Kris M. Havstad

Rotational Grazing on Rangelands: Reconciliation of Perception and Experimental Evidence

Abstract In spite of overwhelming experimental evidence to the contrary, rotational grazing continues to be promoted and implemented as the only viable grazing strategy. The goals of this synthesis are to 1) reevaluate the complexity, underlying assumptions, and ecological processes of grazed ecosystems, 2) summarize plant and animal production responses to rotational and continuous grazing, 3) characterize the prevailing perceptions influencing the assessment of rotational and continuous grazing, and 4) attempt to direct the profession toward a reconciliation of perceptions advocating support for rotational grazing systems with that of the experimental evidence. The ecological relationships of grazing systems have been reasonably well resolved, at the scales investigated, and a continuation of costly grazing experiments adhering to conventional research protocols will yield little additional information. Plant production was equal or greater in continuous compared to rotational grazing in 87% (20 of 23) of the experiments. Similarly, animal production per head and per area were equal or greater in continuous compared to rotational grazing in 92% (35 of 38) and 84% (27 of 32) of the experiments, respectively. These experimental data demonstrate that a set of potentially effective grazing strategies exist, none of which have unique properties that set one apart from the other in terms of ecological effectiveness. The performance of rangeland grazing strategies are similarly constrained by several ecological variables establishing that differences among them are dependent on the effectiveness of management models, rather than the occurrence of unique ecological phenomena. Continued advocacy for rotational grazing as a superior strategy of grazing on rangelands is founded on perception and anecdotal interpretations, rather than an objective assessment of the vast experimental evidence. We recommend that these evidence-based conclusions be explicitly incorporated into management and policy decisions addressing this predominant land use on rangelands.

Unmanned aerial vehicle-based remote sensing for rangeland assessment, monitoring, and management

Rangeland comprises as much as 70% of the Earths land surface area. Much of this vast space is in very remote areas that are expensive and often impossible to access on the ground. Unmanned Aerial Vehicles (UAVs) have great potential for rangeland management. UAVs have several advantages over satellites and piloted aircraft: they can be deployed quickly and repeatedly; they are less costly and safer than piloted aircraft; they are flexible in terms of flying height and timing of missions; and they can obtain imagery at sub-decimeter resolution. This hyperspatial imagery allows for quantification of plant cover, composition, and structure at multiple spatial scales. Our experiments have shown that this capability, from an off-the-shelf mini-UAV, is directly applicable to operational agency needs for measuring and monitoring. For use by operational agencies to carry out their mandated responsibilities, various requirements must be met: an affordable and reliable platform; a capability for autonomous, low altitude flights; takeoff and landing in small areas surrounded by rugged terrain; and an easily applied data analysis methodology. A number of image processing and orthorectification challenges have been or are currently being addressed, but the potential to depict the land surface commensurate with field data perspectives across broader spatial extents is unrivaled.

State-and-Transition Models for Heterogeneous Landscapes: A Strategy for Development and Application

Abstract Interpretation of assessment and monitoring data requires information about how reference conditions and ecological resilience vary in space and time. Reference conditions used as benchmarks are often specified via potential-based land classifications (e.g., ecological sites) that describe the plant communities potentially observed in an area based on soil and climate. State-and-transition models (STMs) coupled to ecological sites specify indicators of ecological resilience and thresholds. Although general concepts surrounding STMs and ecological sites have received increasing attention, strategies to apply and quantify these concepts have not. In this paper, we outline concepts and a practical approach to potential-based land classification and STM development. Quantification emphasizes inventory techniques readily available to natural resource professionals that reveal processes interacting across spatial scales. We recommend a sequence of eight steps for the co-development of ecological sites and STMs, including 1) creation of initial concepts based on literature and workshops; 2) extensive, low-intensity traverses to refine initial concepts and to plan inventory; 3) development of a spatial hierarchy for sampling based on climate, geomorphology, and soils; 4) stratified medium-intensity inventory of plant communities and soils across a broad extent and with large sample sizes; 5) storage of plant and soil data in a single database; 6) model-building and analysis of inventory data to test initial concepts; 7) support and/or refinement of concepts; and 8) high-intensity characterization and monitoring of states. We offer a simple example of how data assembled via our sequence are used to refine ecological site classes and STMs. The linkage of inventory to expert knowledge and site-based mechanistic experiments and monitoring provides a powerful means for specifying management hypotheses and, ultimately, promoting resilience in grassland, shrubland, savanna, and forest ecosystems.

Disentangling Complex Landscapes: New Insights into Arid and Semiarid System Dynamics

Abstract Although desertification is a global phenomenon and numerous studies have provided information on dynamics at specific sites, spatial and temporal variations in response to desertification have led to alternative, and often controversial, hypotheses about the key factors that determine these dynamics. We present a new research framework that includes five interacting elements to explain these variable dynamics: (1) historical legacies, (2) environmental driving variables, (3) a soil-geomorphic template of patterns in local properties and their spatial context, (4) multiple horizontal and vertical transport vectors (water, wind, animals), and (5) redistribution of resources within and among spatial units by the transport vectors, in interaction with other drivers. Interactions and feedbacks among these elements within and across spatial scales generate threshold changes in pattern and dynamics that can result in alternative future states, from grasslands to shrublands, and a reorganization of the landscape. We offer a six-step operational approach that is applicable to many complex landscapes, and illustrate its utility for understanding present-day landscape organization, forecasting future dynamics, and making more effective management decisions.

Field soil aggregate stability kit for soil quality and rangeland health evaluations

Abstract Soil aggregate stability is widely recognized as a key indicator of soil quality and rangeland health. However, few standard methods exist for quantifying soil stability in the field. A stability kit is described which can be inexpensively and easily assembled with minimal tools. It permits up to 18 samples to be evaluated in less than 10 min and eliminates the need for transportation, minimizing damage to soil structure. The kit consists of two 21×10.5×3.5 cm plastic boxes divided into eighteen 3.5×3.5 cm sections, eighteen 2.5-cm diameter sieves with 1.5-mm distance openings and a small spatula used for soil sampling. Soil samples are rated on a scale from one to six based on a combination of ocular observations of slaking during the first 5 min following immersion in distilled water, and the percent remaining on a 1.5-mm sieve after five dipping cycles at the end of the 5-min period. A laboratory comparison yielded a correlation between the stability class and percent aggregate stability based on oven dry weight remaining after treatment using a mechanical sieve. We have applied the method in a wide variety of agricultural and natural ecosystems throughout western North America, including northern Mexico, and have found that it is highly sensitive to differences in management and plant community composition. Although the field kit cannot replace the careful laboratory-based measurements of soil aggregate stability, it can clearly provide valuable information when these more intensive procedures are not possible.

Taking the pulse of a continent: expanding site‐based research infrastructure for regional‐ to continental‐scale ecology

Many of the most dramatic and surprising effects of global change on ecological systems will occur across large spatial extents, from regions to continents. Multiple ecosystem types will be impacted across a range of interacting spatial and temporal scales. The ability of ecologists to understand and predict these dynamics depends, in large part, on existing site-based research infrastructures developed in response to historic events. Here we review how unevenly prepared ecologists are, and more generally, ecology is as a discipline, to address regional- to continental-scale questions given these pre-existing site-based capacities, and we describe the changes that will be needed to pursue these broad-scale questions in the future. We first review the types of approaches commonly used to address questions at broad scales, and identify the research, cyber-infrastructure, and cultural challenges associated with these approaches. These challenges include developing a mechanistic understanding of the drivers and responses of ecosystem dynamics across a large, diverse geographic extent where measurements of fluxes or flows of materials, energy or information across levels of biological organization or spatial units are needed. The diversity of methods, sampling protocols, and data acquisition technologies make post-hoc comparisons of ecosystems challenging, and data collected using standardized methods across sites require coordination and teamwork. Sharing of data and analytics to create derived data products are needed for multi-site studies, but this level of collaboration is not part of the current ecological culture. We then discuss the strengths and limitations of current site-based research infrastructures in meeting these challenges, and describe a path forward for regional- to continental-scale ecological research that integrates existing infrastructures with emerging and potentially new technologies to more effectively address broad-scale questions. This new research infrastructure will be instrumental in developing an “uber network” to allow users to seamlessly identify and select, analyze, and interpret data from sites regardless of network affiliation, funding agency, or political affinity, to cover the spatial variability and extent of regional-to continental-scale questions. Ultimately, scientists must network across institutional boundaries in order to tap and expand these existing network infrastructures before these investments can address critical broad-scale research questions and needs.

Morphological characteristics of shrub coppice dunes in desert grasslands of southern New Mexico derived from scanning LIDAR

Abstract Since the 1880s rangeland vegetation in southern New Mexico has changed dramatically over widespread areas, typically with shrublands displacing native grasslands. Coincident with these changes in vegetation dominance are increases in soil erosion, stream channel cutting, and shrub coppice dune formation on sandy soils. Where marked transitions in vegetation type from grassland to honey mesquite shrubland have occurred, the local topography has been transformed with previously flat mesa becoming rolling duneland. The size, distribution, and morphological characteristics of these dunes have an important impact on fluxes of energy and nutrients at the surface; they also render the land far less useful as grazing land for domestic livestock. These shrub coppice dunes and the mesquite shrubs that grow on them may be considered roughness elements. Quantifying their morphology is important for the calculation of aerodynamic roughness length and displacement height. This article tests the ability of active scanning laser remote sensing techniques to provide accurate estimates of the three-dimensional shapes and areal distributions of dune and interdune areas. It shows that scanning laser with a footprint diameter of 0.38 m and a sampling interval of 1.5 m to 2 m can be used to measure the morphological characteristics of shrub coppice dunes in the desert grasslands of southern New Mexico with acceptable accuracy and precision for a range of uses, including important geomorphological and hydrological applications. The use of scanning laser systems together with optical multispectral data is shown to be highly synergistic, providing information that is not easily obtainable via other surveying methods.

A test of critical thresholds and their indicators in a desertification-prone ecosystem: more resilience than we thought.

Theoretical models predict that drylands can cross critical thresholds, but experimental manipulations to evaluate them are non-existent. We used a long-term (13-year) pulse-perturbation experiment featuring heavy grazing and shrub removal to determine if critical thresholds and their determinants can be demonstrated in Chihuahuan Desert grasslands. We asked if cover values or patch-size metrics could predict vegetation recovery, supporting their use as early-warning indicators. We found that season of grazing, but not the presence of competing shrubs, mediated the severity of grazing impacts on dominant grasses. Recovery occurred at the same rate irrespective of grazing history, suggesting that critical thresholds were not crossed, even at low cover levels. Grass cover, but not patch size metrics, predicted variation in recovery rates. Some transition-prone ecosystems are surprisingly resilient; management of grazing impacts and simple cover measurements can be used to avert undesired transitions and initiate restoration.

Soil-geomorphic heterogeneity governs patchy vegetation dynamics at an arid ecotone.

Soil properties are well known to affect vegetation, but the role of soil heterogeneity in the patterning of vegetation dynamics is poorly documented. We asked whether the location of an ecotone separating grass-dominated and sparsely vegetated areas reflected only historical variation in degradation or was related to variation in inherent soil properties. We then asked whether changes in the cover and spatial organization of vegetated and bare patches assessed using repeat aerial photography reflected self-organizing dynamics unrelated to soil variation or the stable patterning of soil variation. We found that the present-day ecotone was related to a shift from more weakly to more strongly developed soils. Parts of the ecotone were stable over a 60-year period, but shifts between bare and vegetated states, as well as persistently vegetated and bare states, occurred largely in small (<40 m2) patches throughout the study area. The probability that patches were presently vegetated or bare, as well as the probability that vegetation persisted and/or established over the 60-year period, was negatively related to surface calcium carbonate and positively related to subsurface clay content. Thus, only a fraction of the landscape was susceptible to vegetation change, and the sparsely vegetated area probably featured a higher frequency of susceptible soil patches. Patch dynamics and self-organizing processes can be constrained by subtle (and often unrecognized) soil heterogeneity.

Application of Soil Quality to Monitoring and Management

The concept of soil quality was developed in response to public demand for an increased emphasis on susRecent interest in soil quality and rangeland health, and the large tainability and to a recognition by many in the scientific areas set aside under the USDA Conservation Reserve Program, have community that soil management could be improved by contributed to a gradual convergence of assessment, monitoring, and management approaches in croplands and rangelands. The objective taking a more holistic, integrative approach to soils. of this paper is to describe a basis for integrating soils and soil quality These concerns are reflected in SSSA’s definition of soil into rangeland monitoring, and through monitoring, into managequality: “the capacity of a specific kind of soil to funcment. Previous attempts to integrate soil indicators into rangeland tion, within natural or managed ecosystem boundaries, monitoring programs have often failed due to a lack of understanding to sustain plant and animal productivity, maintain or of how to apply those indicators to ecosystem function and manageenhance water and air quality, and support human ment. We discuss four guidelines that we have used to select and health and habitation” (SSSA, 1997). interpret soil and soil quality indicators in rangelands and illustrate The concept of rangeland health was developed in them using a recently developed rangeland monitoring system. The response to similar concerns. Rangeland health is deguidelines include (i) identifying a suite of indicators that are consisfined as, “the degree to which the integrity of the soil tently correlated with the functional status of one or more critical ecosystem processes, including those related to soil stability, soil water and the ecological processes of rangeland ecosystems infiltration, and the capacity of the ecosystem to recover following are sustained” (Natl. Res. Counc., 1994). Rangeland disturbance; (ii) basing indicator selection on inherent soil and site monitoring and assessment systems have traditionally characteristics and on siteor project-specific resource concerns, such focused heavily on plant community composition and as erosion or species invasion; (iii) using spatial variability in develproductivity. Recent interest in rangeland health and a oping and interpreting indicators to make them more representative growing recognition of the importance of soil–vegetaof ecological processes; and (iv) interpreting indicators in the context tion feedbacks in structuring rangelands (Schlesinger et of an understanding of dynamic, nonlinear ecological processes deal., 1990; Tongway and Ludwig, 1994) have led to a refined by thresholds. The approach defined by these guidelines may newed interest in integrating soil information into rangeserve as a paradigm for applying the soil quality concept in other land monitoring and management. ecosystems, including forests and ecosystems managed for annual and perennial crop production. We have found the following guidelines to be useful in developing integrated soil–vegetation monitoring and management systems for rangelands: W farmers often characterize long-term trends 1. Identify a suite of indicators that are consistently in their land in terms of soil productivity, ranchers correlated with the functional status of one or more are more likely to evaluate changes in the dominant critical ecosystem processes. vegetation. These different perspectives reflect the dif2. Base indicator selection on siteor project-specific ferent approaches to assessing and monitoring cropresource concerns and inherent soil and site charlands and rangelands. Recent interest in soil quality and acteristics. rangeland health, and the large previously cropped areas 3. Use spatial variability in developing and interpreset aside under the USDA Conservation Reserve Proting indicators to make them more representative gram, have contributed to a gradual convergence of of ecological processes. assessment, monitoring, and management approaches 4. Interpret indicators in the context of an understandin croplands and rangelands. Many farmers enrolled in ing of dynamic, nonlinear ecological processes. the Conservation Reserve Program, who have traditionIn addition to these guidelines, measurements inally managed annual monocultures, are now managing cluded in monitoring and assessment systems need to perennial polycultures. The objective of this paper is to be rapid, simple, inexpensive, and repeatable. To the describe some of the ways in which soils and soil quality extent possible, indicators should be predictive: They are being integrated into rangeland monitoring, and should reflect early changes in ecological processes and through monitoring, into management. This integration indicate that a more significant change is likely to occur. may serve as a paradigm for applying the soil quality Each of the four guidelines above is illustrated below concept in other areas. using a monitoring system that was recently developed through an informal interagency collaborative effort led J.E. Herrick and K.M. Havstad, USDA-ARS, and J.R. Brown and by USDA-ARS (Herrick and Whitford, 1999). This A.J. Tugel, USDA-NRCS, Jornada Exp. Range, MSC 3JER, New monitoring system is designed to detect long-term Mexico State Univ., Box 30003, Las Cruces, NM 88003; and P.L. trends in three attributes: soil and site stability, hydroShaver, USDA-NRCS, Oregon State Univ., 202 Strand Agric. Hall, Corvallis, OR 97331. Received 22 May 2000. *Corresponding author logic function, and the biotic integrity of the system. (jherrick@nmsu.edu). Abbreviations: SOM, soil organic matter. Published in Agron. J. 94:3–11 (2002).