This article synthesizes the ecological concepts and perspectives underpinning the development and application of state-and-transition models, thresholds, and rangeland health. Introduction of the multiple stable state concept paved the way for the development of these alternative evaluation procedures by hypothesizing that multiple stable plant communities can potentially occupy individual ecological sites. Vegetation evaluation procedures must be able to assess continuous and reversible as well as discontinuous and nonreversible vegetation dynamics because both patterns occur and neither pattern alone provides a complete assessment of vegetation dynamics on all rangelands. Continuous and reversible vegetation dynamics prevail within stable vegetation states, whereas discontinuous and nonreversible dynamics occur when thresholds are surpassed and one stable state replaces another. State-and-transition models can accommodate both categories of vegetation dynamics because they represent vegetation change along several axes, including fire regimes, weather variability, and management prescriptions, in addition to the succession-grazing axis associated with the traditional range model. Ecological thresholds have become a focal point of state-and-transition models because threshold identification is necessary for recognition of the various stable plant communities than can potentially occupy an ecological site. Thresholds are difficult to define and quantify because they represent a complex series of interacting components, rather than discrete boundaries in time and space. Threshold components can be categorized broadly as structural and functional based on compositional and spatial vegetation attributes, and on modification of ecosystem processes, respectively. State-and-transition models and rangeland health procedures have developed in parallel, rather than as components of an integrated framework, because the two procedures primarily rely on structural and functional thresholds, respectively. It may be prudent for rangeland professionals to consider the introduction of these alternative evaluation procedures as the beginning of a long-term developmental process, rather than as an end point marked by the adoption of an alternative set of standardized evaluation procedures.

Drought is common in rangeland environments and an understanding of its impacts on the structure and function of rangeland ecosystems is paramount for developing effective management strategies. This research was the second of a series of studies investigating the impacts of varying seasonal droughts on northern Great Plains rangelands. Research was conducted on native rangeland during the 1998 through 2001 growing seasons. Study plots were twelve 5 × 10 m nonweighing lysimeters. An automated rainout shelter was used to establish drought conditions on 6 lysimeters during April, May, and June of 1998 and 1999. Single-day, flash grazing events were imposed at the beginning of May, June, and July. Grazing treatments were 1) graze during the 2 years of drought and the year after; 2) graze during the 2 years of drought and rest the year after; and 3) rest all years. Results showed that the intense spring drought reduced soil water content in the upper 30 cm of the soil profile and subsequently reduced total herbage production 20% to 40%; cool-season perennial grasses were the primary contributor to the reduction and cool-season annual grasses were secondary. Periodic grazing during drought had minimal impact on herbage production, whereas impacts on nondrought plots ranged from moderate enhancement to moderate suppression, with effects varying depending on functional group. Results also showed that substantial recovery occurred during the 1st postdrought year, with near full recovery realized within 2 years.

Temperate grasslands are a species-rich ecosystem that may be important in mitigating the increase in atmospheric CO₂. The effect of shrub invasion on CO₂ fluxes in Northern Great Plains grasslands is not known. The objectives of this research were to determine CO₂ and water vapor fluxes over a grazed mixed-grass prairie (prairie site) and a mixed-grass prairie that has extensive invasion of shrubs (shrub prairie site). The Bowen ratio/energy balance (BREB) technique was used to determine CO₂ and water vapor (ET) fluxes during a 4-year period from 1 May to 17 October in 1999, 2000, 2001, and 2002. Aboveground biomass and leaf area index (LAI) were measured about every 21 days throughout the growing season. Peak biomass occurred during July to early August and averaged 1 763 kg · ha⁻¹ for the prairie and 1 808 kg · ha⁻¹ for herbaceous locations in the shrub prairie site. LAI of the herbaceous locations averaged 0.39 for the prairie site and 0.56 for the shrub prairie site. LAI for the shrubs in the shrub prairie site averaged 4.28. Total growing season CO₂ fluxes were similar in prairie and shrub prairie sites, averaging about 350 g CO₂ · m⁻² (positive flux is CO₂ uptake). However, the presence of shrubs altered the seasonal pattern of fluxes. Carbon dioxide fluxes over the shrub prairie site were higher than over the prairie site early in the growing season in May and June, and were often lower than those of the prairie site late in the growing season in August, September, and October. Evapotranspiration rates from May to mid-October were higher in the prairie (521 mm) than the shrub prairie site (461 mm). These results suggest that shrub invasion on Northern Great Plains grasslands does not reduce the potential of grasslands to sequester atmospheric CO₂.

A steady state in soil carbon (C) requires that the production of organic C compounds equal their losses through respiration and erosion. How accelerated rates of soil loss affect this balance in western rangelands, where rates of C accumulation without disturbance are relatively slow, is not well understood. The purpose of this study was to determine the effects of soil loss on total, bare soil, and plant respiration rates at shortgrass prairie and sagebrush steppe sites. The experimental design was a split-plot, randomized block, with a factorial arrangement of treatments. The factor levels consisted of 3 levels of soil removal (0, 11.2, and 22.4 t/ha) and 2 range condition classes (good and fair) with 3 replications each at the shortgrass site. Range condition class was not a factor considered at the sagebrush site. Soil CO₂ efflux was measured at weekly intervals during the 1999 and 2000 growing seasons at the shortgrass site but only during the 2000 season at the sagebrush site. Total, bare soil, and plant respiration rates at the sagebrush site varied little (P > 0.10) among the 3 soil removal treatments. While total respiration at the shortgrass site increased significantly (P < 0.10) as soil removal level increased, bare soil respiration did not vary (P > 0.10) among the 3 soil removal levels. Plant respiration rates at the shortgrass site generally increased as soil removal level increased, but this response was affected by environmental factors that influenced plant productivity. Increased total respiration rates observed on the shortgrass prairie resulted primarily from increased plant respiration rather than from changes in bare soil respiration. Thus, changes in plant respiration following disturbance may be more important to total soil CO₂ efflux than soil flora and faunal respiration that appeared to be more resistant to disturbance.

Water supply affects seed germination and seedling establishment of shallow-rooted warm-season grasses. This may explain the difficulty of incorporating Dallisgrass (Paspalum dilatatum Poir.) into humid temperate grasslands through interseeding. We studied the effects of water availability on seed germination and seedling growth under controlled conditions to determine which step of the establishment process was most affected. In a laboratory experiment, seeds were germinated at 0, –0.25, –0.5, –0.75, and –1 MPa water availability generated with solutions of polyethylene glycol. Although both maximum rate and total germination (P < 0.05) significantly decreased with increased water stress, the speed of germination was even more sensitive. In a greenhouse experiment, variations in seedling emergence and morphological characteristics were measured in relation to water availability. Pregerminated and dry seeds were sown in pots that were irrigated at 1-, 2- , 4-, or 7-day intervals. This species showed high sensitivity to water stress during germination and early emergence. High emergence was obtained from the daily irrigation treatment. In all other treatments, where watering frequency was extended, emergence was decreased. Results suggest that rapid germination and early adventitious root growth can be obtained only with reliable availability of water. These conditions, combined with the high temperatures required for breaking seed dormancy, occur infrequently, explaining the difficulty of achieving successful establishment of Dallisgrass. Water availability during seed germination and seedling emergence is a critical factor for survival of this species.

Willows (Salix) are often a key component of riparian ecosystems and are often browsed by both wildlife and livestock. However, little is understood about how the frequency of browsing affects aboveground and belowground willow production. The objectives of this study were to determine how the frequency of simulated browsing events in a controlled environment affected 1) the aboveground, belowground, and total biomass production and 2) the canopy volume of planeleaf willow (Salix planifolia var. planifolia Prush) plants. The experiment was a completely randomized block design consisting of 2 groups of willow plants with different clipping histories. Within each group, plants were randomly assigned to 1 of 11 subgroups. Clipping events comprised of all combinations of early, middle, and late season periods were imposed on the treatment subgroups, while 1 subgroup served as the control. Canopy volume measurements were made before and after each clipping event. Canopy volume change was related to the harvested twig length (cm) and weight (g). Results suggested that frequency of clipping alone did not explain differences in aboveground and belowground willow production. Instead, willow production was influenced by an accumulation of specific combinations of seasonal clipping events and was dependent on the clipping history of the plants. Early season clipping, alone or in combination with other events, was more detrimental to willows with prior clipping histories than middle or late season clipping treatments. Willow with prior clipping histories treated in either the middle or late seasons, and the late/middle combination produced more than the controls. These results should be verified in willow communities subjected to natural environmental variations and browsing.

We examined the influence of the plains vizcacha (Lagostomus maximus) on the vegetation structure and the characteristics of soil in a wet grassland area of Argentina. This rodent lives in social groups that share a communal burrow system that is occupied for several generations. The areas in which the vizcacha live and feed are strongly affected by grazing, trampling, and soil removal, exhibiting extensive biopedturbation. To evaluate these effects, we carried out a vegetation survey, along areas extending outward from active vizcacheras, analyzing abundance, plant diversity, vegetation cover, and biomass. We also established soil properties, analyzing physical and chemical variables from the center of the colonies to the grassland matrix. Our results show that vizcachas have indeed affected vegetation, diminishing plant cover and grass biomass in their grazing areas. Vegetation in both areas without animal activity and those of intense grazing was dominated by a few characteristic species. We verified the hypothesis of greatest diversity in areas of moderate disturbance. Perturbed areas had higher cation exchange capacity and electric conductivity and higher clay and sodium contents than the other areas. The rodents' activity introduces a recurrent disturbance factor to the landscape of this region, the outcome of which is the alteration of both the composition and structure of the botanical communities, and some soil properties, scaled in gradients of decreasing effect from the center of a colony to the periphery.

Guajillo (Acacia berlandieri Benth.) is considered a medium- to high-quality forage for both wild and domestic ruminants. However, studies have shown that guajillo contains phenolic amines and alkaloids, and condensed tannins, which may cause toxicosis and reduced fertility, intake, and nutrient digestibility. To examine the nutritional value of guajillo to white-tailed deer (Odocoileus virginianus Zimmermann) more thoroughly, we present a comparison of mixed diets of 0%, 25%, 50%, and 75% guajillo in male white-tailed deer. Four in vivo metabolism trials were completed with each diet. Dry matter intake and change in body mass did not differ among diets. Gross and digestible energy intakes did not differ among diets, whereas metabolizable energy intake decreased with increased dietary guajillo concentration. Nitrogen balance and digestibility decreased with increased dietary guajillo concentration. Urinary glucuronic acid excretion increased linearly with increased dietary guajillo concentration. Nitrogen requirements for body growth and antler development were met by diets containing < 60% guajillo, whereas energy requirements for maintenance and antler growth were met with diets containing < 20% guajillo. Therefore, concentrations of dietary guajillo < 20% will support the maintenance of white-tailed deer. The primary function of guajillo may be to facilitate maintenance of adult deer, which have fewer obligatory productive processes than young deer, during periods of drought.

Field data collected over a 20-year period at 8 sites in northwestern New Mexico was used to determine Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young) recovery following control with tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-N′-dimethylurea) and to relate understory perennial grass yield to overstory canopy cover. Tebuthiuron killed between 80% and 95% of mature Wyoming big sagebrush plants within 18 months of chemical treatment, but through recruitment plant numbers equaled or exceeded pretreatment density (plants/m²) at 3 of the 8 sites and were increasing at other locations near the study's end. Wyoming big sagebrush canopy cover averaged <2% the first 10 years after herbicide treatment but had returned to near pretreatment levels (>15%) at 2 sites, to between 5% and 10% at 4 sites, and to less than 3% at the remaining 2 sites. Treatment life was projected to exceed 35 years for 6 of the 8 study sites. Higher rates of tebuthiuron generally extended treatment life. Annual average perennial grass yield increased on treated areas relative to untreated rangeland at all study sites over the 20-year study period. Grass yield was highly variable between years, with pronounced increases when weather and environmental conditions were favorable for grass growth. A nonlinear S-shaped curve best described overstory–understory relationships and also defined the time path of Wyoming big sagebrush recovery, which differed by study site.

The optimal frequency of tebuthiuron (N-[5-(dimetylethyl)-1,3,4-thiadiazol-2yl]-N,N′-dimethylurea) treatments was investigated for Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young) when added forage for livestock and wildlife are considered to be the economic benefit of the treatment. Data collected at 8 northwest New Mexico study sites were used to define key relationships for the economic analysis. This long-lived sagebrush control practice was found to be a viable investment for landowners who participate in available cost-share programs. At productive sites, where average herbaceous production increased to over 700 kg/ha following big sagebrush control, the economic value of added forage justified the total cost of the herbicide treatment. Tebuthiuron rates higher than 0.5 kg active ingredient/ha lengthened the expected life of the brush control treatment, but the extended life did not justify the added cost. The threshold abundance of sagebrush needed for economical control was found to be variable, depending on treatment cost, study site, and the economic value of forage. With a 50:50 cost-share arrangement and with forage valued at $7/AUM, the economic sagebrush canopy threshold from the livestock grazing perspective was estimated to range between 6% and 14%, depending on site productivity. A second brush control treatment would optimally be implemented before forage production was fully depleted by the recovering brush canopy. Because some native fauna are closely tied to big sagebrush plant communities and benefit from the shrubs' presence, the trade-off in the desired abundance of big sagebrush must be weighed between economic considerations and other resource values of interest.

Mauto (Lysiloma divaricatum (Jacq.) J. F. Macbr.; Fabaceae) is a thornless, arborescent legume that is abundant in tropical dry forests in northwestern Mexico. To test whether mauto allometry may be used as an indicator of cattle grazing pressure, we compared plant height, canopy cover, and basal trunk diameter between an area where cattle had been excluded for 12 years with an area under continuous heavy cattle grazing. Mauto plants that had mostly avoided grazing grew to 12 m in height, with an average basal trunk diameter of 11 cm. Under intense grazing, many plants appeared as a bonsai, that is, as small pruned trees with a relatively thick trunk. Such differences were expressed in the linearized (log-log) slopes of the height–diameter and cover–diameter allometric relationships, which varied significantly between the grazed and ungrazed areas. Basal trunk diameter increased faster per unit increase in plant height and canopy cover in the grazed area than in the ungrazed area. Therefore, these morphological or allometric relationships of mauto could be useful for quickly assessing cattle grazing pressure in tropical dry forests.

We evaluated Arizona Cooperative Extension's Rangeland Monitoring Program with the use of focus groups and a self-administered mail survey of grazing permittees and natural resource agency employees. Our primary objectives were to 1) determine whether Extension is reaching its target audience, 2) describe the monitoring practices and attitudes of permittees and agency staff, 3) determine whether there is a relationship between permittees' exposure to Cooperative Extension and their monitoring and management practices, and 4) identify the monitoring information needs and preferences of permittees and natural resource agency staff. We found that Arizona's rangeland monitoring Extension program has been effective in reaching a large part of its target audience, and a significant proportion of Arizona permittees monitor on public, private, and state-owned rangelands. However, overall monitoring adoption rates remain low. Extension contact is associated with use of monitoring and other beneficial management practices, and permittees and agency employees report that monitoring increased their knowledge and led to changes in management. Monitoring by permittees improves agency–permittee relationships in many cases. Most permittees and agency employees believe that their respective peers are the most reliable source of monitoring information and prefer to receive information from Extension through face-to-face contact at workshops or personalized on-site assistance. The evaluation revealed important social dimensions of rangeland monitoring. Extension agents play a key role in facilitating the social process of monitoring, as well as providing technical training in monitoring skills. Further study is needed to investigate whether permittee monitoring actually leads to better management, improved economic returns, or increased tenure security.