at 32.5º N and 106.8º W, in New Mexico, USA, the Jornada LTER site is in the Basin and Range province (Peterson 1981), which is characterized by parallel mountain ranges separated by structural basins filled with Cenozoic sediments (Hawley 1986). Elevations at the Jornada range from 1,180 m (3,870 ft) in the Rio Grande floodplain to 2,749 m (9,012 ft) in the Organ Mountains.