A new typology shows that the notion of rangeland carrying capacity has considerable ambiguity even under conditions of high environmental certainty. When these environmental conditions are highly uncertain, rangeland carrying capacity must be reconceived as a Hahn equilibrium in order to be useful for rangeland development and management. A Hahn equilibrium is a state of affairs which does not cause decision-making agents to change the (meta-)-theories which they hold or the (meta-)-policies which they pursue in their decision-making.

Larkspur (Delphinium spp.) kills more cattle on mountain rangelands in the western U.S. than any other plant, disease or predator. The concentration of toxic alkaloids was measured in 4 larkspur species, at 10 locations, at 2-week intervals during the growing season. In addition, multi-year samples from previous studies were analyzed to determine year-to-year variation in toxic alkaloids. Mountain larkspur (D. glaucum Wats.) had the highest concentration of toxic alkaloids averaged over growth stages (1.01% of dry weight), tall, (D. barbeyi (L.) Huth) and waxy larkspur (D. glaucesens Rydb) were intermediate (0.65 and 0.49% respectively), and duncecap (D. occidentale S. Watts) was lowest (0.29%). Toxic alkaloid concentration generally declined as the plants matured. However, toxic alkaloids in tall larkspur at Yampa, Colo. increased slightly in the pod stage, and toxic alkaloids in waxy larkspur increased from the vegetative to the bud stage. Concentration of toxic alkaloids in tall and duncecap larkspur leaves were higher in plants growing in open sunlight than those shaded under aspen or conifer canopy. Toxic alkaloid concentration varied among individual plants (C.V. 20-60%). Knowledge of the toxic alkaloid concentration of larkspur populations can be used to predict the risk of larkspur poisoning.

Seasonal changes in the botanical composition of diets and protein quality of forages consumed by cottontail rabbits (Sylvilagus floridanus) were monitored on disturbed and undisturbed upland hardwood forest-tallgrass prairies in central Oklahoma. Our primary objective was to evaluate the seasonal dynamics of levels of selected amino acid nutrients in forages required for maintenance, growth, or reproduction, and explore bow these changes respond to habitat disturbance resulting from the use of herbicides and fire. Microhistological analyses of stomach digesta indiccated that summer diets were dominated by Panicum oligosanthes Schultes, Croton spp. and Sporobolus asper (Michx.) Kunth; winter diets were dominated by Bromus spp., P. oligosanthes, and Antennaria spp. Differences in the botanical composition and quality of diets between disturbed and undisturbed habitats were of little biological significance. Changes in the concentration of essential amino acids due to plant maturity were minimal in both summer and winter. Estimated levels of nitrogen and essential amino acids in reconstructed diets (based on food habits) appeared to be low, especially for the sulfur-containing amino acids (methionine + cystine) in summer.

Nitrogen enrichment, immobilization, or inhibition of nitrification were used to investigate the influence of available nitrogen on the seedling recruitment of antelope bitterbrush (Purshia tridentata [Pursh] DC) and annual grass competition. The influence of nitrogen enrichment on antelope bitterbrush seedling recruitment depended on the form of nitrogen applied. Ammonium sulfate applications markedly enhanced growth of herbaceous annuals resulting in the loss of all antelope bitterbrush seedlings the first growing season. Enrichment with calcium nitrate marginally enhanced growth of herbaceous annuals and enhanced the growth of antelope bitterbrush seedlings. Immobilization of nitrogen with carbon (sucrose) applications suppressed the growth of herbaceous annuals and produced large, vigorous antelope bitterbrush seedlings. Similar results were obtained by inhibiting nitrification with applications of nitrapyrin or combining nitrapyrin and carbon applications.

We tested the hypothesis that nutrient content of bluebunch wheatgrass (Agropyron spicatum [Pursh] Scribn. &Smith) either cattle grazed in the spring, rested from cattle grazing for a full year, or given long term rest would all be equal during the given season at 1 location. A 3 pasture rest-rotation grazing system and an exclosure on the Mt. Fleece elk winter range in southwestern Montana were studied during 4 seasons over 3 years. Only nitrogen (N) and phosphorus contents were generally greater in the in the spring grazed regrowth pasture. However, regrowth from bluebunch wheatgrass grazed in the spring did not improve the species nutrient content for wildlife the following winter over nongrazed treatments. During winter when elk (Cervus elaphus nelsoni Bailey) are present, N, TDN, and IVDMD were not among the 3 treatments. Elk were determined unlikely to consume enough bluebunch wheatgrass to meet protein maintenance requirements during winter. Our findings resulted from analyses repeated over the 3 years for a complete cycle of a 3 pasture rest-rotation system: however, our hypothesis needs to be tested at other locations before assuming the same results elsewhere.

We studied the diets of desert mule deer (Odocoileus hemionus crooki Mearns) at 3 sites in Arizona and collated this information with that of previous diet studies of desert mule deer across their range in the United States. We documented 96 browse, 69 forbs, 14 succulent, and 6 grass species that each constituted greater than or equal to 1% of the diet during greater or equal to 1 season. The occurrence of individual plant species varied spatially and temporally. Changes in nutrient levels and climatic influence on relative availability and phenology of plant species likely influenced diet. Desert mule deer rely heavily on browse and forbes, which make up the majority of their diet (> 90%). Grasses and succulents were generally < 5% of the diet. Rangeland managers should strive to keep desert rangelands productive with a diversity of forage so animals have opportunities to exercise free choice of diet.

Ashe juniper (Junius ashei, Buckholz) is increasing on most sites across the Edwards Plateau of Texas. It is the purpose of this investigation to document the influence of Ashe juniper tree size on understory vegetation and to evaluate how the interaction between tree size and browsing by domestic and white-tailed deer modifies overstory-understory relationships. Trees were randomly selected from 2 long-term treatments (browsed and unbrowsed) and analysed with univariate analysis of covariance and multivariate repeated-measures analysis. Without browsing, Ashe juniper is more abundant and its individual influence increases as the size of the tree increases; trees with a canopy diameter < 6.0 m expressed minimal influence on understory vegetation compared to larger trees. When browsers are present at sufficient stocking rates to create a browse line on large trees, encroachment of Ashe juniper is slowed, rate of increase of all woody speces is reduced, and large trees cause a shift in species composition directly under the canopy, however cover of all herbaceous species is not reduced. Immediately under the canopy of small browsed trees, herbaceous cover is lower than for unbrowsed trees. Environmental variables rpsponsible for these patterns were litter depth and light penetrating the canopy when the sun is at an angle (during the winter). The increased cover of several herbaceous species under the canopy of large browsed trees and at the canopy edge of browsed and unbrowsed trees, indicates the importance of the interaction between canopy cover and the presence of a browse line. Browse lines on large trees enhance growth and production of cool season species, such as Texas wintergrass (Stipa leucotricha Trin. & Rupr.) and reduce negative influences (low light, thick leaf litter, etc.) on other herbaceous species. At this level of browsing many other palatable species could be reduced or lost from the plant comunity.