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.  

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.  

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.  

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.  

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.  

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 ofstate-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.