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1.
Conifers (Picea and Abies spp.) have replaced aspen (Populus tremuloides Michx.) over much of aspen’s historic range in the western United States. We measured the impact of this change upon the production of understory vegetation potentially useful as forage for livestock and wildlife on two southern Utah national forests. A negative exponential relationship between conifer cover and understory biomass was demonstrated as log(biomass) = 6.25 − 0.03787(% conifer), adjusted R2 = 0.57. Understory production in aspen stands begins to decline under very low levels (10% to 20%) of conifer encroachment. Management implications include loss of forage production capability and wildlife habitat and potential overstocking of livestock grazing allotments if the associated loss of forage is not considered.  相似文献   

2.
Runoff from grazing pasture lands can impact water quality in receiving streams if not well managed. Management consists of conservation practices to reduce runoff and pollutants transport. Simulation models have been effectively used to design and implement these conservation practices. The Agricultural Policy Environmental Extender (APEX), a process-based hydrologic model, was used in this study to simulate the management impacts on surface runoff from three small grazed pasture watersheds located at the North Appalachian Experimental Watersheds near Coshocton, Ohio. Specific objectives of this study were to 1) calibrate the APEX model and test runoff predictions against measured runoff and 2) simulate the long-term impacts of different management scenarios on surface runoff. Results show that the APEX model simulated surface runoff reasonably well with the coefficient of determination (R2) and Nash-Sutcliffe efficiency values varying from 0.49 to 0.72 and from 0.25 to 0.60 for calibration and validation, respectively. After validation, the APEX model was run for 37 yr (1975 ? 2011) for long-term scenarios to analyze the impacts of soil properties and management on surface runoff. Data from this study indicated that keeping the watershed land use as a hay meadow instead of grazing significantly reduced cumulative runoff by 58 ? 67%. Buffer strips of perennial grasses resulted in decreased simulated runoff. To simulate the impacts of soils on runoff, the surface (0 ? 5 cm) soil properties of the toe position were applied to the entire grazed watershed. Subsequently, the increase in soil richness resulted in reduction (≤ 5%) in surface runoff. The simulation results from the present study demonstrate the benefits of hayed meadow over grazed pasture and further predict the decreased trend of runoff due to soil properties change and buffer strips.  相似文献   

3.
In this study, we examined temporal and spatial dynamics of minerals of forage, soil and cattle serum in two savannas (valley and plain) of South Africa. The aims were to explore the relationships between ecosystem components, and plan communal grazing and fodder flow for sustainable livestock production. In each area, grazing sites near, at intermediate distance from (middle) and far from homesteads were selected. In the valley land, site interacted with season to influence local variations (P<0.05) of soil calcium and microelements. In the plain lands, local variations occurred for soil phosphorus and magnesium. At the large scale, most macro-minerals showed differences between the valley and plain lands. In the valley areas, forage calcium differed (P<0.05) locally in the dry season, whereas in the plain areas, potassium and phosphorus showed local variations (P<0.05) in the wet season. Seasonal variations of most forage elements were significant within each local site of both study areas. All forages were deficient in phosphorus and copper. Cattle serum from the valley area had low iron concentrations. The study concluded that biotic and abiotic factors may influence the distribution of soil and forage elements, but their effects may vary between the two areas. Rangeland improvement and supplementation strategies are suggested to sustain animal production.  相似文献   

4.
Dryland ecosystems cover over 41% of the earth’s land surface, and living within these important ecosystems are approximately 2 billion people, a large proportion of whom are subsistence agropastoralists. Improper grazing in drylands can negatively impact ecosystem productivity, soil conservation, hydrologic processes, downstream water quantity and quality, and ultimately human health and economic well-being. Concerns regarding the degraded state of western US rangelands in the 1950s resulted in an interagency committee to study the effects of land use on runoff and erosion processes. In 1953, a federal research group established four paired watersheds in western Colorado to study the interaction of grazing by domestic livestock, runoff, and sediment yield. Exclusion of livestock from half of the watersheds dramatically reduced runoff and sediment yield after the first 10 yr—primarily due to changes in ground cover but not vegetation. Here, we report results of repeated soils and vegetation assessments of the experimental watersheds after more than 50 yr of grazing exclusion. Results show that many of the differences in soil conditions between grazed and ungrazed watersheds observed in the 1950s and 1960s were still present in 2004, despite reduced numbers of livestock: few differences in vegetation cover but large differences in biological soil crusts, soil stability, soil compaction, and soil biogeochemistry. There were differences among soil types in response to grazing history, especially soil lichen cover and soil organic matter, nitrogen, and sodium. Comparisons of ground cover measured in 2004 with those measured in 1953, 1966, and 1972 suggest much of the differences between grazed and ungrazed watersheds likely were driven by high sheep numbers during droughts in the 1950s. Persistence of these differences, despite large reductions in stocking rates, suggest the combination of overgrazing and drought may have pushed these salt desert ecosystems into a persistent, degraded ecological state.  相似文献   

5.
There is rapidly growing national interest in grazing lands’ soil health, which has been motivated by the current soil health renaissance in cropland agriculture. In contrast to intensively managed croplands, soil health for grazing lands, especially rangelands, is tempered by limited scientific evidence clearly illustrating positive feedbacks between soil health and grazing land resilience, or sustainability. Opportunities exist for improving soil health on grazing lands with intensively managed plant communities (e.g., pasture systems) and formerly cultivated or degraded lands. Therefore, the goal of this paper is to provide direction and recommendations for incorporating soil health into grazing management considerations on grazing lands. We argue that the current soil health renaissance should not focus on improvement of soil health on grazing lands where potential is limited but rather forward science-based management for improving grazing lands’ resilience to environmental change via 1) refocusing grazing management on fundamental ecological processes (water and nutrient cycling and energy flow) rather than maximum short-term profit or livestock production; 2) emphasizing goal-based management with adaptive decision making informed by specific objectives incorporating maintenance of soil health at a minimum and directly relevant monitoring attributes; 3) advancing holistic and integrated approaches for soil health that highlight social-ecological-economic interdependencies of these systems, with particular emphasis on human dimensions; 4) building cross-institutional partnerships on grazing lands’ soil health to enhance technical capacities of students, land managers, and natural resource professionals; and 5) creating a cross-region, living laboratory network of case studies involving producers using soil health as part of their grazing land management. Collectively, these efforts could foster transformational changes by strengthening the link between natural resources stewardship and sustainable grazing lands management through management-science partnerships in a social-ecological systems framework.  相似文献   

6.
围封与放牧对土壤微生物和酶活性的影响   总被引：3，自引：0，他引：3  
以阿拉善高原微温干旱温带半荒漠类草地为对象,研究了围封与放牧对土壤微生物量碳、氮及土壤酶活性的影响,旨在为荒漠区退化草地的恢复与重建提供科学依据。结果表明,1）围封与放牧地土壤微生物量碳含量无显著差异（P＞0.05）,而围封样地内土壤微生物量氮含量显著高于放牧样地（P＜0.05）,从而使得其土壤微生物量碳氮比显著低于放牧样地,且围封与放牧条件下不同土层的土壤微生物量碳、氮含量变化规律均为表层（0~10 cm）土壤高于下层（10~20 cm）土壤。2）围封样地表层土壤脲酶和碱性磷酸酶的活性均高于放牧样地,下层土壤的碱性磷酸酶的活性也高于放牧样地,而脲酶活性则显著低于放牧样地。对于土壤蔗糖酶活性,围封样地仅下层土壤显著高于放牧样地,而表层土壤无差异。另外,围封和放牧条件下3种酶的活性在不同土层间的高低规律也不一致。3）围封与放牧条件下土壤微生物量碳和氮含量与土壤酶活性的关系表现不同,围封条件下,仅土壤微生物量碳含量与土壤脲酶活性呈显著正相关,而与土壤蔗糖酶活性呈显著负相关。放牧条件下,土壤微生物量碳和氮均与土壤脲酶活性呈显著负相关,而与蔗糖酶和碱性磷酸酶活性呈显著正相关。另外,土壤微生物量碳氮比与脲酶活性在放牧条件下也呈显著正相关性。  相似文献   

7.
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8.
We assessed plant interspaces in July 2007 using continuous line intercepts in twice-replicated pastures of northern mixed-grass prairie with contrasting grazing treatments: 1) long-term (25 yr) heavily grazed, dominated by the bunchgrass blue grama (Bouteloua gracilis), and 2) ungrazed, dominated by the rhizomatous grass western wheatgrass (Pascopyrum smithii). The number of plant interspaces was 26% higher in pastures heavily grazed, but the amount of soil surface occupied by plant interspaces was 27% greater without grazing. Plant interspaces were larger without grazing (14.8 ±  cm, mean ± 1 SE) than heavily grazed (8.9 ±  cm). Plant interspaces represented 87% and 68% of the total soil surface in the ungrazed and heavily grazed communities, respectively. The percentage of soil surface covered by plant interspaces < 20 cm was higher for the heavily grazed (94%) compared to the ungrazed (79%). Litter cover in the plant interspaces was higher without grazing (80 ± 1%) compared to the heavily grazed (57 ± 3%). Grazing-induced structural changes from a rhizomatous- to a bunchgrass-dominated vegetation community were manifest in the size and distribution of plant interspaces. Ecological consequences for erosion from raindrop impacts in larger plant interspaces in the ungrazed community are likely offset by greater litter cover in these communities; conversely, lower litter cover in heavily grazed pastures may increase erosion potential despite occurrence of smaller plant interspaces and less proportion of the soil surface covered by interspaces. Management practices that increase the cover of litter in plant interspaces should reduce the potential of erosion from water and wind in this semiarid rangeland.  相似文献   

9.
Veld management in Ciskei is based predominantly on a communal land tenure system, and livestock is produced under a ‘Group Ranch’ system. Stocking rates are excessively high and consequently veld degradation is severe, resulting in a lowered carrying capacity and low efficiency indicators such as weaning percentage. Veld rehabilitation programmes during the 1960s resulted in some improvement in veld condition, but also caused bush encroachment and selective grazing. Factors limiting veld rehabilitation are of a political, socio‐economic and technical nature, the most important being overstocking, the land tenure system and incorrect land use. A solution to the continued degradation of the veld depends on the commitment of the government to a development strategy which addresses factors such as freehold tenure, education and the provision of incentive for profitable production.  相似文献   

10.
Changes in soil and vegetation due to livestock grazing are occurring in arid lands throughout the world. The most extreme cases result in desertification, which is seen as largely irreversible, because of altered soil properties. To understand better how long-term livestock removal affects soil properties and vegetation, we compared water-infiltration rates, soil bulk density, and perennial grass cover inside and outside a long-term livestock exclosure in an arid grassland site in southeastern Arizona, United States. The site had not been desertified at the time of this study. Exclusion of livestock for 40 yr was associated with lower bulk density and higher water infiltration in both the dry and wet seasons. Perennial grass cover was higher and two native grasses, Eragrostis intermedia and Bouteloua hirsuta were significantly more common (P < 0.05) in the ungrazed area. These findings parallel our results from a desertified site and suggest that changes in soil physical properties associated with long-term livestock removal are not an artifact of desertification and can take place in a system that has remained in a grassland state. Our data suggest that, although significant changes in species composition have occurred, this grassland is relatively resilient to substantial changes in soil physical properties.  相似文献   

11.
Sera from 173 apparently healthy, unvaccinated dogs from 4 widely separated communal lands in Zimbabwe were tested by ELISA for antibodies against canine distemper virus. Overall, 82% were positive with high prevalences found in each communal land. The highest seroprevalence was in dogs between 1 and 2 years of age (91%; 49/54). These results show dogs in the communal lands of Zimbabwe are commonly exposed to canine distemper virus and that a substantial number survive infection. The role that the virus might play in the high mortality rate of the dog population on communal land warrants further investigation.  相似文献   

12.
The Common Agricultural Policy supports the use of free-ranging cattle herds to control woody encroachment and fire hazards in Europe. There is, however, little empirical evidence about the effectiveness of extensive grazing to preserve open landscapes in the Mediterranean Basin. In this work, we evaluated the effects of extensive beef cattle grazing on the vegetation structure in a Mediterranean ecosystem using a twofold framework: 1) analyzing temporal changes in the forest, shrub, and grassland cover in areas under different grazing pressures for 16 yr (1993 ? 2009) and 2) studying diet selection to assess the impact of cattle on the local Mediterranean vegetation. Our landscape structure analyses revealed a remarkable change in land cover over the study period. However, woody community dynamics seemed to be more related to natural vegetation succession than to cattle effects. Extensive grazing seemed to preserve grasslands but only at high stocking rates. On the other hand, the diet analyses supported the lack of a role for cattle in encroachment control. Beef cattle diets were based on herbaceous plants (59%) with lower contribution of woody ones (41%). Cattle only showed a significant preference (P < 0.05) for few woody species (Erica multiflora, Olea europaea, Quercus ilex, and Rosmarinus officinalis), mostly at high-density stocking rates. Hence, our results support the idea that extensive cattle grazing alone exerts a negligible effect on shrub encroachment and thus on the risk of fire in the studied Mediterranean area. We urge a redesign of current research to truly integrate extensive cattle grazing as High Nature Value farming in European policies to successfully meet its putative goals, such as shrub encroachment control and wildfire risk prevention.  相似文献   

13.
Wearable sensors have been explored as an alternative for real-time monitoring of cattle feeding behavior in grazing systems. To evaluate the performance of predictive models such as machine learning (ML) techniques, data cross-validation (CV) approaches are often employed. However, due to data dependencies and confounding effects, poorly performed validation strategies may significantly inflate the prediction quality. In this context, our objective was to evaluate the effect of different CV strategies on the prediction of grazing activities in cattle using wearable sensor (accelerometer) data and ML algorithms. Six Nellore bulls (average live weight of 345 ± 21 kg) had their behavior visually classified as grazing or not-grazing for a period of 15 d. Elastic Net Generalized Linear Model (GLM), Random Forest (RF), and Artificial Neural Network (ANN) were employed to predict grazing activity (grazing or not-grazing) using 3-axis accelerometer data. For each analytical method, three CV strategies were evaluated: holdout, leave-one-animal-out (LOAO), and leave-one-day-out (LODO). Algorithms were trained using similar dataset sizes (holdout: n = 57,862; LOAO: n = 56,786; LODO: n = 56,672). Overall, GLM delivered the worst prediction accuracy (53%) compared with the ML techniques (65% for both RF and ANN), and ANN performed slightly better than RF for LOAO (73%) and LODO (64%) across CV strategies. The holdout yielded the highest nominal accuracy values for all three ML approaches (GLM: 59%, RF: 76%, and ANN: 74%), followed by LODO (GLM: 49%, RF: 61%, and ANN: 63%) and LOAO (GLM: 52%, RF: 57%, and ANN: 57%). With a larger dataset (i.e., more animals and grazing management scenarios), it is expected that accuracy could be increased. Most importantly, the greater prediction accuracy observed for holdout CV may simply indicate a lack of data independence and the presence of carry-over effects from animals and grazing management. Our results suggest that generalizing predictive models to unknown (not used for training) animals or grazing management may incur poor prediction quality. The results highlight the need for using management knowledge to define the validation strategy that is closer to the real-life situation, i.e., the intended application of the predictive model.  相似文献   

14.
Woodland encroachment on United States rangelands has altered the structure and function of shrub steppe ecosystems. The potential community structure is one where trees dominate, shrub and herbaceous species decline, and rock cover and bare soil area increase and become more interconnected. Research from the Desert Southwest United States has demonstrated areas under tree canopies effectively store water and soil resources, whereas areas between canopies (intercanopy) generate significantly more runoff and erosion. We investigated these relationships and the impacts of tree encroachment on runoff and erosion processes at two woodland sites in the Intermountain West, USA. Rainfall simulation and concentrated flow methodologies were employed to measure infiltration, runoff, and erosion from intercanopy and canopy areas at small-plot (0.5 m2) and large-plot (13 m2) scales. Soil water repellency and vegetative and ground cover factors that influence runoff and erosion were quantified. Runoff and erosion from rainsplash, sheet flow, and concentrated flow processes were significantly greater from intercanopy than canopy areas across small- and large-plot scales, and site-specific erodibility differences were observed. Runoff and erosion were primarily dictated by the type and quantity of ground cover. Litter offered protection from rainsplash effects, provided rainfall storage, mitigated soil water repellency impacts on infiltration, and contributed to aggregate stability. Runoff and erosion increased exponentially (r2 = 0.75 and 0.64) where bare soil and rock cover exceeded 50%. Sediment yield was strongly correlated (r2 = 0.87) with runoff and increased linearly where runoff exceeded 20 mm·h?1. Measured runoff and erosion rates suggest tree canopies represent areas of hydrologic stability, whereas intercanopy areas are vulnerable to runoff and erosion. Results indicate the overall hydrologic vulnerability of sagebrush steppe following woodland encroachment depends on the potential influence of tree dominance on bare intercanopy expanse and connectivity and the potential erodibility of intercanopy areas.  相似文献   

15.
Drastic changes have occurred in Mongolia’s grazing land management over the last two decades, but their effects on rangelands are ambiguous. Temporal trends in Mongolia’s rangeland condition have not been well documented relative to the effects of long-term management changes. This study examined changes in grazing land use and rangeland biomass associated with the transition from the socialist collective to the current management systems in the Tsahiriin tal area of northern Mongolia. Grazing lands in Tsahiriin tal that were formerly managed by the socialist collective are now used by numerous nomadic households with their privately owned herds, although the lands remain publicly owned. Grazing pressure has more than tripled and herd distribution has changed from a few spatially clustered large herds of sheep to numerous smaller herds of multiple species. Landsat image–derived normalized-difference vegetation index estimates suggest that rangeland biomass significantly decreased (P < 0.001) from the collective to the postcollective periods. The observed decrease was significantly correlated with changes in the grazing management system and increased stocking density (P < 0.001), even when potential climate-induced changes were considered. Furthermore, field- and Satellite Pour l’Observation de la Terre imagery–based rangeland assessments in 2007 and 2008 indicate that current rangeland biomass is low. Spatial pattern analyses show that the low biomass is uniform throughout the study site. The observed decrease in rangeland biomass might be further accelerated if current grazing land use continues with no formal rangeland management institution or organized, well-structured efforts by the local herding households.  相似文献   

16.
A field study in Randall County, Texas, was conducted to determine how soil bulk density and plant cover change over time in response to deferment following a high-density, high-intensity, short-term grazing/trampling event. Green Sprangletop (Leptocloa dubia Kunth.) and Kleingrass (Panicum coloratum L.) were broadcasted at 4.5 kg ha−1 pure live seed (PLS) on former cropland that had a partial stand of WW-Spar Bluestem (Bothriochloa ischaemum L.). A high-density, high-intensity trampling event was achieved with twenty-four 408-kg Bos taurus heifers occupying four 0.10-ha plots (97 920 kg live weight ha−1) for 10 h, with four adjacent 0.10-ha control plots left untrampled. Canopy and basal cover were determined by plant functional group using the Daubenmire method after rainfall events of > 0.254 cm, and a 5.08 × 7.62 cm core was collected to determine soil bulk density. Strips of supplemental plant material were applied in March to test the effects of 100% soil cover on seedling recruitment. Trampled treatments had 30% less vegetative cover (P < 0.01) and average soil bulk densities that were 0.20 g cm−³ higher (P < 0.01) than untrampled plots post trampling. Bulk density decreased with deferral until there were no significant differences between treatments (240 d). However, WW-Spar basal cover increased in both treatments, with no differences between treatments. Trampling did not affect seedling recruitment, but supplemental cover increased seedling density on three of five subsequent sampling dates (P < 0.05). Canopy cover of warm season perennial grasses in trampled treatments surpassed that of the untrampled treatments during the early growing season of 2016 (P < 0.01) but were no different after mid-June. Hydrologic function can be maintained with high stock densities by providing adequate deferment to reestablish sufficient cover and allow natural processes to restore porosity.  相似文献   

17.
Runoff and sediment yield from rangelands are extremely important variables that affect productivity, but are difficult to quantify. Studies have been conducted to assess erosion on rangelands, but very little has been done to determine if microchannels (rills) affect runoff and sediment yield. Rainfall simulations were used to quantify the effects of microchannels on runoff and sediment loss on a shortgrass prairie with two types of range conditions (good and fair). Natural flow paths within plots in the two range conditions were defined and then enhanced with an ellipse-shaped hoe to create microchannels. Soil from plots was removed at two rates (11.2 t· ha−1 and 22.4 t· ha−1) to create three soil surface configurations. Soil was removed by vacuuming to create either a single microchannel or multiple microchannels down the plot, and the third treatment was uniform soil removal over the entire plot (sheet). Results showed significantly greater total runoff from both single and multiple microchannel treatments compared with sheet soil removal. The microchannels resulted in significantly less sediment yield per unit of runoff compared with the sheet soil removal treatment. Both runoff and sediment yield were affected by range condition. Plots that were in fair range condition, dominated by buffalo grass (Buchloe dactyloides [Nutt.] Engleman), had a greater amount of total runoff (double) but less sediment yield (75%) than plots in good range condition that were dominated by blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Stued.). The dense buffalo grass sod protected the soil surface from erosion, but water flowed freely across the sod. This study has provided a greater understanding of how microchannels affect runoff and sediment yield under different rangeland conditions, and has illustrated how plant species composition and soil surface features relate to several hydrologic functions.  相似文献   

18.
Juniper encroachment into otherwise treeless shrub lands and grasslands is one of the most pronounced environmental changes observed in rangelands of western North America in recent decades. Most studies on juniper change are conducted over small areas, although encroachment is occurring throughout regions. Whether changes in juniper cover can be assessed over large areas with the use of long-term satellite data is an important methodological question. A fundamental challenge in using satellite imagery to determine tree abundance in rangelands is that a mix of trees, sagebrush, and herbaceous cover types can occur within a given image pixel. Our objective was to determine if spectral mixture analysis could be used to estimate changes in Rocky Mountain juniper (Juniperus scopulorum Sarg) and Utah juniper (Juniperus osteosperma [Torr.] Little) cover over 20 yr and 20000 ha in southeast Idaho with the use of Landsat imagery. We also examined the spatial patterns and variation of encroachment within our study area using Geographic Information Systems–based data sets of grazing use, land-cover types, and topography. Juniper cover determined from 15-cm-resolution digital aerial orthophotography was used to train and validate juniper presence/absence classification in 1985 and 2005 Landsat images. The two classified images were then compared to detect changes in juniper cover. The estimated rate of juniper encroachment over our study area was 22–30% between 1985 and 2005, consistent with previous ground-based studies. Moran’s I analysis indicated that juniper encroachment pattern was spatially random rather than clustered or uniform. Juniper encroachment was significantly greater in grazed areas (P = 0.02), and in particular in grazed shrub land cover type (P = 0.06), compared to ungrazed areas. Juniper encroachment was also greater on intermediate slopes (10–35% slopes) compared to steeper or flatter terrain, and encroachment was somewhat less on north-facing (P = 0.03) and more on west-facing (P = 0.02) slopes compared to other aspects.  相似文献   

19.
植被覆盖度对流域有机质和氮素径流流失的影响   总被引：12，自引：0，他引：12  
张兴昌  邵明安 《草地学报》2000,8(3):198-203
以8.27km2纸坊沟流域1:400模型为对象,在人工控制的条件下,模拟天然降雨(降雨强度2mm/min,历时30min),研究不同植被覆盖度对小流域土壤氮素随径流流失的规律。结果表明,当覆盖度为60%、40%、20%和0%时,土壤铵态氮流失量为87.08、44.31、25.16和13.71kg/km2,硝态氮流失量为85.50、74.05、63.95和56.23kg/km2,有机质流失量为15.67、24.02、44.68和164.87t/km2,全氮流失量为0.81、1.18、1.98和7.5t/km2。泥沙氮素含量随着产流时间的延长呈下降趋势,而流失累积量测呈逐渐增大趋势。植被覆盖虽能有效地减少土壤侵蚀和全氮的流失,却能增加土壤矿质氮的流失。  相似文献   

20.
Dryland ecosystems are particularly vulnerable to erosion generated by livestock grazing. Quantifying this risk across a variety of landscape settings is essential for successful adaptive management, particularly in light of a changing climate. In the Upper Colorado River Basin, there are nearly 25 000 km2 of rangelands with underlying soils derived from Mancos Shale, an erodible and saline geologic parent material. Salinity is a major concern within the Colorado River watershed, much of which is attributed to runoff and leaching from Mancos Shale deposits. In a 60-yr paired-watershed experiment in western Colorado, we used silt fences to measure differences in saline hillslope erosion, including both total sediment yield and concentrations of primary saline constituents (Na and Se), in watersheds that were either exposed to grazing or where livestock was excluded. After accounting for the strong effects of soil type, slope, and antecedent precipitation, we found that grazing increased sediment loss by ≈50% across our 8-yr time series (0.1–1.5 tn ha−1), consistent with levels reported at the watershed scale in early published work from studies at the same location. Eroded sediment Se levels were low and unaffected by grazing history, but Na concentrations were significantly reduced on grazed hillslopes, likely due to depletion of surface Na in soils exposed to chronic soil disturbance by livestock. Variable selection and path analysis identified that biological soil crust (BSC) cover, more than any other variable, explained the differences in sediment yields between grazed and ungrazed watersheds, partially through the enhancement of soil aggregate stability. Our results suggest that BSC cover should be granted heightened consideration in rangeland decision support tools (e.g., state-and-transition models) and that measures to reduce surface disturbance from livestock such as altering the timing or intensity of grazing may be effective for reducing downstream impacts.  相似文献   

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