In LTER V (2000-2006, we propose to expand on the Jornada desertification model in order to understand and predict variation in ecosystem properties and dynamics across multiple scales, rather than focusing only on average conditions through time or across space. This shift in focus to connections across scales was necessitated by our inability to predict temporal and spatial variation in vegetation dynamics (i.e., shrub invasion success and grass persistence and recovery) in the previous model. In particular, we have observed that fine-scale events associated with woody plant invasion can amplify or cascade to result in desertification over increasingly larger areas through time (Peters et al. 2004).
However, important questions remain unresolved, including:
- How do we integrate diverse observations about flora, fauna, soils, hydrology, climate, and human populations across spatial and temporal scales to improve our ability to understand current and historic patterns and dynamics?
- How do processes interact across a range of scales and under different conditions to drive desertification dynamics and constrain the conservation of biological resources? More specifically, under what conditions do fine-scale processes cascade to affect larger spatial scales, and under what conditions do broad-scale drivers constrain or overwhelm fine-scale processes to influence system dynamics?
- How do we disentangle interactions driving landscape dynamics such that we can predict spatial and temporal variation in ecosystem properties related to desertification?
- How can we use knowledge of desertification dynamics to more effectively promote the conservation of biological resources and the recovery of grasslands?
In addressing these questions, JRN LTER V will focus on quantifying interactions between ecosystem processes and patch structure (i.e., area or size, composition, spatial arrangement of bare and vegetated patches at multiple scales) as a means of improving our mechanistic understanding and ability to integrate, predict, and extrapolate across spatial and temporal scales up to and including those relevant to land management and policy.
Our overall hypothesis is that spatial and temporal variation in ecosystem dynamics is the result of patch structure interacting with transport vectors (wind, water, animals) and environmental drivers (e.g., precipitation, temperature, human activities) to influence cross-scale resource redistribution. These interactions feed back to patch structure and dynamics to cause cascading events (Fig. 3) with affects on ecosystem goods and services (Fig. 4). Historic legacies and geomorphic templates are important modifiers of this relationship.
Here we describe specific hypotheses to be tested by integrating long-term data with a strategic suite of new multi-scale experiments designed to elucidate how interactions and feedbacks play out across scales to determine pattern-process relationships and spatial and temporal variation in system dynamics. We also describe how we will use our spatially explicit, multi-scale, multi-transport vector ENSEMBLE simulation model to synthesize and integrate this information in order to generate new testable hypotheses and to predict future system dynamics under alternative environmental conditions and management regimes (Figs. 5, 6).
LTER V seeks to elaborate on the landscape linkages framework that emerged in the latter stages of LTER IV by:
- testing specific elements of our framework using existing long-term studies,
- conducting a suite of new integrated, cross-scale experiments for three geomorphic units and a nearby suburban interface with the Jornada, and
- 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.
Our conceptual framework builds on previous frameworks that seek to quantify the redistribution of resources within and among a hierarchy of spatial units (Fig. 7) (Peters et al. in review). We hypothesize that interactions among five key elements connect spatial units across scales to generate complex dynamics:
- historical legacies that include climate and past disturbances,
- environmental drivers, such as weather, current disturbance regimes, and human activities,
- a geomorphic template that includes both local properties, such as soil texture and geomorphology, and the context and arrangement of spatial units, and
- multiple horizontal and vertical transport vectors (fluvial, aeolian, animal) that interact to
- redistribute resources within and among spatial units (Fig. 3) and (Fig. 4).
Interactions and feedbacks among these elements within and across spatial scales generate threshold changes in patch structure and dynamics that result in cascading events and associated broad-scale conversion from perennial grasslands to shrub dominance. Feedback mechanisms operate at multiple scales: via plants, animals, and soils to influence transport vectors and resource redistribution within each spatial scale, and via the spatial arrangement of vegetation patches, transport vectors, and spatial connectivity in resource redistribution among scales. Complexity, contingency, and the interdependence of system components are major obstacles to prediction in ecosystem science. We believe that the integration of the above elements is a powerful approach for advancing our understanding and forecasting ecosystem dynamics.
We propose to test the proposition that an explicit accounting of processes in the context of patch and geomorphic structure, spatial context, and cross-scale interactions will improve our predictive capabilities by resolving what heretofore has been a large pool of unexplained variance. Our landscape linkages framework addresses three aspects largely missing from previous desertification frameworks.
First, dominant processes and dominant vectors interact and change through time and across space. These cross-scale interactions often generate unexpected dynamics (Peters et al. 2004a). For example, both wind and water can operate as broad-scale drivers for soil, nutrient, litter, and seed redistribution. In arid systems, wind is often the dominant broad-scale driver on sandy soils with little topographic relief, and water is the dominant broad-scale driver on fine-textured soils along topographic gradients. At fine scales, the importance of each driver depends on factors such as surface soil texture, patch structure of the vegetation, and topographic position. In addition, large and small animals redistribute resources and seeds within and among spatial units across a range of scales. Interactions among these multiple transport vectors operating across spatial and temporal scales determine the relative importance of within versus among spatial unit processes to ecosystem dynamics.
Second, spatial context (i.e., location or adjacency of spatial units) and patch structure influence resource redistribution both within and among spatial scales. In arid systems, a key characteristic is the influence of bare patches on horizontal resource redistribution by wind and water. Landscapes with highly connected bare soil patches are expected to promote the rapid movement of materials and disturbances over greater distances, whereas landscapes with low connectivity may have barriers or spatial configurations that restrict horizontal movement of materials (e.g., Ludwig et al. 2005). Highly connected landscapes for one vector may have low connectivity for another vector. For example, grasslands with many small bare patches have low potential for wind and water erosion, yet can be highly connected for grass seed dispersal. By contrast, shrub-dominated systems with large bare patches are highly connected for wind and water erosion and shrub seed dispersal, yet have limited connectivity for grass seed dispersal.
Third, soil-geomorphic organization is a primary determinant of the importance of particular vectors and spatial context that controls resource redistribution. For example, sites on sandy soils with high infiltration rates are expected to experience relatively short distances of horizontal redistribution of nutrients by water compared with sites located on slopes with silty soils and physical soil crusts that limit infiltration rates and promote overland flow. Further, the spatial context of geomorphic units in arid and semiarid systems is predictable within physiographic regions, and these units have predictable relationships with climate and soil development (Monger and Bestelmeyer in press).Read Less
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.Read Less
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.Read Less