Showing papers in "Journal of Range Management in 1991"

Stable states and thresholds of range condition on North American rangelands: A viewpoint

Abstract: The concepts of relatively stable multiple states and thresholds or transitions between these states has received little attention in range management until recently On North American rangelands lower successional stable states occur in sagebrush and other shrub-dominated vegetation types in the Great Basin, the shortgrass steppe, the Southwestern desert grasslands, and communities dominated by annual grasses in California and southern Idaho Recognition of these stable states and models describing them are needed to develop new concepts about range condition The model presently used assumes a single stable state (climax) and that the stages of secondary succession on improving rangelands are the reverse of the stages of retrogession Alternative models presented include the 'cup in ball' analogy, the state-and-transition model, and others While much theoretical work needs to be done before any of these models can be incorporated into range condition standards, it is important for range managers to recognize that multiple steady states exist for many vegetation types One assumption of the current range condition model is that a reduction in grazing pressure and an improvement in grazing management will result in range improvement If a vegetation type is in a stable lower successional state, it normally will not respond to change in grazing or even removal of grazing Managers must recognize this situation when it occurs so that false expectations of improvement are not fostered

Range condition assessment and the concept of thresholds: a viewpoint.

Abstract: Dissatisfaction persists with current approaches to range condition and trend assessment. Sometimes assessed condition does not truly represent the past or the potential of range. One of the likely causes is a failure to re-examine and change if necessary the theoretical basis of assessment, in line with developing understanding of ecological processes. The concept of thresholds of environmental change appears to provide a reasonable alternative in some circumstances to the concepts of gradual retrogression and secondary succession which are currently accepted. I suggest that environmental change can be discontinuous, with thresholds between alternative states. Once it threshold is crossed to a more degraded state, the former state cannot be attained without significant management effort, such as prescribed burning, ploughing, or herbicide application, rather than simple grazing control. Examination of data from extensive monitoring programs and from a study of grazing impact, as well as more general sources of information, indicates that thresholds of change may be identifiable in arid rangelands. A practical means of monitoring proximity to thresholds is available and, with the aid of multivariate analysis, the effects of spatial variability and season can be separated from those of management. The potential of this approach deserves investigation in a wider variety of environments.

Spatial components of plant-herbivore interactions in pastoral, ranching, and native ungulate ecosystems.

Abstract: Thespatialeomponent ofherbivory remainsenigmatic although it is a central aspect of domestic and native ungulate ecosystems. The effects of ungulate movement on plants have not been clearly established in either range or wildlife management. While livestock movement systems have been implemented to cope with increases in livestock density, restrictions on movement, and overgrazing, B large number of studies have disputed the effectiveness of different livestock movement patterns. Traditional pastoralism, particularly nomndism, has been perceived as irrational and even destructive, hut many studies have documented features of traditional pastoral land use that would promote sustainability. Disruptions of wild ungulate movements have been blamed for wildlife owrgrazing and population declines, but actual patterns and mechanisms of disrupted movement and population responses have been poorly documented. Models that integrate plant growth, ungulate movement, and foraging are suggested as a way to improve analyses of spatial plant-herbivore systems. Models must give due attention to nonforage constraints on herbivore distribution, such as topography. Models should assess the significnnee of movement 8s a mans Of coping with local climatic variation (patchy rainfall). Models that distribute an aggregate population over a landscape in relation to the distribution of habitat features deemphasize aspects of ungulate movements and population responses that inevitably cause nonideal distributions, particularly in natural ecosystems. Individual based models describe movement and foraging processes more accurately, hut these models are difficult to apply over large areas. Both top-down and bottom-up approaches to spatial herbiwry are needed. To model plant responses to movement, it is important to account for small scale phenomena such as tiller defoliation patterns, patch grazing, and grazing lawns as well as large scale patterns such as rotation and migration. Herbivory patterns at these difTerent scales are interrelated. Managers of wildlife and domestic livestock populations confront similar problems as they attempt to interpret ungulate spatial distribution patterns and their effects on plants. Overgrazing and subsequentecosystemdegradation onrangelandsorpastoralgrazing areas arc often attributed to inappropriate management of livestock spatial distributions. Overabundances of ungulates in wildlife preserves, and consequent overgrazing, arc often attributed to human interference with natural ungulate migrations or dispersal patterns. These management problems are analogous in that ungulate spatial utilization patterns determine how grazing impacts are distributed in space and in time. Ecosystem sustainability is affected by interactions among animal movement and abundance, plant growth, plant response to grazing, and the physical structure of the landscape. The consequences of herbivory for ecosystems depend, of course, upon herbivore abundance. However, herbivore abundance is expressed in terms of numbers of animals per unit of land, per unit of plant production, or per unit of land per unit time. These measures have been distinguished as stocking density, grazing pressure, and stocking rate, respectively (Heitschmidt and Taylor 1991). Until recently, research and management of ungulate herbivores have treated the denominators of these expressions as homogeneous. The aim of this paper is to explore heterogeneities in these denominators that are normally averaged out.

Grazing impacts on litter and soil organic matter in mixed prairie and fescue grassland ecosystems of Alberta.

Abstract: Impacts of long-term cattle grazing on litter and soil organic matter were assessed in mixed prairie, parkland fescue, and foothills fescue grasslands of Alberta, Canada. Grazing regimes were of light to very heavy intensities, grazed early, late, and continuously during the growing season. Litter and soil organic matter were sampled in O.l-rnz quadrats and removed as live vegetation, standing Utter, fallen litter, and soil organic matter. Litter and organic matter samples were air dried and sorted by size using sieves and an automatic sieve shaker. Organic carbon content was determined by thermal oxidation. Ground cover was determined using point frames, and heights of standing litter and fallen litter were measured. Heavy intensity and/or early season grazing had greater neptive impacts on litter and soil organic matter than did light intensity and/or late season grazing. Under the former regimes there were significant reductions in heights of standing and fallen litter, decreases in live vegetative cover and organic matter mass, and increases in bare ground. More large particle-sized organic matter, particularly standing litter, occurred in controls than in grazed treatments since it would not be removed or trampled by grazing animals. More medium and small particle-sized organic matter occurred in grazed treatments than in ungrazed controls since vegetation likely decomposed more rapidly when it was trampled and broken down as animals grazed.

Plant-animal interactions affecting plant establishment and persistence on revegetated rangeland

Abstract: The role of ungulate grazing in shaping rangeland ecosystems is well known relative to other important plant-animal interactions such as pollination, seed dispersal, granivory, and belowground herbivory. Successful rangeland revegetation may be enhanced by strategies that favor certain groups of animals and discourage others. Many perennial forbs and shrubs require animals for successful pollination, reproduction, and subsequent maintenance of species on a site; however, pollination biology of many rangeland plants and pollinator abundances at potential revegetation sites are largely unknown. Granivory may be significant in some locations and planning and design of revegetation areas may be improved by implementing principles of seed escape mechanisms, such as predator satiation, seed escape in space (low perimeter-to-area ratio for revegetation site), and seed escape in time (synchronous or staggered timing for nearby revegetation sites). Seedling establishment may be associated with invertebrate population levels which need to be considered in future revegetation projects. Timing and site preparation are important in limiting belowground herbivory. Animals can serve as dispersal agents of seeds. Livestock dosed with desirable seeds can disperse them in their dung across the landscape, thereby creating patches of desirable plants. If revegetation sites will be grazed by livestock, then managers should choose plant species that tolerate rather than avoid grazing and should apply adequate management to establish and maintain plant populations. Seeds inoculated with mutualistic species such as mycorrhizae, nitrogen-fixing bacteria, or actinomycetes may enhance establishment, productivity, and nutrient quality of rangeland species while increasing rates of succession.

Perspectives and processes in revegetation of arid and semiarid rangelands.

Abstract: Range revegetation research has been dominated by empirical studies that provide some information about what works or does not work under a given set of conditions, but tell us little or nothing about the underlying ecological processes. Research has emphasized the establishment of vigorous exotic grasses on specific sites rather than the establishment of persistent, biologically diverse plant communities. A more mechanistic research approach is needed to better understand factors governing germination, seedling establishment, and plant community development in natural and synthetic systems to guide revegetation toward biological diversity. This paper evaluates selected aspects of the present knowledge of revegetation science on arid and semiarid lands, and attempts to identify areas for future research direction. Specific concepts and aspects of succession and plant community development, such as seedbed ecology, temporal and spatial patterns of resource availability and use, species life history traits, and species interactions are important areas of research. Continuous measurement of detailed environmental and biological data at the appropriate scale (down to the size of small seeds) will allow development of mechanistic models which can be used to predict plant establishment and community development for different environmental conditions.

Beef cattle distribution patterns on foothill range.

Abstract: A 3-year experiment designed to quantify the spatial and temporal utilization patterns of range sites by beef cattle on summer foothill range was conducted on the Wick Brothers Management Unit of the Wyoming Game and Fish Commission, 8 km w. of Arlington, Wyo. The grazing seasons, in replicate pastures, were from 15 July-9 August, 15 June-26 July, and 15 June-2 August in 1980, 1981, and 1982, respectively. Daily observations were made of radio-telemetry collared cattle (3 per pasture). Cattle dispersion was constrained by the spatial distribution of water and slope. Across 3 seasons, 77% of observed use was within 366 m of water. Approximately 65% of the land area was beyond 723 m from water and sustained only 12% of observed use. Cattle concentrated use (79%) on slopes less than 7%. Consequently 35% of the area, on or surrounded by slopes > 10%, received only 7% of observed use. Loamy, grazable woodland and wetland/subirrigated range sites were most preferred and accounted for over 65% of observed use while occupying less than 35% of the land area. Overall, coarse upland, very shallow and shallow loamy sites were not preferred; however, site preference varied as areas further from water were utilized. Observed use was significantly (P < 0.10) correlated (r 0.41 to 0.69) with standing crop and crude protein standing crop over various growth form characteristics of the forage component. Associated stepwise regression models accounted for 44 to 73% of the variation in observed use over the 1982 grazing season. As the forage complex became more similar, in terms of standing crop and crude protein content, significantly less (P < 0.05) variation in use was accounted for by the forage variables (0-37%).

Water holding capacity of litter and soil organic matter in mixed prairie and fescue grassland ecosystems of Alberta.

Abstract: Litter and organic matter accumuiations can reduce soil water through interception of precipitation and subsequent evaporation of absorbed water. Interception varies with mass and water hoiding capacity (WHC) of litter and organic matter, and is highest from small precipitation events. WHC varies with vegetation type, which is affected by grazing regime. Thus long-term grazing could affect WHC of litter and organic matter and would be important in the hydrologic assessment of rangelands subjected to many small precipitation events throughout the growing season. The study was conducted in mixed prairie, parkland fescue, and foothills fescue grasslands in Alberta, Canada. Grazing regimes were of light to very heavy intensities, grazed early, Me, and continuously during the growing season. Litter and organic matter were sorted by sieving into various sized categories. Litter-soil cores were also evaluated. WHC of litter and organic matter was lower in mixed prairie than in fescue grasslands. WHC increased with increazed particle size, being higher for roots and standing and fallen litter than for organic matter. WHC of iarge particle-sized material decreased with heavy intensity and/or early season grazing. WHC was affected more by intensity than season of grazing. Grazing affected WHC through species composition changes, since species have different WHC, and through trampling which affected particle size. It was concluded that litter and organic matter WHC were important in rangeland hydrologic assessments.

Overgrazing: present or absent?

Abstract: This paper discusses the criteria needed for quantitative evidence of overgrazing and outlines some of the main pasture and external factors that promote overgrazing by herbivores. Overgrazing is defined as occurring where there is a concomitant vegetation change and loss of animal productivity arising from the grazing of land by herbivores. Confirmation of the loss of productivity requires the measurement of departures from the linear relationship between animal productivity and stocking rate for any given animal-pasture system. In the ex-ante situation of an experiment, overgrazing will be observed as a loss of linearity with time. in the ex-poste situation of a comparison between 2 paddocks of the some range type, but different grazing history, overgrazing will be observed as a difference in the optimum stocking rate. The outcome of a species change in terms of productivity is shown to be complex because of the interaction of the quality and quantity influences in both pasture and product. Influences that promote lower stocking rates include low cost-price margins and a negative relationship between product quality and grazing intensity. Conversely, higher stocking rates are promoted by the use of mineral supplements and products such as wool that have a positive relationship between product quality and stocking rate.

Maintaining the long-term productivity of Pacific Northwest forest ecosystems

Abstract: Sustainability continues to be a major issue in forest management policy, and the papers presented in this volume summarize how various management practices affect forest health in the diverse Northwest forest regions, providing guidelines for the present and future.

Influence of temperature and cheatgrass competition on seedling development of two bunchgrasses

Abstract: Rapid seedling growth and ability to compete against cheatgrass (Bromus tectorum L.) are 2 characteristics that perennial grasses must have for successful establishment on semiarid western rangelands. This study was conducted to determine the effects of temperature and competition from cheatgrass on seedling root and shoot growth of 'Hycrest' crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult. X A. cristatum (L.) Gaert.] and 'Whitmar' bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) Loeve). For the temperature study, seedlings were grown in growth chambers with alternate 12-h day/night temperatures of 10/5, 15/5, and 20/5 degrees C. Seedlings were destructively harvested on 6 dates and evaluated for 14 root and shoot characteristics. Leaf development, leaf area, total root length, and number and length of the second group of seminal roots were greater (P < 0.05) for cheatgrass than Hycrest and Whitmar at all dates and temperatures. Cheatgrass elongated adventitious roots earlier and at colder temperatures (10/5 degrees C) than Hycrest, and Whitmar did not produce adventitious roots at low temperatures. This would favor the establishment of cheatgrass at low temperatures. For the competition study, seedlings were established in pots in a greenhouse with planting ratios of Hycrest to cheatgrass and Whitmar to cheatgrass of 1:0, 2:0, and 5:0 (32, 64, and 159 plants m(-2) respectively) and compared with planting ratios of 1:1 and 1:4. Fourteen shoot and root characteristics were evaluated 15, 30, and 50 days after seedling emergence. Hycrest had greater shoot and root development than Whitmar for all seedling characteristics. Competition from cheatgrass reduced growth of Hycrest and Whitmar seedlings. At a planting ratio of 1:4, Hycrest-cheatgrass mixtures depleted soil moisture to lower soil water potentials than Whitmar-cheatgrass mixtures. These results indicate that Hycrest seedlings are more effective competitors with cheatgrass than Whitmar seedlings.

Woody and herbaceous aboveground production of a Patagonian steppe.

Abstract: Aboveground net primary production (ANPP) of the Patagonian steppe in southwestern Chubut (Argentina) was estimated using a harvest technique to assess the herbaceous (mainly grass) component and a double sampling technique to evaluate shrub production. The latter requires the measurement of plant dimensions and the harvest of shrub biomass in small plots. This technique, by virtue of having an explicit biological model which considers both shrub size and production per unit surface of plant, allows comparisons among years, sites, and treatments. Detailed estimates of ANPP yielded a value of 79 g of dry matter (DM) m-2 yr-1 (SE = 19 g DM m-2 yr-1) for an annual rainfall of 191 mm. Our estimates fits (+/- 17%) predictions of 4 models relating primary production to annual precipitation. Two thirds of production were accounted for by perennial grasses and one third by shrubs. A less detailed method, which uses only peak biomass, gave ANPP estimates for 4 additional years ranging from 21 to 75 g DM m-2 yr-1 while annual precipitation during this period ranged from 55 to 167 mm. There was a large reduction in ANPP during a year of extreme drought; however, there were no increases in ANPP during years with above-average precipitation. This suggests that the carrying capacity for the Patagonian steppe may not be linearly related to precipitation.

Rangeland experiments to parameterize the water erosion prediction project model: vegetation canopy cover effects

Abstract: The Water Erosion Prediction Project (WEPP) is a new water erosion prediction technology being developed by the USDA-Agricultural Research Service to replace the Universal Soil Loss Equation. Rangeland field experiments were designed to parameterize the WEPP rangeland erosion model. Included in the field experiments were plot treatments designed to separate direct from indirect effects of vegetation canopy on runoff and soil erosion. Nine rangeland sites from a wide range of soil and vegetation types were evaluated using rainfall simulation techniques. Natural versus clipped treatment surface characteristics and runoff and erosion responses were compared using regression analyses. These analyses showed that there were no significant differences between natural and clipped plot surface characteristics, runoff ratios, final infiltration rates, or initial rainfall abstractions. Erosion rates were different between treatments with the clipped plots having slightly less erosion than the natural plots. Results indicated that, under the rainfall conditions simulated, canopy cover was not directly contributing to initial abstractions through rainfall interception loss or significantly affecting runoff or erosion.

Competition between cheatgrass and two native species after fire: Implications from observations and measurements of root distribution

Abstract: During 1987 and 1988, a study was conducted in northern Nevada to examine root growth of cheatgrass (Bromus tectorum L.) and 2 native species, needle-and-thread grass (Stipa comata Trin. & Rupr.) and rabbitbrush (Chrysothamnus viscidiflorus (Hook.) NW.), after fire. Profile wall maps were used to determine the distribution of roots in the soil profile for the 2 native species. Root morphology differed between the 2 species: needle-and-thread grass had a flabelliform root system, whereas rabbitbrush had a main tap root with 2-4 major lateral roots. Although total root biomass differed between the species, more than half the root biomass was in the top 0.2 m of soil for both needle-and-thread grass and rabbitbrush. Measurements of root length density were used to evahmte the interaction between root systems of cheatgrass and the native species. Root production of plots with only the native species was not significantly different from that of plots with both the native species and cheatgrass for the first 2 years after fire. Furthermore, root production of plots in a recently burned area was also not significantly different from that in an area burned 12 years prior to our study. Thus, root systems of these species rapidly occupied the belowground space and competed for soil resources

Variability of near-surface soil temperature on sagebrush rangeland.

Abstract: Models used to simulate plant growth and insect development on rangelands often assume that soil temperature is homogeneous over the entire area of interest. This simplifying assumption is made because few data are available on the magnitude and structure of the spatial variability of soil temperature within rangeland communities. The influence of sagebrush on the spatial variability and diurnal fluctuations of near-surface soil temperature was examined within a sagebrush-grass plant community. Hourly soil temperatures were measured at 1-, 5-, and 10-cm depths at 30-cm intervals along a 12.3-m north-south transect over a 6-day period in March, 1989. Both classical and geostatistical techniques were used to quantify and model the magnitude and structure of the spatial and temporal variability. Maximum soil temperatures at the 1-cm depth varied from 7 to 23 degrees C under sagebrush and bare interspace, respectively. Periodic spatial patterns in soil temperature were found for all measured depths with a wavelength of periodicity approximately equal to the separation distance between sagebrush plants along the transect. Diurnal variability in near surface soil temperature was much greater in interspace areas compared to under sagebrush plants. The amplitude of diurnal variability in soil temperature at the 1-cm depth under sagebrush was similar to the amplitude of the diurnal variability at the 10-cm depth within the interspaces.

Pasture characteristics affecting spatial distribution of utilization by cattle in mixed brush communities.

Abstract: Utilization patterns of cattle were related to pasture characteristics in a nonrandom and complex manner. Six mixed brush pastures on the Rio Grande Plains (244-356 ha) that were topographically flat and homogeneous in soil type and range sites were studied. Two experiments were conducted: the first experiment was conducted when green forage was abundant and the second under conditions of little vegetative regrowth. A total of 340 random points were characterized for amount, frequency, and greenness of both grasses and forbs, brush and shade tree density, and distance to nearest fence, road, and water. These are variables that can be altered with management practices. When green forage was abundant, factor analysis identified 5 orthogonal factors (green herbage availability, grass quantity, brush abundance, remoteness from roads, and water availability) which accounted for 70% of the communal variation. Six factors (brush abundance, grass quantity, green forb frequency, road location, fence proximity, and water availability) accounted for 70% of the communal variation when herbage was limited. Regression analyses predicting percent utilization from the orthogonal factors indicated that when green forage was abundant, utilization was related largely to green herbage availability, grass quantity, brush abundance, and remoteness (R2 = 0.54, RSD = 0.114). Remoteness, brush abundance, green forb frequency, and water availability were the factors associated with utilization when forage was limited (R2 = 0.45, RSD = 0.152). Green herbage availability was less important under conditions of limited forage. In mixed brush communities, the actual amount of grass, brush abundance, and remoteness were the major factors affecting utilization.

Effects of established perennial grasses on yields of associated annual weeds

Abstract: Perennial grasses are needed for seeding annual grasslands in the Mediterranean/maritime climatic regime of southwest Oregon. Selection of plants for reseeding purposes would be facilitated by identification of perennial grasses that, once established, are able to suppress resident annual plant production. Perennial grasses were transplanted and allowed to establish in the absence of competition for the first growing season at 2 sites in the foothills of southwest Oregon. After the first growing season, resident annual plants were allowed to reinvade. Perennial grasses such as Berber orchardgrass (Dactylis glomerata L. var. Berber) and Idaho fescue (Festuca idahoensis Elmer) that begin growth early suppressed annuals more effectively than later growing perennial grasses such as intermediate and tall wheatgrasses (Agropyron intermedium (Host.) Beauv. and A. elongatum (Host.) Beauv., respectively). Of the perennial grasses adapted to these sites, those which initiated growth earliest, maintained some growth through winter months, and matured earliest were the best competitors.

Vegetational responses of a mixed-grass prairie site following exclusion of prairie dogs and bison.

Abstract: Combined grazing by black-tailed prairie dogs (Cynomys ludovicianus) and bison (Bison) produces and maintains a series of changes in the vegetation of prairie dog colonies. However, because their grazing patterns differ in frequency and intensity through time, their individual impacts may be different. The objective of this study was to determine the individual and combined influences of these 2 herbivores in maintaining selected vegetation characteristics of a prairie dog colony in a mixed-grass prairie at Wind Cave National Park, S.D. This was assessed by monitoring plant responses during 2 years following exclusion from grazing by 1 or both species. In spite of their different grazing patterns, prairie dogs and bison had similar and independent (i.e., additive) effects in maintaining plant community structure. For example, total above-ground biomass increased 32-36% within 2 years of removal of each species, primarily as a result of increases in accumulation of graminoid biomass. Plant species diversity, equitability, and dominance concentration were similar in all treatments both years, although there were slight decreases in relative abundance of forbs and increases in relative abundance of grominoids in the second year after removal of grazers. Mean graminoid leaf nitrogen concentration (May to September) declined slightly but significantly after removal of prairie dogs (1.49 to 1.38%) in 1985, and after bison exclusion (1.64 to 1.50%) in 1986. We suggest that rate of vegetation change following removal of grazers depends upon weather conditions, plant species composition, and prior intensity and duration of grazing.

Effects of tree canopies on soil characteristics of annual rangeland

Abstract: In the central California region of annual rangeland, herbage production beneath blue oak (Quercus douglasii Hook & Arn.) canopies is greater and production beneath the canopies of interior live oak (Quercus wislizenii DC) and digger pine (Pinus sabiniana Dougl.) is less than that in adjacent open grassland. The objective of this investigation was to assess the impact of these major overstory species on soil-associated characteristics in an effort to explain this tree-herbage production relationship. Greater amounts of organic carbon (OC), greater cation exchange capacity (CEC), lower bulk density, and greater concentrations of some nutrients were found beneath blue oak canopies than in open grassland. This explains, at least in part, the increased herbage production beneath blue oak canopy.

Influence of seedbed microsite characteristics on grass seedling emergence.

Abstract: Successful germination and establishment of grass seedlings from surface-sown seeds requires a microsite which provides adequate soil water and temperature conditions, among other species-specific requirements. The microsite where these requirements are met has been termed a "safesite". Safesites may occur naturally as cracks and depressions in the soil surface, gravel, and plant litter, or be prepared by seedbed equipment and livestock trampling. A greenhouse study was conducted to determine the influence of seedbed microsite characteristics and soil water treatments on seedling emergence of 'Vaughn' sideoats grama (Bouteloua curtipendula (Michx.) Torr.), 'A-130' blue panic (Panicum antidotale Retz.), and 'Cochise' Atherstone lovegrass (Eragrostis lehmanniana Nees X E. tricophera Coss and Dur.). Although there were several interactions, in general, emergence of all 3 species was highest from gravel, followed by litter, cracks, and finally the bare soil surface. Bare surface sites decreased in water content more quickly than the other sites. Cochise lovegrass had high emergence in gravel under all water treatments. Small-seeded species such as Cochise lovegrass broadcast on coarse-textured surface soils may establish with minimal seedbed preparation, provided summer precipitation is adequate.

Evaporation from rangeland with and without honey mesquite.

Abstract: The Bowen ratio/energy balance technique was used to estimate evaporation (E) from honey mesquite (Prosopis glandulosa Torr. var. glandulosa) as the difference in total E (plant and soil) between that of adjacent mesquite-dominated and mesquite-free rangeland areas near Throckmorton, Tex. After treatment with diesel in July 1988 to defoliate the honey mesquite, E from the treated area decreased to a minimum value of about 40% of that from the untreated area. In 1989, seasonal E totals from the untreated and treated areas were, respectively, 190 and 176 mm-a 7% reduction in E due to mesquite defoliation. Total E for the herbaceous and honey mesquite vegetation in 1989 in the untreated area was 117 and 73 mm, respectively. Honey mesquite foliar cover was 15% in the untreated area, but it accounted for 38% of the total E. These honey mesquite E data were supported by independent measurements of sap flow. While honey mesquite used substantial amounts of water, E from the rangeland from which it was removed was just slightly lower due to increased herbaceous evaporation associated with increases in standing crop. Under the circumstances of low grazing intensity and low runoff potential, honey mesquite removal would provide little if any additional water for off-site uses in the short-term and, therefore, the removal of this species for purely hydrological purposes would not be justified. Increases in off-site water availability may, however, result from honey mesquite control under grazing regimes which preclude accumulation of additional herbaceous standing crop or at site with greater runoff potential.

Effects of cattle trampling and mechanical seedbed preparation on grass seedling emergence.

Abstract: Cattle trampling has been recommended to bury seeds and encourage seedling establishment but has not been compared with traditional seedbed preparation techniques. We compared seedling emergence of broadcast-seeded 'Vaughn' sideoats grams [Bouteloua curtipendula (Michx.) Torr], 'A-130' blue panic (Panicum antidotale Retz.), 'A-68' Lehmann lovegrass (Eragrostis lehmanniana Nees), and 'Cochise' atherstone lovegrass (Eragrostis lehmanniana Nees X E. tricophera Coss and Dur.) on lightly and heavily trampled seedbeds with that on undisturbed, land imprinted, and root-plowed or ripped seedbeds. We seeded and applied the treatments prior to summer rains on a sandy loam soil in southern Arizona for 3 years. In a wet year (1987) when surface soil water was estimated to be available for at least 24 consecutive days, heavy trampling and land imprinting increased emergence of blue panic and land imprinting increased emergence of Cochise lovegrass. In that year, lovegrass emergence was high even on undisturbed plots. In a moderately wet year (1988), surface soil water was available for periods of 6-9 days during seedling emergence and greater disturbance, either by heavy trampling, land imprinting and/or root plowing or ripping produced higher emergence than light trampling and nondisturbance. In a dry year (1989), surface soil water was available for periods of 2-3 days and seedling emergence was low and generally similar for all treatments. Sideoats grama emergence was low all 3 years, but was highest in 1988 when initial thunderstorms were followed closely by subsequent storms. Seedbed disturbance by cattle and mechanical methods may enhance revegetation in the Southwest in years of moderate precipitation but may be unnecessary in wet years or futile in dry years, depending on species and soils.

Economic evaluation of spotted knapweed [Centaurea maculosa] control using picloram.

Abstract: Spotted knapweed is the most serious range weed problem in western Montana. Although picloram is often used to control knapweed, the economic feasibility of the practice has not been evaluated. We developed a model to economically evaluate spotted knapweed control on rangeland. Model functions describing the dynamics of the plant community preceding and following treatment were derived from field observations in western Montana. Economic returns per management unit were calculated for 3 scenarios: (1) no treatment, (2) containment, and (3) eradication of spotted knapweed. After tax costs and benefits of treatments were analyzed for a 20-year period and discounted to the present. An economic loss in current dollars of S2.38/ha was incurred under the no treatment strategy when 25% of the management unit was initiaily inlested with spotted knapweed and the weed was spreading to new acres and replacing desirable forage. After-tax present value of added AUMs in the eradication strategy was greater than the after-tax present value of added costs, S3.41/ha and Sl .99/ha, respectively. As site productivity, value of an AUM, and rate of knapweed spread to new acres increased, economic returns increased relative to treatment costs. In contrast, herbicide treatment became leas cost-effective as knapweed utilization by livestock increased. Thus, economic feasibility of spotted knapweed control varied with economic and biologic conditions.

The grass seedling: When is it established?

Abstract: Adventitious roots of sufficient length and diameter must develop to assure that the photosynthetic surfaces receive sufficient water and nutrients before grass seedlings can be considered established. We evaluated development of crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.] and blue grama [Bouteloua gracilis (H.B.K.) Lag.] seedlings in the field to decide when they were established. Blue grams and crested wheatgrass seedlings, under the environmental conditions of this study, were considered established 21 days after emergence. At this time, crested wheatgrass seedlings had 4 leaves, 2 adventitious roots penetrating to a depth of at least 80 mm into the soil, and 1 tiller per plant. Blue grama seedlings had about 6 leaves, 2 adventitious roots penetrating to a depth of at least 100 mm into the soil, and 2 tillers per plant. Most seedlings that reached this stage by the end of the first growing season overwintered and survived the following growing season and provided adequate stands for both species.

Correcting estimates of net primary production: are we overestimating plant production in rangelands?

Abstract: This paper addresses the issue of the effect of random errors in field estimates of net primary production (NPP). This is a critical subject in range management because field estimates of plant production are regularly used to determine stocking rates, range condition, and animal consumption. What we show in this paper is that random errors associated with field estimates of NPP can result in a positive bias and thus an overestimation of NPP. Depending on the case, this overestimation has been reported as high as 700%. We present examples with overestimations in the 200% to 400% range. The overestimation in NPP increases with increases in biomass variances, frequency of sampling, and number of taxonomic (species) and tissue (live, dead, etc) components sampled. We (1) outline in nonmathematical terms the reasons behind overestimation in NPP and the analytical solutions designed to correct them; and (2) present applications of the analytical solution for adjustments to concrete cases. The adjustments for overestimation outlined in this paper do not guarantee an accurate estimate of NPP but eliminate an unneeded source of error. A computer program (for IBM(TM) compatible) designed to implement the necessary adjustments is available from the authors free of charge (send a blank diskette).

Effects of competition on spatial distribution of roots of blue grama

Abstract: The spatial distribution of roots of the perennial grass blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Griffiths] was evaluated under 2 competitive conditions. The radioistope 14C was used to label roots of blue grama plants growing with and without neighboring plants of the same lifeform. The majority of labeled blue grama roots (>75%) were found within 5 cm of the plant and within 10 cm of the soil surface. Labeled roots extended at least 30 cm from the edge of the plant and to depths of at least 90 cm. Root system morphology was insensitive to changes in competitive conditions. Based on our estimates of the depth and breadth of the root system of an average blue grama plant, roots associated with at least 4 other blue grama plants of average size and separated by average distances of 10 cm might occur within the volume of soil associated with roots of this plant. The distribution of total root biomass was not representative of the distribution of labeled roots, even when neighboring grasses were removed.

Response of cottontail rabbit populations to herbicide and fire applications on cross timbers rangeland

Abstract: Knowledge of how resident wildlife populations respond to brush management strategies is especially limited for rangelands in the cross timbers vegetation type of Oklahoma. We examined how cottontail rabbit (Sylvilagus floridanus) density and habitat use were influenced by applications of tebuthiuron or triclopyr, with and without annual burning, on cross timbers rangeland. Line transect flush-counts, mark-recapture livetrapping, and fecal pellet counts were used to evaluate seasonal differences in population density among 5 brush control treatments. Cottontail rabbits (n = 225) were flushed along 362 km of line transects during 5 census periods. Density in winter was consistently lower than summer for all treatments, except for the untreated control in winter 1987. Line transect density estimates varied from 0 to 1.975 rabbits/ha and suggested that herbicide and annual burning treatments had a positive influence on cottontail rabbit populations compared to untreated controls. Mark-recapture density estimates did not differ among treatments. Fecal pellet counts were greater on herbicide-treated pastures than an untreated control in both spring and fall. Prairie-eastern redcedar (Juniperus virginiana L.) and forest-prairie ecotone habitats were utilized greater than expected by cottontail rabbits. Mature hardwood overstory and mixed-brush habitats were avoided. Tebuthiuron and triclopyr effectively deceased hardwood overstory and increased preferred habitats for cottontail rabbits.

Root morphological development in relation to shoot growth in seedlings of four range grasses

Abstract: Successful seedling establishment in arid and semiarid rangelands depends on seedling root characteristics and on the relationship between shoot and root development. This study was conducted to determine seedling shoot and root developmental characteristics of 'Hycrest', a hybrid cultivar of crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult. X Agropyron cristatum (L.) Gaert.]; 'Whitmar', a cultivar of bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Love subspecies inermis (Scrib. and Smith) Love]; 'Secar', a cultivar of Snake River wheatgrass [Elymus lanceolatus (Scribner & J.G. Smith) Gould]; and cheatgrass (Bromus tectorum L.) under favorable growth conditions. Seedlings were grown in 20-cm X 20-cm pots filled with sandy loam soil in a greenhouse and were destructively harvested 9, 17, 24, 31, 39, and 45 days after emergence. Cheatgrass had greater (P<0.05) plant height, leaf area, total shoot dry weight, primary root length, number and order of branching of the second group of seminal roots, order of branching of the first group of adventitious roots, and total root dry weight than Hycrest, Whitmar, and Secar. Hycrest had greater (P<0.05) seedling growth than Whitmar and Secar. The pattern of root and shoot development was similar in the 4 species; however, species differed in the cumulative growing degree days required to initiate elongation and branching of siminal and adventitious roots. The close association between the pattern of root development and shoot growth in the 4 species may be useful in deriving models of root morphological development based on shoot development.