Throughout southern New Mexico where the Jornada research site is located, large areas upland areas that were formerly dominated by perennial grasses, including black grama (Bouteloua eriopoda) and mesa dropseed (Sporobolus flexuosus), have been replaced by desert shrubland species, in particular creosotebush (Larrea tridentata) and honey mesquite (Prosopis glandulosa)(Fig. 1). Historical accounts of the region report significant changes in vegetation starting in the late 1800’s coincident with the expansion of livestock grazing (Smith 1899, Wooton 1908). By 1912, the changes were sufficiently dramatic that area scientists and private land owners convinced the U.S. government to set aside Public Domain Lands for the Jornada Range Reserve for the purpose of scientific investigation on shrub invasion and subsequent loss of forage grasses. Much of this early research focused on quantifying utilization levels for forage species, developing livestock production strategies to deal with drought, and developing methods for shrub control and grass recovery. Exclosures were constructed and long-term plots were established throughout the range to monitor the continued expansion of shrubs across the landscape. Over the decades, numerous trials of various remediation approaches were put into place, from manual and mechanical shrub removal to herbicide application to construction of terraces or other means of redirecting surface flow of runoff.
In the 1970’s, the research focus shifted to an ecosystems approach with the selection of the Jornada Basin for desert grassland and shrubland sites within the International Biological Programme (IBP). In 1982, the Jornada Basin was selected as one of the first LTER sites funded by the National Science Foundation. Although the overall goal of the Jornada Basin LTER is to identify key factors controlling ecosystem dynamics and biotic patterns in Chihuahuan Desert landscapes, the impetus for the work has certainly been influenced by the historical changes that have occurred in this region, and the uncertainty in the causes of those changes.
At the start of LTER I (1982-1989), historic livestock grazing and periodic drought were thought to be the main drivers of vegetation change. The overall hypothesis tested during LTER I was that spatial differences in nitrogen and water availability would impose lags in ecosystem response that would differ between communities located along a topographic gradient. These studies confirmed that nitrogen availability in interaction with water availability can limit primary production, and that spatial and temporal patterns of water and nitrogen availability are related to patterns of water and organic matter transport across the landscape. We continue to derive information on effects of water availability, and effects of release from livestock grazing, on plant community dynamics from the long-term transects set up during LTER I. Although these studies documented important patterns and processes, a conceptual model of the causes and consequences of desertification had not yet been developed.
Original Jornada desertification model -- As part of LTER II (1989-1993), we developed a resource-redistribution model that incorporates spatial variation in the physical environment and disturbance regime as well as positive plant-soil feedbacks to explain desertification dynamics (Schlesinger et al. 1990). This “teeter-totter” model explains shifts between arid shrublands and semiarid grasslands as a result of human activities (such as livestock grazing) or climate change, including drought (Fig. 2). The model focuses on small patches dominated either by a grass plant, shrub or bare area. As grasslands degrade following shrub invasion, patches of bare area appear. Wind and water remove soil nutrients from these bare areas and accumulate nutrients under shrubs to form “islands of fertility”. Thus, at a local scale, desertification results in a shift from spatially homogeneous resources associated with uniformly distributed grass plants to a non-uniform distribution of water and nitrogen associated with individual shrub plants. Grazing and drought are key factors that shift the system towards desertified shrublands, whereas the exclusion of grazers and extended wet periods should favor grasses. These shifts between grasses and shrubs also have important consequences at the regional to global scale where loss of vegetation can lead to higher runoff of rainwater, greater losses of soil nutrients, and the persistence of regional desertification. Research studies at the JRN were expanded to examine small-scale patterns of resource availability in the 5 major ecosystem types (black grama grasslands, playa grasslands, and creosotebush, honey mesquite, and tarbush shrublands), to test the major tenets of the Jornada desertification model. A network of 15 intensive study sites was established to represent the range of variation within each of the 5 major ecosystem types.
As part of LTER III (1994-2000), we continued to test hypotheses associated with the Jornada desertification model by adding two major efforts: (1) extensive studies were added to improve our understanding of the role of physical processes (wind, water) in desertification, and (2) two long-term experiments were established on the relationship between biodiversity and ecosystem function, and the effects of interactions among multiple stressors on ecosystem dynamics.
In LTER IV (2000-2006) we made a natural and essential progression from the plant – interspace model to the exploration of the implications of resource redistribution at larger scales. We hypothesized that if shrub dominance increases the rate at which water and nutrients move within an ecosystem, and if shrub-occupied patches fail to intercept and accumulate all such fluxes, then shrub-occupied landscapes may be overall sources for export of resources. Thus, we initiated several areas of work to better document and understand fluxes of water and nutrients at the landscape scale. We are using our interactive landscape model (see below) as a framework to generate testable hypotheses about ecosystem structure and dynamics, and to predict system behavior. Much of our research since 2000 has focused on quantifying transfers of materials among spatial units across a range of scales for different vectors and for each of the 5 major vegetation types. We demonstrated that a spatial accounting of connections and interactions among landscape components was needed to extract trends and signals from what had previously been regarded as noise.
Aeolian movement of dust and particles is a conspicuous vector for large-scale transport. During LTER III, we documented that the five primary ecosystem types differed significantly in rate of dust and sand generation. In particular, mesquite-dominated ecosystems are the most important sources of this material, behind bare human-disturbed soil surfaces; other shrublands and grass-dominated ecosystems appear to generate only small amounts of dust. Both the extent of bare inter-dune areas, and their spatial arrangement (e.g., into long ‘streets’ oriented along the prevailing wind direction), appear to contribute to dust generation from mesquite sites. In LTER IV, we moved to better understand source-sink relationships for all five ecosystem types, by monitoring particle deposition in each 15 sites monitored for above ground net primary production (NPP) and by analyzing the biogeochemical composition of those particles.
Hydrologic flows represent another primary vector for resource redistribution investigated in LTER IV. Hydrologic work during LTER II and III focused primarily on small scale runoff, plant – interplant space exchanges of water (as influenced by canopy interception, stemflow, microtopography, and infiltration), and the dynamics of streamflow in ephemeral channels. We expanded our work to better understand sheet flow (a major mechanism of water redistribution) and to quantify water transport at a larger, landscape scale. We developed and refined mini-flumes, small instruments deployed to monitor and capture sheet-flow in a large number of locations across the landscape. The mini-flume network focuses on ecotonal areas to assess the frequency and rate of water fluxes across ecosystem contact zones. Other new efforts include detailed monitoring of water levels in stock tanks, in order to better understand how precipitation across the landscape translates into actu al accumulations of water in both natural and human-made low points.
Redistribution of materials (e.g., seeds, nutrients) by small and large animals is the third vector that we studied, primarily in collaboration with USDA ARS scientists. We investigated both the local affects of animals (e.g., granivory, herbivory, trampling) as well as the redistribution of materials within and among spatial units with feedbacks to animal behavior, diversity, and production. These studies are being conducted both within major vegetation types as well as at their ecotones or boundaries between vegetation types.
The evolution from LTER I through IV brings us to the LTER V thesis: a more complete understanding of how patch structure and connectivity interact with environmental and anthropogenic drivers across scales to influence transport vectors and resource redistribution is needed to predict ecosystem states and dynamics. LTER V seeks to elaborate on the landscape linkages framework that emerged in the latter stages of LTER IV by: (1) testing specific elements of our framework using existing long-term studies, (2) conducting a suite of new integrated, cross-scale experiments for three geomorphic units and a nearby suburban interface with the Jornada, and (3) forecasting alternative future landscapes under a changing environment that includes socioeconomic processes and explicit interactions with ecosystems. We will also expand our modeling efforts by integrating a fine-scale model of vegetation and soil water dynamics (ECOTONE, Peters 2000) with existing transport models of wind (SWEMO, Okin et al. 2006), water (Műller 2004), and animal dynamics (under development with ARS funding). This multi-scale, spatially-explicit ENSEMBLE model complements our field studies, and will be used to improve our understanding of complex interactions among system components and to make predictions about future states and dynamics.