Remediation Research in the Jornada Basin: Past and Future

Land degradation in most of the Chihuahuan Desert is characterized by a shift from grass- to shrub-dominated plant communities (Ballín Cortés 1987; Grover and Musick 1990; Fredrickson et al. 1998; see also chapter 10). This shift is associated with increased soil resource redistribution and spatial variability at the plant-interspace scale (Schlesinger et al. 1990; see also chapter 6). Earlier descriptions focused more specifically on the loss of plant species, such as black grama (Bouteloua eriopoda), which were palatable to livestock (Nelson 1934). In 1958, it was estimated that one section (3.2 km2) of black grama grassland could support 18 animal units yearlong, while a similar area dominated by mesquite (Prosopis glandulosa) dunes could support just three animal units (Jornada Experimental Range Staff 1958; see also chapter 13). It was recognized that overgrazing facilitated the increase of less palatable species, including shrubs. Consequently, the objectives of the first organized rangeland research in the Southwest were to identify proper techniques to restore grasslands that had been overgrazed (Jardine and Hurtt 1917; Havstad 1996). Today, we recognize the importance of multiple, interacting factors in addition to overgrazing, and research is more broadly focused on the recovery of ecosystem functions necessary to support multiple ecosystem services. This chapter details this extensive history of research to identify and develop technologies to revegetate, restore, reclaim, rehabilitate, or more generally remediate degraded rangelands. The Society for Ecological Restoration considers that “an ecosystem has recovered when it contains sufficient biotic and abiotic resources to continue its development without assistance or subsidy. It will demonstrate resilience to normal ranges of environmental stress and disturbance. It will interact with contiguous ecosystems in terms of biotic and abiotic flows and cultural interactions” (Society for Ecological Restoration Science and Policy Working Group 2002). Although restoration of perennial grasslands is often cited as the ultimate objective of management intervention in the Southwest, we recognize that in many if not most cases complete restoration of a preexisting plant and animal community is impossible, even if we had perfect knowledge of all of the elements they contained. We also recognize that many of the historic management interventions discussed herein had more limited objectives.

Plant Communities in the Jornada Basin: The Dynamic Landscape

Plant communities of the Jornada Basin are characteristic of the northern Chihuahuan Desert both in structure and dynamics. Although a number of plant communities can be differentiated, five major vegetation types are often distinguished that differ in plant species cover and composition, as well as other factors, such as animal populations, soil properties, and elevation. These five types are black grama (Bouteloua eriopoda) grasslands, playa grasslands, tarbush (Flourensia cernua) shrublands, creosotebush (Larrea tridentata) shrublands, and mesquite (Prosopis grandulosa) shrublands. Similar to many other parts of the Chihuahuan Desert, these plant communities have experienced major shifts in vegetation composition over the past 50–150 years (York and Dick-Peddie 1969). The most dramatic changes in vegetation and associated ecosystem processes have occurred as a result of a shift in life form due to woody plant encroachment into perennial grasslands (Grover and Musick 1990; Bahre and Shelton 1993). This encroachment of shrubs has occurred in many arid and semiarid regions of the world, including the Western United States, northern Mexico, southern Africa, South America, New Zealand, and Australia (McPherson 1997; Scholes and Archer 1997). A number of drivers have been implicated in these grass–shrub dynamics, including various combinations of livestock grazing, small animal activity, drought, changes in fire regime, and changes in climate (Humphrey 1958; Archer 1989; Allred 1996; Reynolds et al. 1997; Van Auken 2000). The causes of shrub invasion are quite variable and often poorly understood, although the consequences consistently lead to the process of desertification (Schlesinger et al. 1990). This chapter describes the characteristics of each vegetation type and the documented changes in each type at the Jornada Basin. We then discuss the key drivers influencing these dynamics. Vegetation in the Chihuahuan Desert region has been classified as desert-grassland transition (Shreve 1917), desert savanna (Shantz and Zon 1924), desert plains grasslands (Clements 1920), desert shrub grassland (Darrow 1944), and shrubsteppe (Kuchler 1964). Desert grassland is often used as a general descriptive name for the area (McClaran 1995), although landscapes at the Jornada and throughout the northern Chihuahuan Desert often consist of a mosaic of desert grasslands, Chihuahuan Desert shrublands, and plains-mesa sand scrub (Dick-Peddie 1993).

Eolian Processes on the Jornada Basin

In arid and semiarid lands, soil erosion by wind is an important process that affects both the surface features and the biological potential of the ecosystem. The eolian flux of soil nutrients into or out of an ecosystem results in enrichment or impoverishment of its biological potential. In the Jornada Basin, wind erosion is the only significant mechanism for the net loss of soil materials because fluvial processes do not remove materials from the basin. Vigorous wind erosion leads to topographic changes, altering the growing conditions for plants and animals. Examples of such changes in topography are the formation of sand dunes or the removal of whole soil horizons. Our goal in this chapter is to describe the construction of a mathematical model for wind erosion and dust production for the Jornada Basin. The model attempts to answer the following questions: 1. Which soils are affected by wind erosion? 2. How does wind erosion occur on Jornada soils? 3. Does changing vegetation cover lead to a change in the source/sink relationship? 4. Is the Jornada a source or sink of eolian materials? If it is a source, what materials are lost? 5. How does wind erosion change the soil-forming process? We will provide provisional answers for the questions and outline work that will more clearly define these answers. Airborne dust has a significant residence time in the atmosphere and acts to modify the radiative properties of the atmosphere, mainly by back-scattering the incoming solar radiation (Andreae 1996). Changing land uses in arid and semiarid areas (e.g., overgrazing and cultivation) can drastically alter the dust emissions to the atmosphere (Tegen et al. 1996). The climatic effects of soil-derived dust were investigated in an experiment in central Asia (Golitsyn and Gillette 1993). Using measured size distributions for emitted dust (Sviridenkov et al. 1993) and various real and imaginary indices of refraction (Sokolik et al. 1993), Sokolik and Golitsyn (1993) calculated climatic effects. Atmospheric dust decreased the total radiative balance of the underlying surface and at the same time induced general warming of the underlying surface–atmosphere system due to a decrease in the system albedo over the arid zones.

Biogeochemical Fluxes across Piedmont Slopes of the Jornada Basin

This chapter is an overview of recent studies of the movement of water, sediment, and nutrients across a principle piedmont slope, or bajada, of the Jornada Basin. Bajadas are extensive, gently sloping surfaces formed by the coalescence of alluvial fans and are a major landscape component of the basin and range province. Over the past four decades a considerable body of research has elucidated the form and function of alluvial fans (Bull 1977; Blair and McPherson 1994; Harvey 1997), but less attention has been paid to bajadas. In particular, the bajadas most neglected are those where channels converge and diverge at irregular intervals downslope. This type of bajada is found at the base of Summerford Mountain, the northernmost peak of the Doña Ana Mountains on the western edge of the Jornada Basin. For convenience, this bajada is hereafter referred to as the Summerford bajada. The research has involved rainfall simulation experiments on small plots, monitoring of two small watersheds on this bajada, and computer modeling of the processes operating in these watersheds and over the bajada as a whole. A detailed understanding of the hydrology and hydraulics of overland flow on this bajada requires a numerical model of the rainfall-runoff process. The objective of this chapter is to detail the model and draw conclusions from model simulations about hydrologic transports of sediment and nutrients across this bajada. Because these piedmonts are important surfaces in this desert (chapter 2) an understanding of their hydrologic and biogeochemical dynamics is crucial to understanding landscape dynamics in the basin and throughout arid regions. Summerford Mountain is a steep-sided, rocky inselberg (i.e., isolated mountain) that rises 380 m above the surrounding bajada to an elevation of 1,780 m. The mountain is composed of monzonite porphyry of Oligocene age (Seager et al. 1976) and has a fringing bajada on its northern and eastern sides. This study focuses on the bajada to the east, which extends 2.5 km to the basin floor at an average gradient of 4%.

Future Directions in Jornada Research: Applying an Interactive Landscape Model to Solve Problems

The long history of research at the Jornada Basin (through the Agricultural Research Service [ARS] since 1912, New Mexico State University in the late 1920s, and joined by the Long-Term Ecological Research [LTER] program in 1981) has provided a wealth of information on the dynamics of arid and semiarid ecosystems. However, gaps in our knowledge still remain. One of the most perplexing issues is the variation in ecosystem dynamics across landscapes. In this concluding chapter to this volume, we propose a new conceptual model of arid and semiarid landscapes that focuses explicitly on the processes and properties that generate spatial variation in ecosystem dynamics. We also describe how our framework leads to future research directions. Many studies have documented variable rates and patterns of shrub invasion at the Jornada as well as at other semiarid and arid regions of the world, including the Western United States, northern Mexico, southern Africa, South America, New Zealand, Australia, and China (York and Dick-Peddie 1969; Grover and Musick 1990; McPherson 1997; Scholes and Archer 1997; see also chapter 10). In some cases, shrub invasion occurred very rapidly: At the Jornada, areas dominated by perennial grasses decreased from 25% to < 7% from 1915 to 1998 with most of this conversion occurring prior to 1950 (Gibbens et al. 2005; Yao et al. 2002a). In other cases, shrub invasion occurred slowly, and sites were very resistant to invasion; for example, perennial grasses still dominate on 12 out of 57 research quadrats originally established in black grama (Bouteloua eropoda) grasslands in the early twentieth century (Yao et al. 2002b). Soil texture, grazing history, and precipitation patterns are insufficient to account for this variation in grass persistence through time (Yao et al. 2002a). It is equally perplexing that although many attempts to remediate these shrublands back to perennial grasses have led to failure, some methods worked well, albeit with long (> 50 year) time lags (Rango et al. 2002; see also chapter 14). Although variations in vegetation dynamics and shrub invasion are the most well known, other lesser known aspects of arid and semiarid systems have been found to be quite variable as well.

Chihuahuan Desert Fauna: Effects on Ecosystem Properties and Processes

This chapter focuses on the direct and indirect effects of animals on ecosystem processes and/or their effects on ecosystem properties. This set of effects has been the primary focus of animal studies on the Jornada Experimental Range (JER) and the Chihuahuan Desert Rangeland Research Center (CDRRC) during the twentieth century. Early studies dealt with animal species that were thought to reduce the amount of primary production that was available to support livestock. With the establishment of the International Biological Programme (IBP) in the late 1960s and its premise that ecosystems could be modeled based on energy flow, animal studies were designed to measure energy flow through consumer populations. Those studies yielded estimates of consumption of live plant biomass between 1% and 10% of the annual net primary production (NPP) (Turner and Chew 1981). From these studies Chew (1974) concluded that in most ecosystems consumers process only a small fraction of the NPP as live plant material but play important roles in ecosystems as regulators of ecosystem processes rather than energy flow. Chew’s hypothesis was then the focus of animal studies in the Jornada Basin for nearly 30 years. Studies of animals as regulators of ecosystem processes led to the expansion of Chew’s hypothesis to include the effects of animals on ecosystem properties, such as patchiness. Many of the studies examined in this chapter support the hypothesis that animals affect spatiotemporal heterogeneity and in turn are affected by it. Because this research focused on the role of animals in ecosystems, studies of animal populations were conducted simultaneously with functional studies. Population and behavioral studies were considered an integral part of the central theme because they supported an understanding of the spatial and temporal variation of desert ecosystem properties. We review animal studies that focused on spatial patterns in the distribution and ecosystem effects of several taxa and guilds. Large-scale ecosystem degradation and vegetation changes in the Jornada Basin occurred prior to studies of animal populations (Buffington and Herbel 1965). Therefore, it is important to bear in mind that the published data on animal populations reflect vegetation and ecosystem conditions that are very different from the conditions in which many Chihuahuan Desert species existed only a century before (see chapter 10).

Soil Development in the Jornada Basin

Soils of the Jornada Basin are the substrate on which Jornada ecosystems reside and interact. Understanding soils and plant–soil feedback processes have been integral to understanding vegetation change and desertification (Buffington and Herbel 1965; Schlesinger et al. 1990). Formal studies of Jornada soils extend back to 1918. The most detailed study of Jornada soils is the USDA-SCS Desert Soil-Geomorphology Project (Gile et al. 1981), a 400-mi2 study area that includes the southernmost areas of the Jornada Experimental Range (JER) and Chihuahuan Desert Rangeland Research Center (CDRRC). This chapter highlights findings of soil and geomorphology studies, discusses factors and processes of soil development, and lists several ways soils of the Jornada Basin carry a memory of past climates. In addition to the Veatch (1918) study and the Desert Soil-Geomorphology Project, other investigations of soil types in the Jornada Basin include three soil surveys by the Soil Conservation Service: the first was of Jornada Experimental Range (SCS 1963), the second was of the White Sands Missile Range that includes the eastern Jornada Basin and San Andres Mountains (Neher and Bailey 1976), and the third was of Doña Ana County (Bulloch and Neher 1980). The 1918 investigation by J.O. Veatch of soils of the Jornada Basin was a reconnaissance study of the Jornada physical landscape. The purpose of the investigation was to make observations on the relation between soils and native vegetation and of the effect of overgrazing on different soil types. Veatch divided the study area into the higher mountain slopes, the foothills, and the Jornada Plain (as he described it, the plain included the currently recognized basin floor and piedmont slope). He recognized that the Jornada Plain was of Pleistocene age and contained extinct lakes with gypsum precipitated from desiccating water. He wrote that little change existed between the soil and subsoil, that “in reality a description of ‘soils’ here is but little more than a description of the various lithologic phases, appearing at the surface of a recent geologic formation.”

Climate and Climatological Variations in the Jornada Basin

The purpose of this chapter is to review the climatic data for the Jornada Basin over the period for which instrumental records exist. Over this time period, up to 83 years in the case of the Jornada Experimental Range (JER), we can deduce both the long-term mean characteristics and variability on a range of different spatial and temporal scales. Short-term variability is seen in individual rainstorms. Longer-term patterns are controlled spatially by factors such as large-scale circulation patterns and basin and regional orography and temporally by the large-scale fluctuations in atmospheric and oceanic circulation patterns. Variability can have significant impacts on the biogeography of a region (Neilson 1986) or its geomorphic processes (Cooke and Reeves 1976), which may set in motion a series of feedbacks, most important those referring to desertification (Schlesinger et al. 1990; Conley et al. 1992). Understanding the frequency and magnitude of such variability is therefore fundamental in explaining the observed landscape changes in areas such as the Jornada Basin. The patterns observed for different climatic variables within the available instrumental records for the Jornada Basin are defined in a hierarchical series of temporal scales, starting with the patterns that emerge from long-term average conditions and moving to seasonal and monthly, daily, and subdaily time scales. Two further analyses are made because of their potential importance to the hydrological and ecological characteristics of the basin, namely, the occurrence of extreme rainfall events and of longer-term changes. The effects of El Niño events in controlling the rainfall over decadal time scales will be addressed in particular. Spatial variability is an additional important concern, especially when characterizing dryland areas such as the Jornada Basin, where spatial variability tends to be high. The overall climate of the basin can be defined according to the Köppen classification as being cool and arid, belonging to the midlatitude desert zone (BWk). However, interannual variability is important, and occasionally, the annual conditions are more characteristic of the semiarid steppe (BSk) zone. The higher rainfall rates in the higher altitudes of the basin are also more characteristic of semiarid conditions.

Introduction

Arid lands throughout the world are often degraded or increasingly at risk of degradation. These lands, including those at the border of arid regions, commonly exhibit accelerated soil erosion, losses of productivity, and impaired economic potential to support human populations. Human history is replete with the collapse of great civilizations of the hot and dry subtropics that suffered severe soil resource depletions in their midst or at their margins. Given that over 1 billion people currently inhabit the arid lands of the world, it is critical that we have the knowledge and resulting technologies to mitigate our impacts and improve environmental conditions of these lands and their resources. This book describes our understanding of basic processes of arid ecosystems resulting from nearly a century of research in one desert locale, the Jornada Basin of southern New Mexico. Much of our understanding comes from both extensive and intensive studies in a landscape that has drastically changed over that time. The loss of ecological, economic, and social capital is called “desertification” (Dregne et al. 1991). The 1992 United Nations Desertification Convention defined desertification as “land degradation in arid, semiarid and dry subhumid areas resulting from various factors, including climatic variations and human activities.” In the future, we can expect that the shifting border between arid and semiarid lands will be one of the most sensitive indicators of global change. Desertification involves human and environmental drivers of change but is a regional symptom that emerges from degradation at finer spatial scales (Reynolds and Stafford Smith 2002). Desertification does not describe cyclic phenomena, as when decadal variations of precipitation lead to periods of drought and to losses of vegetation cover that are fully restored when rains return (Tucker et al. 1994). An updated and revised desertification paradigm has been developed by Reynolds et al. (2003; table 1-1). An important feature of this conceptual model is that both biophysical and socioeconomic factors are jointly involved in desertification. This paradigm clearly recognizes critical points, or thresholds, in system dynamics, yet these points may be manageable for increasing system resilience.

A Holistic View of an an Arid Ecosystem: A Synthesis of Research and Its Applications

A primary objective of the Jornada Basin research program has been to provide a broad view of arid land ecology. Architects of the program, more recently scientists with the Jornada Basin Long-Term Ecological Research (LTER) program, felt that existing ecological data sets were usually of too short a duration and represented too few ecosystem components to provide a foundation for predicting dynamics in response to disturbances (NSF 1979). This recognition gave rise to the approach of using long-term and multidisciplinary research at particular places to advance a holistic and broad-scale but also mechanistic view of ecological dynamics. Such a view is essential to applying ecological research to natural resources management (Golley 1993; Li 2000). In this synthesis chapter we ask: What has this approach taught us about the structure and function of an arid ecosystem? How should this knowledge change the way we manage arid ecosystems? What gaps in our knowledge still exist and why? The Jornada Basin LTER was established in 1981 with the primary aim of using ecological science to understand the progressive loss of semiarid grasslands and their replacement with shrublands. This motivation echoed that which initiated the Jornada Experimental Range (JER) 69 years earlier. The combined, century-long body of research offers a unique perspective on several core ideas in ecology, including the existence of equilibria in ecosystems, the role of scale, landscape heterogeneity and historic events in ecosystem processes and trajectories, and the linkage between ecosystem processes and biodiversity. From this perspective, we examine key assumptions of this research tradition, including the value of the ecosystem concept and the ability to extrapolate site-based conclusions across a biome. The Jornada Basin research program is also uncommon in its close ties to long-term, management-oriented research. The research questions first asked by the U.S. Forest Service and later by the Agricultural Research Service (ARS), such as how to manage livestock operations, frame much of the Jornada Basin research. This allows us to consider the contributions of this research and synthesis toward answering management questions.