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The goal of this synthesis is to initiate development of a unified framework for threshold assessment that is able to link ecological theory and processes with management knowledge and application. Specific objectives include the investigation of threshold mechanisms, elaboration of threshold components, introduction of threshold categories and trajectories, and presentation of an operational definition of ecological thresholds. A greater understanding of ecological thresholds is essential because they have become a focal point within the state-and-transition framework and their occurrence has critical consequences for land management. Threshold occurrence may be best interpreted as a switch from the dominance of negative feedbacks that maintain ecosystem resilience to the dominance of positive feedbacks that degrade resilience and promote the development of post-threshold states on individual ecological sites. Threshold categories have been identified to serve as ecological benchmarks to describe the extent of threshold progression and increase insight into feedback mechanisms that determine threshold reversibility. Threshold trajectories describe the developmental pathway that post-threshold states may follow once a threshold has been exceeded. These trajectories may produce a continuum of potential post-threshold states, but the majority of them may be organized into four broad states. This framework lends itself to management application by providing an operational definition of thresholds that is based on a probabilistic interpretation. Probabilities associated with 1) the occurrence of triggers that initiate threshold progression, 2) the trajectory of post-threshold states, and 3) threshold reversibility will provide an operational procedure for threshold assessment and application. If thresholds are to play a central role in rangeland ecology and management, then the rangeland profession must accept responsibility for their conceptual development, ecological validity, and managerial effectiveness.
Following the 1999 Railroad Fire in Tintic Valley, Utah, we initiated a large-scale fire rehabilitation study comparing a predominately introduced species seed mix used by the US Department of Interior–Bureau of Land Management (BLM), a mix of native and introduced species provided by the US Department of Agriculture–Agricultural Research Service (ARS), and 2 native seed mixes (high and low diversity). Mixes were seeded with a rangeland drill on the big sagebrush (Artemisia tridentata var. wyomingensis [Beetle & A. Young] Welsh) study area whereas the pinyon–juniper (Pinus edulis Engelm.–Juniperus osteosperma [Torr.] Little) woodland study area was aerially seeded followed by 1-way chaining. On drill-seeded plots and by the third year after seeding the native high-diversity mix (16.4 kg pure live seed [PLS]·ha−1) had the highest seeded species cover (11.5%) and density (14 plants·m−2). Both the BLM (9.3 kg PLS·ha−1) and ARS (9.1 kg PLS·ha−1) seed mixes had higher seeded species cover (BLM = 8.5%, ARS = 8.2%) and density (BLM = 8.4 and ARS = 7.2 plants·m−2) than plots seeded to the low-diversity native mix (8 kg PLS·ha−1, cover = 3.8%, density = 3.6 plants·m−2). Indian ricegrass (Achnatherum hymenoides [Roemer and J. A. Schultes] Barkworth ‘Nezpar’) in the native high-diversity mix was especially successful on the sandy soils of the drill site, whereas seeds of other species may have been buried too deep for optimum emergence. Aerially-seeded and chained plots had similar and successful seeded species frequency, cover, and density (third-year average = 10.6% cover, 17.2 plants·m−2) among all species mixes. All seeded plots had lower cover of annual species than unseeded plots, indicating that revegetation is necessary to reduce weed invasion following catastrophic wildfire in big sagebrush communities lacking residual perennial understory vegetation.
We evaluated the adequacy of rotational grazing to improve rangeland condition in the Flooding Pampa region, eastern Argentina, comparing the floristic composition dynamic of the 2 main plant communities under rotational and continuous grazing over a study period of 4 years (1993–1996). The experiment was conducted in commercial farms located in 4 sites of the Flooding Pampa region. In each site, a couple of farms, one managed under rotational grazing (implemented in 1989) and an adjacent one managed under continuous grazing at a similar stocking rate (1 AU·ha−1), constituted the replications of the experiment. Basal cover of species, litter, and bare soil were monitored in midslope and lowland grassland communities on each farm. Total plant basal cover in midslope and in lowland communities remained unchanged over the whole experimental period under both grazing methods. Under rotational grazing, litter cover was higher in both communities while the amount of bare soil showed a significant reduction in lowlands and a tendency to be lower in midslope. Basal cover of legumes, C3 annual and C3 perennial grasses was higher, while cover of C4 prostrate grasses was lower under rotational grazing in the midslope community. In the lowland community, rotational grazing effects were evident only in the drier years, when higher cover of hydrophytic grasses and legumes and lower cover of forbs occurred. Plant species diversity did not change in response to grazing. In conclusion, rotational grazing promoted functional groups composed of high forage value species and reduced bare soil through the accumulation of litter. These changes indicate an improvement in rangeland condition and in carrying capacity. As the stocking rate was approximately 60% higher than the average stocking rate of the Flooding Pampa region, we believe that productivity and sustainability may be compatible by replacing continuous with rotational grazing.
There is increased interest in the use of summer-season fires to limit woody plant encroachment on southern prairie grasslands, but collateral effects of these fires on grasses are poorly understood. We quantified effects of repeated winter fires, repeated summer fires, simulated grazing (clipping), and their interaction on yields of the C4 midgrass, sideoats grama (Bouteloua curtipendula) in northern Texas. Monoculture patches of sideoats grama were exposed to 1 of 3 fire treatments: 1) no burn, 2) 2 winter fires in 3 years, or 3) 2 summer fires in 3 years; and to 1 of 2 clip treatments (no clip or clip once each spring). Total yield (live standing dead), live yield, percent live tissue, and foliar cover were measured in spring and late-growing season (late-season) over a 7-year period. In unclipped plots, late-season total yield did not fully recover until 2 growing seasons after winter fires and 3 growing seasons after summer fires. By 5 years postfire, total yield was greater in both fire treatments than in the no burn. Live yields recovered more quickly than total yields following summer fires but never exceeded the no burn. Percent live tissue was greater in both fire treatments than in the no burn for up to 2 years postfire. Clipping reduced total and live yields in the no burn and winter-fire treatments but not in the summer-fire treatment. By 5 years postfire, total and live yields were greater in the summer fire clip than the no burn clip or winter fire clip treatments. Results suggest that 1) sideoats grama is tolerant of summer fires but full recovery may require at least 3 years, and 2) in the long-term, summer fire clipping may stimulate sideoats grama production more than winter fire clipping or clipping alone.
Tropospheric ozone (O3) is a phytotoxic air pollutant widespread in industrialized nations of the world. Ozone is produced by the photo-oxidation of hydrocarbons released into the atmosphere by combustion of fossil fuels. Studies demonstrate O3 can be transported from metropolitan areas to rural areas important to agricultural and forestry practices. Reports regarding O3 effects have focused on vegetation important to food production or agronomic crops of economic importance. However, relatively little is known about O3 effects on native plant species. The effects of tropospheric O3 on two warm-season grasses, eastern gamagrass (Tripsacum dactyloides L.) and big bluestem (Andropogon gerardii Vitman), were examined during June–September of 2003. Plants were fumigated with three levels of O3 in a randomized-block experiment with three replicates of each treatment. Grasses were grown in open-top chambers with introduced carbon-filtered (CF) air, characteristic of clean air quality; non-filtered (NF) air, representative of quality in Auburn, AL; and air with double (2×) the ambient concentration of O3. Because forage quality can be as important as quantity, we determined various effects on nutritive quality characteristics in addition to biomass yield. Big bluestem exhibited little response to O3 exposure. For eastern gamagrass, we generally found decreased nutritive quality with increasing O3 exposure as evidenced by increased concentrations of cell wall constituents and decreased concentrations of N. Regrowth of both species exhibited little treatment effect which emphasizes the importance of timing and duration of O3 exposures in relation to physiological stage of plant development. Decreased nutritive quality parameters observed for eastern gamagrass may have implications to diet selection and nutrient intake by ruminant herbivores. In addition, range managers can use species-specific information regarding O3 sensitivity to make decisions about mechanical harvesting and grazing regimes of these forages growing in areas exposed to elevated O3 concentrations.
Extending grazing into the winter, as opposed to feeding of harvested forages, can increase the sustainability of ranching in the western US. This study was conducted to determine the economic value of grazing stockpiled forage kochia (Kochia prostrata [L.] Scrad.) and crested wheatgrass (Agropyron desertorum [Fisch. Ex Link] Schultes) during the fall and winter. Changes in cow body weight, body condition score, and ultrasound backfat were compared for late-gestation cows grazing forage kochia–crested wheatgrass pastures vs. those fed alfalfa (Medicago sativa L.) hay in drylot. The study was conducted from early November to late January for 2 consecutive years near Promontory, Utah. Forage availability and nutritional quality were monitored throughout the experiment. Cows grazing stockpiled forages did not receive any protein or energy supplements. Forage kochia comprised approximately 70% of available forage, with November crude protein content of 116 and 76 g·kg−1 in years 1 and 2, respectively. Nutritional quality declined throughout the season, presumably mostly because of removal of higher-quality forage by preferential grazing as opposed to weathering. Averaged over years, cows grazing forage kochia–grass gained body weight (19 kg), increased in body condition (0.3 points), and maintained backfat thickness, finishing well within the range considered optimum for onset of calving and return to estrus. Pasture- vs. drylot-fed cows did not differ with regard to changes in body weight or body condition score. Both treatments increased backfat in year 1, when initial backfat was less than 0.5 cm, but both treatments resulted in loss of backfat in year 2, when initial backfat was greater than 1.0 cm. Grazing was more economical, costing $0.24·cow−1·d−1 less than feeding alfalfa hay in drylot. Forage kochia can be used on western rangelands to extend grazing into the fall and winter, thereby improving the profitability of beef production.
Revegetation and sustainable cattle grazing are major objectives in the reclamation of mine tailings at the Highland Valley Copper mine in British Columbia, Canada. A total of 150 cows with their calves grazed forage extremely high in molybdenum (Mo) for 5–6 weeks in the summer and fall for 3 consecutive years (2002–2004). The average stocking rate was 0.89 ha per animal unit month. The animals' diet consisted primarily of alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) containing 100–400 ppm Mo. Each year, the herd was divided into 2 groups of approximately 25 cow–calf pairs. One group was supplemented with 2.5% copper sulphate (CuSO4·5H2O) in loose salt, and the other group was only given loose salt. Clinical signs of Mo toxicity, including lameness, diarrhea, and a faded hair coat, were significantly reduced in the supplement cows compared to control cows, which demonstrated the efficacy of the copper supplement treatment. Cattle also developed a tolerance to grazing high-Mo forage, as lameness and diarrhea were reduced in cows that had previous exposure to the site. However, lameness, the primary sign, and diarrhea were resolved in all cows by the end of each trial without treatment and hair coats returned to normal by the following spring. Only 4 calves showed signs of lameness or diarrhea in the 3-year study. Cattle grazing is a viable option for the end land use plan for Highland Valley Copper, provided a CuSO4·5H2O supplement is available to counteract the toxic effects of the high-Mo forage. As well, animals with previous exposure to the site should be utilized, as they appear to develop a tolerance to the Mo in the forage.
Six dual-flow continuous culture fermenters were used over a 10-day experimental period to compare the digestibilities of dry matter (DM) and fiber, ruminal fermentation profile, and nitrogen metabolism of rumen microbial populations obtained from an inoculum of chamois (Rupicapra pyrenaica pyrenaica) and cattle (Bos taurus). The aim of this work was to test the fermenters with inoculum from chamois and to compare the function of ruminal microbial population of chamois and cattle under the same conditions of diet and passage rate. Fermenters were fed 56 g DM daily of a diet consisting of 75% Italian ryegrass hay (Lolium multiflorum Lam. var. westerwoldicum) and 25% alfalfa hay (Medicago sativa L.). Both inocula were adapted to the in vitro conditions after 6 days. There were no differences in DM and fiber digestibilities, and total volatile fatty acids concentration (VFA) between cattle and chamois inocula. The proportion of propionate was higher (25.8 vs. 17.9 mol/100 mol), and that of butyrate was lower (9.9 vs. 13.8 mol/100 mol) in chamois inoculum than in cattle inoculum. Ammonia nitrogen concentration was greater (10.6 vs. 6.7 mg/100 ml), but microbial protein synthesis was lower (41.1 vs. 55.3% of total nitrogen flow) in chamois inoculum than in cattle inoculum. These results suggest that microbial populations from chamois and cattle inocula, maintained under the same conditions of diet and passage rate, have a similar degradation capacity, producing the same amounts of VFA, but with a different fermentation profile, and different microbial protein synthesis. The dual-flow continuous culture fermenters could be a useful tool to study ruminal fermentation in wild species such as Pyrenean chamois.
Near infrared reflectance spectroscopy (NIRS) of feces for the prediction of diet quality in several species of livestock and wildlife has been reported. The technique has not been reported in deer. This study was conducted to determine the ability of fecal NIRS to determine dietary crude protein (CP), digestible organic matter (DOM), and phosphorus (P) in white-tailed deer (Odocoileus virginianus). Seventy-six diet reference chemistry:fecal spectrum (D:F) pairs were created ranging from 6.00 to 18.95% CP, 26.64 to 76.08% DOM, and 0.08 to 0.48% P. Calibration results (R2 and SE cross validation) were: 0.95 and 1.17, 0.88 and 3.62, 0.83 and 0.04 for CP, DOM, and P, respectively. These equations were used to predict a validation D:F set (n = 11). Results (R2 and SE prediction) were: 0.79 and 1.53, 0.49 and 5.46, 0.67 and 0.03 for CP, DOM, and P, respectively. These two D:F sets were combined and calibrations reformulated. Results (R2 and SE cross validation) were: 0.84 and 1.40, 0.89 and 3.55, 0.83 and 0.04 for CP, DOM, and P, respectively. These combined calibrations were used to predict diet quality characteristics using 11 fecal samples from wild deer. The diet quality characteristics were compared to NDVI greenness values for the study area in winter, spring and summer. High correlation (R2 > 0.7) between fecal NIRS predicted diet quality and NDVI greenness was observed with the exception of P in summer (R2 = 0.25). Fecal NIRS can be used to determine diet quality in white-tailed deer and thus become another tool to evaluate habitat suitability.
Scaled quail (Callipepla squamata pallida Vigors) are closely associated with semidesert grasslands of the southwestern United States, and populations have declined by as much as 50% since 1960. Livestock grazing, shrub encroachment, and exotic grass invasion are considered important factors reducing scaled quail distribution and density in Arizona. We investigated habitat use by scaled quail across their range in southeastern Arizona to determine the habitat conditions important for survival and reproduction. Pointing dogs located quail during autumn and winter of 2002–2003 and 2003–2004, and we measured habitat characteristics at 52 flush sites and 54 nonuse plots, where scaled quail were not found. We recorded information on landform, substrate, vegetation, and cover. Scaled quail used areas with grass canopy cover ≥ 26%, tree canopy cover ≤ 10%, and higher grass species richness than randomly available. Short (≤ 50 cm tall) visual obstruction (i.e., cover), usually associated with low shrubs, cacti, and bunchgrass, was greater at use sites than at nonuse plots. A logistic-regression equation, including visual obstruction and tree canopy variables, correctly predicted ≥ 91% of quail use sites. Greater amounts of visual obstruction and lower percentages of tree canopy cover best-predicted scaled quail sites. Land management practices that reduce grass species richness and cover and increase tree cover may reduce scaled quail habitat quality and availability in southeastern Arizona. Based on habitat use patterns of scaled quail, we recommend that semidesert grassland habitats contain a maximum tree canopy of < 6% and > 25% grass canopy cover at the 20-cm height to provide optimum cover availability.
Manure amendments to rangelands may alter soil functions related to nutrient recycling. We investigated the influence of grazing and cattle manure on soil carbon, nitrogen, Mehlich 3 phosphorus, and activities of alkaline phosphatase and dehydrogenase. Fertilizer treatments (unamended, manure, or urea potassium dihydrogen phosphate [KH2PO4] fertilizer) were imposed under grazed and nongrazed conditions in a short-grass native rangeland. Manure was applied at rates of ∼ 125 kg N ha−1 and ∼ 42 kg P ha−1, and urea KH2PO4 was applied at 75 kg N ha−1 and 20 kg P ha−1, respectively. Total aboveground biomass and soil samples at 4 depths (0–200 mm) were collected throughout 2 growing seasons. A controlled environment study also evaluated fertilizer source effects on enzymatic activities at 5 P rates (0–120 mg·kg−1 P as manure or urea KH2PO4). Amendments significantly (P < 0.05) increased extractable P following the second application for the 3 uppermost depth increments. Extractable P was greatest on manure-amended plots, increasing 44% from February 1999 to July 2000 at the surface. However, increases in P extractability as a proportion of total P applied were similar for manure and KH2PO4. Enzymatic activities were significantly (P < 0.001) influenced by sampling date and soil depth. There were no consistent grazing effects on enzyme activities. Amendments did not influence dehydrogenase activities in the field; however, in the controlled environment, activities averaged 16% greater across all rates for manure-amended soil as compared with urea KH2PO4–amended soil (P = 0.025). Phosphatase activities increased significantly following manure applications under both field (P = 0.007) and controlled environment (P = 0.003) conditions. Elevated phosphatase activities following manure applications probably led to enhanced P mineralization and similar P extractabilities as a proportion of total P applied for manure- and KH2PO4–amended soils. Therefore, when determining applications rates, total manure P should be considered bioavailable.
Data on the effects of livestock grazing on soil nutrient availability are virtually nonexistent for meadow systems. We measured the effect of livestock grazing on soil, emphasizing soil-solution chemistry, in a Sierra Nevada riparian meadow. Treatments were livestock exclusion (begun in 1989) and grazing to leave 1 000 kg·ha−1 of vegetation. Ceramic tension lysimeters were placed in the treatments (2 replicates) by landscape position (stream edge, midfloodplain, and forest edge), and by depth (approximately 0.1, 0.6, and 1.2 m below the soil surface). Lysimeter water was extracted twice monthly in April, May, and June of 1990 through 1993, and cations and anions were quantified. In addition, KCl-extractable NO3− and NH4; bicarbonate-extractable ortho-P; available Mn, Cu, Fe, and Zn; and root-length density (RLD) were quantified in soils by treatment, landscape position, and soil depth in July 1991 and September 1993. RLD was not affected by grazing. Significant (P ≤ 0.05) treatment effects were largely limited to the forest edge. The grazed treatment had greater lysimeter-extractable Na, Ca 2, Mg 2, and NO3−; higher pH; and less K and NH4 than the excluded treatment. Compared with corresponding excluded treatments, bicarbonate-extractable P was significantly greater on the grazed forest edge, and available Mn was significantly greater at the grazed stream-edge position in 1991. Extractable NO3− was significantly higher in the 0–25 cm depth increment of the grazed treatment, and available Zn was significantly greater on the grazed midfloodplain position in 1993. Grazing did not result in more anoxic soil conditions than the excluded treatment. Grazing effects were most pronounced at the forest edge, possibly as a result of spatial transfer of nutrients via cow urine and feces. Management goals to sustain high-elevation meadows should emphasize maintenance of high RLD to sequester soil nutrients.
The red imported fire ant (Solenopsis invicta Buren) is an invasive pest that causes ecological disturbance and economic damage to habitats worldwide. Since its introduction to the United States 75 years ago, the ant has spread across the southeast through Texas into California and causes over $6.5 billion in damages. Conventional control techniques have not proven effective or long-lasting, leading to a search for alternative methods. We examined the ability of WW-B.Dahl Old World bluestem (Bothriochloa bladhii [Retz] S.T. Blake), increasingly used in pastures, to limit or reduce ant infestations. Pastures planted with WW-B.Dahl had about one-third the fire ant mounds found in adjacent pastures of native grass or coastal bermuda (Cynodon dactylon [L.] Pers) grass, and the difference is statistically significant (P = 0.0006). No difference was found in the number of ants collected in bait cups or in mound vitality ratings, suggesting that more than one measure of ant infestation is needed to accurately assess ant populations. A reduction in fire ant mounds can improve the efficiency of haying operations and reduce wildlife impacts, suggesting broad uses for WW-B.Dahl in ant-infested areas.
An improved global positioning system (GPS)–based animal tracking system is needed to meet quickly evolving demands of ecological research, range livestock production, and natural resource management. Commercially available tracking systems lack the data storage capacity needed to frequently collect animal location data (e.g., 15-minute intervals or less) over long-term deployment periods (e.g., 1 year or more). Some commercial systems have remote data–download capabilities, reducing the need to recapture tagged animals for data retrieval, but these systems download data via satellite (Argos), global system for mobile communications (GSM) cellular telephone, or telemetry radio frequencies. Satellite systems are excessively expensive, and GSM cellular coverage is extremely limited within the United States. Radio-based systems use narrow-band very-high– or ultra-high frequencies requiring the user to obtain frequency allocations. None of these existing systems were designed to provide continual, real-time data access. The Clark GPS Animal Tracking System (Clark ATS) was developed to meet the evolving demands of animal ethologists, ecologists, natural resource managers, and livestock producers. The Clark ATS uses memory-card technology for expandable data storage from 16 megabytes to 8 gigabytes. Remote data downloading and program uploading is accomplished using spread-spectrum radio transceivers, which do not require narrow-band radio frequency allocations. These radios also transmit, at a user-defined time interval, a real-time, GPS-location beacon to any Clark ATS base station within range (about 24 km or 15 miles line of sight). Advances incorporated into the Clark ATS make it possible to evaluate animal behavior at very fine spatial- and temporal-resolution over long periods of time. The real-time monitoring provided by this system enables researchers to accurately examine animal distribution and activity responses to acute, short-term disturbances relative to longer-term behavioral patterns. The Clark ATS also provides a huge time- and cost-savings to researchers and natural resource managers attempting to relocate a tagged animal in the field for direct observation or other operations.