Joel R. Brown

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.

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.

Water Relations of a Perennial Grass and Seedling vs Adult Woody Plants in a Subtropical Savanna, Texas

Over 52 ? 16% (Mean ? SE) of the seeds of the arborescent legume Prosopis glandulosa var. glandulosa germinated within two weeks of dissemination in plots dominated by a perennial grass (Chloris cucullata) in July 1984 and 63 ? 7% of those germinating survived through September 1985. Our observations spanned a period of normal temperatures and below-normal precipitation, suggesting the apparent increased abundance of Prosopis on this site in recent times has probably not been episodic with regard to moisture. Over 60% of the herbaceous root biomass occurred in the upper 30 cm of soil. In contrast, tap roots of Prosopis seedlings had penetrated beyond 40 cm within 4 months of germination and their mean proportion of toal biomass belowground increased from 0.27 ? 0.09 in May to 0.52 ? 0.15 in August. Net photosynthesis (Pn) and conductance (g) of Chloris were closely coupled to fluctuations in moisture in the upper soil horizons ( 90 cm.

Patterns of tree dieback in Queensland, Australia: the importance of drought stress and the role of resistance to cavitation

During the extreme 1992–1997 El Niño drought event, widespread stem mortality, or tree “dieback”, of both mature and juvenile eucalypts occurred within the tropical savannas of northeast Australia. Most of the dieback occurred in individuals of the ironbark species complex (Eucalyptus crebra – E. xanthoclada) while individuals of the bloodwood species Corymbia erythrophloia, exhibited significantly less stem mortality. Indicative of greater water stress, predawn and midday xylem water potentials of ironbark adults and saplings were significantly more negative than predawn values of bloodwoods. The very negative xylem water potentials in ironbarks suggest that stem mortality in both adult and juvenile ironbarks results from drought-induced embolism and that ironbarks perhaps have a shallower and less extensive root system than bloodwoods. Although predawn and midday water potentials for ironbark adults and saplings were similar, a census of mature and juvenile ironbark trees indicated that mortality was higher in adult trees. Cavitation vulnerability curves indicated that ironbark saplings may be better buffered against cavitation than adult trees. If they possess smaller root systems, saplings are more likely than adults to experience low xylem water potentials, even in non-drought years. Xylem conduits produced in adult trees during periods of normal rainfall, although perhaps more efficient in water conduction, may be more vulnerable to cavitation during infrequent severe droughts.

Climate Change and North American Rangelands: Trends, Projections, and Implications

Abstract The amplified “greenhouse effect” associated with increasing concentrations of greenhouse gases has increased atmospheric temperature by 1°C since industrialization (around 1750), and it is anticipated to cause an additional 2°C increase by mid-century. Increased biospheric warming is also projected to modify the amount and distribution of annual precipitation and increase the occurrence of both drought and heat waves. The ecological consequences of climate change will vary substantially among ecoregions because of regional differences in antecedent environmental conditions; the rate and magnitude of change in the primary climate change drivers, including elevated carbon dioxide (CO2), warming and precipitation modification; and nonadditive effects among climate drivers. Elevated atmospheric CO2 will directly stimulate plant growth and reduce negative effects of drying in a warmer climate by increasing plant water use efficiency; however, the CO2 effect is mediated by environmental conditions, especially soil water availability. Warming and drying are anticipated to reduce soil water availability, net primary productivity, and other ecosystem processes in the southern Great Plains, the Southwest, and northern Mexico, but warmer and generally wetter conditions will likely enhance these processes in the northern Plains and southern Canada. The Northwest will warm considerably, but annual precipitation is projected to change little despite a large decrease in summer precipitation. Reduced winter snowpack and earlier snowmelt will affect hydrology and riparian systems in the Northwest. Specific consequences of climate change will be numerous and varied and include modifications to forage quantity and quality and livestock production systems, soil C content, fire regimes, livestock metabolism, and plant community composition and species distributions, including range contraction and expansion of invasive species. Recent trends and model projections indicate continued directional change and increasing variability in climate that will substantially affect the provision of ecosystem services on North American rangelands.

Climate change and biotic invasions: a case history of a tropical woody vine

The impacts of climate change in the potential distribution and relative abundance of a C3 shrubby vine, Cryptostegia grandiflora, were investigated using the CLIMEX modelling package. Based upon its current naturalised distribution, C. grandiflora appears to occupy only a small fraction of its potential distribution in Australia under current climatic conditions; mostly in apparently sub-optimal habitat. The potential distribution of C. grandiflora is sensitive towards changes in climate and atmospheric chemistry in the expected range of this century, particularly those that result in increased temperature and water use efficiency. Climate change is likely to increase the potential distribution and abundance of the plant, further increasing the area at risk of invasion, and threatening the viability of current control strategies markedly. By identifying areas at risk of invasion, and vulnerabilities of control strategies, this analysis demonstrates the utility of climate models for providing information suitable to help formulate large-scale, long-term strategic plans for controlling biotic invasions. The effects of climate change upon the potential distribution of C. grandiflora are sufficiently great that strategic control plans for biotic invasions should routinely include their consideration. Whilst the effect of climate change upon the efficacy of introduced biological control agents remain unknown, their possible effect in the potential distribution of C. grandiflora will likely depend not only upon their effects on the population dynamics of C. grandiflora, but also on the gradient of climatic suitability adjacent to each segment of the range boundary.

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).

Climate Change and North American Rangelands: Assessment of Mitigation and Adaptation Strategies

Abstract Recent climatic trends and climate model projections indicate that climate change will modify rangeland ecosystem functions and the services and livelihoods that they provision. Recent history has demonstrated that climatic variability has a strong influence on both ecological and social components of rangeland systems and that these systems possess substantial capacity to adapt to climatic variability. Specific objectives of this synthesis are to: 1) evaluate options to mitigate greenhouse gas emissions and future climate change; 2) survey actions that individuals, enterprises, and social organizations can use to adapt to climate change; and 3) assess options for system transformation when adaptation is no longer sufficient to contend with climate change. Mitigation for carbon sequestration does not appear economically viable, given the small and highly variable carbon dioxide fluxes of rangeland ecosystems and the high transaction costs that would be incurred. In contrast, adaptation strategies are numerous and provide a means to manage risks associated with climate change. Adaptation strategies are diverse, including altered risk perception by individuals, greater flexibility of production enterprises, and modifications to social organizations that emphasize climatic variability, rather than consistency. Many adaptations represent “no regrets” actions because their implementation can be justified without emphasis on pending climate change. Adaptations specific to livestock production systems can include flexible herd management, alternative livestock breeds or species, innovative pest management, modified enterprise structures, and geographic relocation. Social-ecological systems in which adaptation is insufficient to counter the adverse consequences of climate change might undergo transformative change to produce alternative ecosystem services, production enterprises, and livelihoods. The rangeland profession is in a pivotal position to provide leadership on this global challenge because it represents the intersection of management and scientific knowledge, includes diverse stakeholders who derive their livelihoods from rangelands, and interacts with organizations responsible for rangeland stewardship.

How herbivory affects grazing tolerant and sensitive grasses in a central Texas grassland: integrating plant response across hierarchical levels

The hypothesis that herbivore selectivity, rather than plant response to defoliation, is the overriding factor in determining community level responses to grazing was tested in a Texas grassland. We monitored defoliation intensity and subsequent regrowth and reproduction of herbaceous dominants Schizachyrium scoparium and Paspalum plicatulum at the individual tiller, population, and community levels of organization. While long-term observations indicate that Schizachyrium declines and Paspalum increases in response to herbivory, individual Schizachyrium tillers exhibited little response to any level of defoliation and were able to fully compensate for lost tissue on a seasonal basis (...)

SPAnDX: a process-based population dynamics model to explore management and climate change impacts on an invasive alien plant, Acacia nilotica

This paper describes a process-based metapopulation dynamics and phenology model of prickly acacia, Acacia nilotica, an invasive alien species in Australia. The model, SPAnDX, describes the interactions between riparian and upland sub-populations of A. nilotica within livestock paddocks, including the effects of extrinsic factors such as temperature, soil moisture availability and atmospheric concentrations of carbon dioxide. The model includes the effects of management events such as changing the livestock species or stocking rate, applying fire, and herbicide application. The predicted population behaviour of A. nilotica was sensitive to climate. Using 35 years daily weather datasets for five representative sites spanning the range of conditions that A. nilotica is found in Australia, the model predicted biomass levels that closely accord with expected values at each site. SPAnDX can be used as a decision-support tool in integrated weed management, and to explore the sensitivity of cultural management practices to climate change throughout the range of A. nilotica. The cohort-based DYMEX modelling package used to build and run SPAnDX provided several advantages over more traditional population modelling approaches (e.g. an appropriate specific formalism (discrete time, cohort-based, process-oriented), user-friendly graphical environment, extensible library of reusable components, and useful and flexible input/output support framework).