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Historically, ecosystems in the southwestern United States derived much of their nitrogen (N) from N-fixation in biological soil crusts. Today, these regions have highly reduced crust cover, and atmospheric deposition may be the dominant source of N. This study investigates the effects of increased nitrogen deposition on nitrogen uptake, photosynthesis, and growth of the two main forage grasses on the Colorado Plateau, galleta (Hilaria jamesii [Torr.] Benth.) and Indian ricegrass (Oryzopsis hymenoides, [Roemer & J.S. Schultes] Ricker ex Piper). Plots were fertilized for 2 years with 0, 10, 20, and 40 kg nitrogen ha−1 annually, up to 4× the estimated current annual deposition rate, in 2 applications per year (spring and summer). Half-plots were fertilized with either (NH4)2SO4 in KCl solution or with KNO3 solution to determine possible differences in the effects of NH4 and NO3− in this system. Neither grass increased leaf photosynthesis or tiller size due to supplemental N. Galleta also did not increase tiller density, while estimated live tiller density in Indian ricegrass increased up to 50% in the second year. Nitrogen applications accelerated the onset of water stress in both species presumably through stimulating ecosystem transpiration. Nitrogen form did not significantly affect any aspect of grass physiological performance or growth. However, leaf nitrogen in NH4-fertilized plants was significantly more isotopically enriched than in NO3−-fertilized plants, suggesting that both species incorporated NH4-N only after it had been enriched by soil turnover. Seedlings of Russian Thistle (Salsola iberica, Sennen & Pau), a noxious annual invasive weed on western rangelands, grew rapidly in the first summer on plots with 40 kg nitrogen ha−1 per annum, and more so on plots fertilized with NO3− than with NH4. The study suggests that changes in the timing and amount of nitrogen input may alter community composition through facilitating the invasions of summer-active noxious weeds.
The resilience of willow (Salix monticola Bebb, Salix geyeriana Anderss., Salix planifolia Pursh) stems released from intense elk (Cervus elaphus) browsing in Rocky Mountain National Park, Colorado, was quantified in 1998 with a retrospective study that compared biomass, number, and length of segments on willow stems located inside (protected) and outside (browsed) elk exclosures. Segment biomass increased each year after protection by about 3–12 g year−1 on browsed stems and 10–27 g year−1 on protected stems. The number of segments on stems was similar for browsed and protected stems in the first 2 years after exclusion but differed in the next 3 years, when they increased exponentially on protected stems. Nearly 80% of segments on browsed stems were < 5 cm in length in 1994–1997, which caused stems to develop a short-hedged morphology. Protected stems had more long segments and fewer short segments than browsed stems for the first 3 years, but then increased their number of short segments as stems became tall and bushy. Thus, evidence suggests short-hedged willow stems are highly resilient and can rapidly recover height and vigor after protection from intense elk browsing.
The distribution characteristics of willow (genus Salix) can be evaluated with the use of remote sensing, geographic information systems (GIS) technologies, and spatial analysis. This information can be used to better understand willow ecology, such as willow community composition, species relationships, and associations with other landscape attributes (i.e., soils, elevation gradients, water sources, and landscape position). Aerial photographs of a willow-dominated riparian area located in southeastern Oregon were taken in November 1999 at a 1:2 400 scale. Four basic techniques were used to separate willow species from each other in both color and infrared images using ERDAS Imagine® GIS. Willow species included Geyers willow (Salix geyeriana Anderson), Booth willow (Salix boothii Dorn), and Lemmon's willow (Salix lemmonii Bebb). The techniques used to analyze aerial photographs included image resampling (degrade), image filtering (smoothing), unsupervised classification, and supervised classification. Highest accuracy was obtained using a supervised classification of the color images smoothed with a low-pass convolution filter (84.6% accuracy). Spectral samples were collected using a polygon digitizing method, which had superior results compared to the seed or region growing method. The infrared image was least effective in separating the plants into species classes (58.6% accuracy). This may be due to the lack of the blue band in the infrared image.
Quantitative assessment of vegetation change is often conducted by digitally analyzing time series of aerial or vertical photography. Change analysis conducted using repeated oblique or landscape photography, however, has been limited to qualitative assessments. The purpose of this study was to develop sampling and analysis techniques for using a time series of digitized landscape photography to quantify vegetation change on rangeland landscapes. Digital images were created from black-and-white landscape photographs acquired in 1917, 1962, and 2000 near Whiskey Mountain in the Reynolds Creek Experimental Watershed of southwestern Idaho. Images were spatially registered to each other using control points and a polynomial transformation algorithm. Thirty random pixels along each of 30 random image lines were selected as point samples (n = 900) from within each image. The landscape feature represented in each selected pixel was classified into 1 of 15 cover types. Cover-type classification accuracy for the 2000 image was estimated to be 92.2% based on ground-truth data collected in the field. Classification accuracy was increased to 98.9% by combining rare or poorly separable cover-type classes. Image cover of vegetation cover types was quantified for each photography acquisition date. Changes in image cover of each cover type and direction of cover-type conversions were determined for each intervening time period. Analysis of image cover using repeated landscape photography is constrained by limitations imposed by oblique view angles and variable image quality. Repeat landscape photography, however, can be used to quantitatively assess long-term dynamics of vegetation cover on rangeland landscapes with visually distinct vegetation types.
New methods of image acquisition and analysis are advancing rangeland assessment techniques. Most image-analysis programs require users to adjust detection thresholds for color or object classification, a subjective process we postulated would be influenced by human error and variation. We developed a ground-cover–measurement calibration procedure, the digital grid overlay (DGO), which is similar to image point sampling (dot grid) advanced by earlier researchers. We asked 21 rangeland professionals to measure ground cover using 2 subjective visual-estimate methods (threshold adjustment process, or TAP, and external [to the software] visual estimate, or EVE) and the DGO on 5 different nadir-view images of rangeland. We also compared cover measurements made by DGO-calibrated software in automated batch processing against DGO manual-only measurements. We found an unacceptable range of variation among rangeland professionals using TAP. The DGO and EVE values were more closely aligned. We discovered an age-related bias in bare-ground measurements: all users over 50 years of age classified more bare ground than did all users under 50 years of age when using TAP. One explanation for this bias is age-related yellowing of the eye lens. Manual DGO measurements required up to 15 minutes per image compared to about 1 second per image for automated computer analysis after software calibration. The greatest bare-ground difference between the DGO-calibrated software and manual DGO measurements for the data sets analyzed was 5.6% and the correlations imply that reasonably accurate automated measurements can be used for bare-ground measurements from digital-image data sets. The exception is where the software cannot adequately separate litter and bare ground. The digital methods we tested need improvement. However, external calibration (DGO or EVE) of current-generation image-analysis algorithms bring economical, statistically adequate monitoring of extensive land areas within the realm of practical application.
The effects of chemical creosotebush (Larrea tridentata [D.C.] Cov.) control on infiltration rates, wetting-front depth, and sediment production were examined in the Chihuahuan Desert of southern New Mexico. Study sites were treated with aerial applications of tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea) as part of a brush control program administered by the US Bureau of Land Management. Study sites were equally distributed among 3 geographic regions and included 6 sites treated 5–9 years ago, 6 sites treated 15–18 years ago, and 12 contiguous nontreated sites. Paired 1-m2 rainfall simulation plots encompassed the creosotebush canopy zone and adjacent interspace area with runs made using simulated rainfall at 9.1 cm·hr−1 for 30 minutes. Runoff and sediment were collected every 5 minutes, and wetting-front depths were measured at the end of rainfall simulations. Infiltration rates were significantly higher in the canopy zone than in interspace areas and were highest in 5–9-year-old sites, intermediate in nontreated sites, and lowest in 15–18-year-old sites. Regression equations showed that infiltration rates within the canopy zone were positively correlated with litter mass, and cover of litter, shrubs, and grasses. Within interspace areas, infiltration rates were most correlated with aggregate stability and cover of litter, rocks, and grasses. Wetting-front depths were significantly deeper in the canopy zone than in interspace areas for all treatments. Sediment concentration (kg·L−1) was higher in the canopy zone than interspace, but total sediment yield was not significantly different between these areas or affected by any treatments.
This study was conducted to determine the effect of prescribed fire on surface hydrology in Great Basin pinyon-juniper (Pinus monophylla Torr. & Frém.—Juniperus osteosperma Torr.) woodlands. Infiltration rates were measured using a single ring infiltrometer over an elevation gradient (2 103, 2 225, and 2 347 m) at 3 microsites (tree canopy, shrub canopy, and interspace) and 2 tree cover types (intermediate and high) in August 2001, before a spring prescribed burn conducted in May 2002, and then following the prescribed burn in August 2002. Infiltration experiments were used to calculate saturated hydraulic conductivity (K[θs]) rates, and water drop penetration times were determined to evaluate the development of water-repellent soils. Infiltration rates before the burn were greater at the low elevation than at the mid and high-elevation study sites because of differences in measured soil texture. Before burning, the infiltration and saturated hydraulic conductivity (K[θs]) rates measured on interspace and shrub canopy microsites were less than on tree canopy microsites at the midelevation study site (2 225 m). Following burning, the intermediate tree cover tree canopy microsites had greater infiltration rates than interspace microsites; all other microsites were similar to each other. No significant differences in K(θs) rates existed among the microsites after burning. However, on the higher elevation study site before the burn, the interspace microsites had final infiltration rates less than the tree canopy microsites, and burning caused no deviation from this trend. Saturated hydraulic conductivity rates at the high elevation did not differ by microsite before the burn, but after burning interspace microsites had K(θs) rates less than tree canopy microsites. Burning increased water repellency of surface soils (0–3 cm) for all cover types. Spring burning in Pinyon-juniper woodlands may produce a hydrologic response depending on surface soil texture and vegetation cover.
Surface water is an important limiting factor for wildlife populations in desert environments where water sources are uncommon or have been lost or degraded due to human activities. To address this need, wildlife water developments have been constructed in many areas of the southwestern United States, particularly in the Sonoran Desert. Previous studies of wildlife water developments are limited and critics have asserted that water quality at these facilities may be deleterious to animal health. Water quality was evaluated at natural, modified natural, and constructed water sources in the Sonoran Desert of southwestern Arizona and southeastern California. Samples were taken from primary sources of surface water available to wildlife, including natural tinajas (rock basins), modified tinajas, springs, rainwater catchments (“guzzlers”), and wells. Water samples were tested for 21 chemical constituents known to affect animal health, blue-green algal toxins, and a presumed waterborne pathogen, the protozoan avian parasite Trichomonas gallinae. Seven chemical constituents were absent or below detection limits. The majority of constituents detected (10/13, 77%) occurred at levels below recommended guidelines for domestic animals. Elevated pH, alkalinity, and fluoride were found in rainwater catchments, springs, and wells, respectively, but at relatively low levels unlikely to affect animal health. Blue-green algal toxins were not detected and there was no evidence of Trichomonas. Although specific water quality guidelines for wildlife are lacking, these results do not support hypothesized negative impacts to wildlife populations from developed water sources.
Sheep are more resistant than cattle to larkspur poisoning and thus may be used as a biological tool to graze larkspur to reduce cattle poisoning. Sheep readily graze larkspur in its mature stages, but if they are to be an effective management tool, they must graze it in the early growth stages before cattle enter the allotment. The objective of this study was to determine if sheep could be positively conditioned to graze duncecap larkspur (Delphinium occidentale (S.Wats) S. Wats) early in its growth stages. Eighteen ewes were divided into 3 groups of 6 ewes each. During conditioning, group 1 was offered potted larkspur plants then were gavaged with glucose, the second group was exposed to larkspur plants together as a group (social facilitation), and the third group was an untreated control. In the preference test, the glucose group ate more duncecap larkspur than the social facilitation and control groups. The glucose and control groups were taken to duncecap larkspur-infested mountain rangeland to test the conditioning. In the field grazing trial, the glucose group consumed more larkspur than the control group, but it occurred later in the grazing trial when larkspur was in flower and after desirable forages had been consumed. High levels of diterpenoid alkaloids in larkspur and other alternative palatable forages may have caused ewes to reject larkspur at the beginning of the trial. The sheep were positively conditioned to graze larkspur, but the amount consumed and the timing of consumption was not sufficient to prevent potential cattle poisoning.
Perennial pepperweed (Lepidium latifolium L.) is an aggressive perennial forb that is infesting much of western North America. Grazing may provide an alternative to chemical and mechanical control of perennial pepperweed. However, if livestock are used in control efforts, they may spread weeds by depositing viable seeds in fecal pats in uninfested areas. This study consisted of 2 experiments using fistulated steers to estimate the effect of ruminant digestion on germination of perennial pepperweed seeds. In Experiment 1, we tested the hypothesis that ruminal incubation (for 0, 48, and 96 hours) affects perennial pepperweed germination. In Experiment 2, we tested the hypothesis that type of incubation (no incubation, water only, or total digestive tract) affects perennial pepperweed germination. In Experiment 1, germination was 17 and 15 times greater for the 48- and 96-hour incubation treatments compared to the control, respectively. In Experiment 2, germination was 23 and 19 times greater for the water and total tract incubation treatments compared to the control, respectively. Effects were attributed to a combination of seed hydration and seed coat scarification. Results from this study suggest that grazing should occur prior to seed set or that livestock which have grazed perennial pepperweed bearing viable seed should be quarantined before being moved to uninfested areas. These results also suggest that control of perennial pepperweed is especially important where moving water may transport seeds off site.
Festuca campestris Rydb. (mountain rough fescue) is a dominant grass species in the montane grasslands of western Canada. Little is known about the genetic diversity of this plant and the effects of long-term grazing on the genetics of populations. The amplified fragment length polymorphism (AFLP) technique was applied to compare the genetic diversity of fescue plants at adjacent grazed and protected areas for 3 populations spread across a longitudinal range: Stavely in the foothills, Milroy in the Rocky Mountain trench, and Goose Lake on the interior plateau. Five AFLP primer pairs were used to screen the tiller samples of about 39 plants in each grazed (or ungrazed) area, and 139 polymorphic AFLP bands were scored for each individual sample. These scored bands had frequencies ranging from 0.03 to 0.98 with an average of 0.56. About 81% of the total AFLP variation resided within the populations. The Goose Lake population had the lowest level of AFLP variation, but genetically was the most distinct. Four AFLP bands were possibly associated with chromosomal segments significant for grazing resistance. Comparisons of AFLP variation between grazing and nongrazing samples revealed variable and relatively small impacts of the long-term grazing on the genetic diversity of the grazed populations. The AFLP variation of grazed samples was 1.5% lower at Goose Lake, 2.2% higher at Milroy, and not different at Stavely. If developing diverse germplasm for rangeland seedings is desired, one should sample across geographic space rather than combining materials with and without historical grazing pressure.
Native seed mixes for rangeland seeding, prairie restoration, or cultivated pasture can benefit from a greater variety of forbs that more closely reflect the original vegetation of the southern Great Plains. Fifteen native, perennial herbaceous legumes were collected in central Texas and evaluated for herbage production, mineral content, and fiber concentration of established plants in research plots over 2 years. Downy milk-pea (Galactia volubilis [L.] Britton) was productive, regardless of rainfall, whereas prairie acacia (Acacia angustissima [Mill.] Kuntze var. hirta [Nutt.] B.L. Rob.) and Illinois bundle-flower (Desmanthus illinoensis [Michx.] MacMill. Ex. B.L. Rob. & Fernald) out-yielded others in year 3 when rainfall was the greatest. Herbage crude protein averaged approximately 100 g·kg−1 for bush-clovers (Lespedeza spp.) compared to bundle-flowers (Desmanthus spp.), which exceeded 200 g·kg−1; the latter also was high in herbage phosphorus. Herbage neutral detergent fiber ranged from 300 to more than 500 g·kg−1, acid detergent fiber ranged from 140 to 360 g·kg−1, and acid detergent lignin ranged from 36 to 140 g·kg−1, a wide range from which to select if animal nutrition is a primary criterion. Seed production was evaluated within a subset of 8 entries submitted to periodic herbage removal or left intact throughout the season. Three bundle-flowers yielded the greatest mass and seed number, but were negatively affected by harvest, unlike prairie acacia. Herbage and seed characteristics indicate there are promising perennial herbaceous legumes in the southern Great Plains that can be included in native seed mixes.
Willow (Salix) communities are important components of riparian ecosystems. However, browsing by livestock and wildlife species can negatively impact willow size and abundance, and make restoration efforts difficult. A common solution has been fencing of affected willows to exclude ungulates, but fencing is expensive and may not complement desirable land management strategies. An alternative to fencing is the use of structures that limit access to streamside willows, without excluding ungulate access to the entire riparian zone. We examined the use of felled western juniper trees (Juniperus occidentalis Hook) placed over streamside willow shrubs. Four replicates of felled western juniper treatments (covered) and noncovered treatments were applied to a 1.2-km length of stream in southeastern Oregon. Willows (< 2 m) within treatment areas were censused, tagged, examined for evidence of browse-use, and measured for maximum height during August 2002, before treatment. Posttreatment measurements were made in August and October 2003. Results indicate that by August 2003 (posttreatment) the average growth of willows in covered treatments was 25 cm (480%) greater than in noncovered treatments. By October 2003 (posttreatment), more shrubs were browsed in noncovered (84%) than covered (39%) treatments. Our data suggest that covering small willow shrubs (< 2 m tall) with felled western juniper is an effective deterrent to browsing.