Insights from Long-Term Ungrazed and Grazed Watersheds in a Salt Desert Colorado Plateau Ecosystem

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.     

Grazing Density Effects on Cover,Species Composition,and Nitrogen Fixation of Biological Soil Crust in an Inner Mongolia Steppe

Biological soil crusts (BSCs) are important in many arid and semiarid ecosystems for their abilities to fix atmospheric nitrogen (N) and stabilize surface soil. Grazing disturbance has a profound influence on abundance, species composition, and ecological functioning of BSCs. To elucidate the effects of grazing on BSCs in Inner Mongolia grasslands, an investigation was conducted in a typical steppe that had previously been subjected to long-term grazing with six grazing densities (control: 0 sheep · ha^?1, very light: 4 sheep ·ha^?1, light: 8 sheep · ha^?1, medium: 12 sheep · ha^?1, heavy: 16 sheep · ha^?1, and very heavy: 20 sheep · ha^?1). Cover, species composition, potential N-fixing activity, and potential N input of BSC indicate that long-term grazing significantly reduced the importance of BSC in N input and soil stabilization. Such reductions were highly related to grazing density. Very light grazing had no significant effect on the role of BSC in soil stabilization, but resulted in a 13.3% reduction in BSC N input potential. Heavy and very heavy grazing led to a decrease of potential N input by one order of magnitude, and a decline of BSC function via a shift from high coverage of an attached group–dominated BSC community to a low coverage of a vagrant group–dominated community. Constraining grazing level to a very light density—and to a lesser extent, a light grazing density—is likely a preferred practice for conserving BSC and the ecological services it provides in N fixation and soil stabilization.     

Long-Term Vegetation Trends on Grazed and Ungrazed Chihuahuan Desert Rangelands

Long-term information on the effects of managed grazing versus excluded grazing effects on vegetation composition of desert rangelands is limited. Our study objectives were to evaluate changes in frequency of vegetation components and ecological condition scores under managed livestock grazing and excluded livestock grazing over a 38-yr period at various locations in the Chihuahuan Desert of southwestern New Mexico. Sampling occurred in 1962, 1981, 1992, 1998, 1999, and 2000. Range sites of loamy (1), gravelly (2), sandy (2), and shallow sandy (2) soils were used as replications. Black grama (Bouteloua eriopoda Torr.) was the primary vegetation component at the seven locations. Dyksterhuis quantitative climax procedures were used to determine trends in plant frequency based on a 1.91-cm loop and rangeland ecological condition scores. Frequency measures of total perennial grass, black grama, tobosa (Hilaria mutica Buckley), total shrubs, honey mesquite (Prosopis glandulosa Torr.), and other vegetation components were similar on both grazed and ungrazed treatments (P > 0.1) at the beginning and end of the study. The amount of change in rangeland ecological condition scores was the same positive increase (14%) for both grazed and ungrazed treatments. Major changes (P < 0.1) occurred within this 38-yr study period in ecological condition scores and frequency of total perennial grasses and black grama in response to annual fluctuations in precipitation. Based on this research, managed livestock grazing and excluded livestock grazing had the same long-term effects on change in plant frequency and rangeland ecological condition; thus, it appears that managed livestock grazing is sustainable on Chihuahuan desert rangelands receiving over 25 cm annual precipitation.     

Prescribed Fire,Grazing, and Herbaceous Plant Production in Shortgrass Steppe

We examined the independent and combined effects of prescribed fire and livestock grazing on herbaceous plant production in shortgrass steppe of northeastern Colorado in the North American Great Plains. Burning was implemented in March, before the onset of the growing season. During the first postburn growing season, burning had no influence on soil moisture, nor did it affect soil nitrogen (N) availability in spring (April–May), but it significantly enhanced soil N availability in summer (June–July). Burning had no influence on herbaceous plant production in the first postburn growing season but enhanced in vitro dry matter digestibility of blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths) forage sampled in late May. For the second postburn growing season, we found no difference in herbaceous plant production between sites that were burned and grazed in the previous year versus sites that were burned and protected from grazing in the previous year. Our results provide further evidence that prescribed burns conducted in late winter in dormant vegetation can have neutral or positive consequences for livestock production because of a neutral effect on forage quantity and a short-term enhancement of forage quality. In addition, our results indicate that with conservative stocking rates, deferment of grazing during the first postburn growing season may not be necessary to sustain plant productivity.     

黄土高原侵蚀区生物结皮的人工培育及其水土保持效应

探讨生物结皮的人工培育技术,并对其发育初期的水保功能进行评价。结果表明:撒播粉碎后的自然生物结皮,培育1个雨季即可形成盖度高达30%-60%的人工生物结皮,其主成分不变;人工生物结皮具有较好的水保效应,其中室内培育的生物结皮在坡度5°、雨强46.8mm°h^-1、历时1h的模拟降雨条件下可减少49%-64%的径流,消除土壤侵蚀;同条件下野外培育的生物结皮在无植被时对径流影响不明显,但全年可减少26%的土壤侵蚀,有柠条植被时全年可减少11%的径流和39%的土壤侵蚀;在黄土高原侵蚀区以人工生物结皮进行大规模的水保治理成本较低,切实可行。     

Simulating Runoff from Small Grazed Pasture Watersheds Located at North Appalachian Experimental Watershed in Ohio

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.     

Plant Interspaces Resulting From Contrasting Grazing Management in Northern Mixed-Grass Prairie: Implications for Ecosystem Function

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.     

Influence of 90 Years of Protection From Grazing on Plant and Soil Processes in the Subalpine of the Wasatch Plateau,USA

Human communities in the Intermountain West depend heavily on subalpine rangelands because of their importance in providing water for irrigation and forage for wildlife and livestock. In addition, many constituencies are looking to managed ecosystems to sequester carbon in plant biomass and soil C to reduce the impact of anthropogenic CO₂ on climate. This work builds on a 90-year-old grazing experiment in mountain meadows on the Wasatch Plateau in central Utah. The purpose of this study was to evaluate the influence of 90 years of protection from grazing on processes controlling the input, output, and storage of C in subalpine rangelands. Long-term grazing significantly reduced maximum biomass in all years compared with plots within grazing exclosures. For grazed plots, interannual variability in aboveground biomass was correlated with July precipitation and temperature (R² = 0.51), while there was a weak correlation between July precipitation and biomass in ungrazed plots (R² = 0.24). Livestock grazing had no statistically significant impacts on total soil C or particulate organic matter (POM), although grazing did increase active soil C and decrease soil moisture. Grazing significantly increased the proportion of total soil C pools that were potentially mineralizable in the laboratory, with soils from grazed plots evolving 4.6% of total soil C in 1 year while ungrazed plots lost 3.3% of total soil C. Volumetric soil moisture was consistently higher in ungrazed plots than grazed plots. The changes in soil C chemistry may have implications for how these ecosystems will respond to forecast climate change. Because grazing has resulted in an accumulation of easily decomposable organic material, if temperatures warm and summer precipitation increases as is anticipated, these soils may become net sources of CO₂ to the atmosphere creating a positive feedback between climate change and atmospheric CO₂.     

青藏高原东北缘放牧草地土壤矿物元素含量及分布特征

为科学评价放牧草地土壤矿物元素含量、分布特征以及对土-草-畜矿物生态系统的潜在影响,对青藏高原东北缘的4个县(天祝、大通、玛曲、若尔盖)的土壤特征和矿物元素进行分析调查。共计采集256个来自该地区不同土壤类型(高山草甸土、亚高山草甸土、沼泽土)的表层土壤样品,分析了土壤中11种矿物元素的有效含量(Ca、P、S、K、Mg、Na、Fe、Mn、Zn、Cu和Co)和2种矿物元素(Mo和Se)全部含量。分析发现,土壤中多数矿物元素含量随着地区和土壤类型的不同而表现出一定的差异性,土壤中多数矿物元素含量与土壤有机碳和全氮呈显著正相关,相反与土壤pH有一定的负相关性。土壤中矿物元素磷和镁的有效含量均低于动物可能出现缺乏的临界值,土壤中全量硒的含量严重低于动物硒缺乏的临界值。土壤中磷、镁、硒的缺乏可能会引起该地区放牧家畜该矿物元素缺乏。另外,土壤中元素铁的有效含量远高于推荐值,土壤铁的含量过高可以引起牧草中铁的含量上升,进而影响家畜对其他元素的吸收。然而,要确认家畜是否存在该类矿物元素缺乏和不平衡问题,还需要进一步分析该地区牧草和家畜组织矿物元素含量以及家畜的生产力水平。     

Postfire Succession in Big Sagebrush Steppe With Livestock Grazing

Prescribed fire in rangeland ecosystems is applied for a variety of management objectives, including enhancing productivity of forage species for domestic livestock. In the big sagebrush (Artemisia tridentata Nutt.) steppe of the western United States, fire has been a natural and prescribed disturbance, temporarily shifting vegetation from shrub–grass codominance to grass dominance. There is limited information on the impacts of grazing to community dynamics following fire in big sagebrush steppe. This study evaluated cattle grazing impacts over four growing seasons after prescribed fire on Wyoming big sagebrush (Artemisia tridentata subsp. Wyomingensis [Beetle & Young] Welsh) steppe in eastern Oregon. Treatments included no grazing on burned and unburned sagebrush steppe, two summer-grazing applications after fire, and two spring-grazing applications after fire. Treatment plots were burned in fall 2002. Grazing trials were applied from 2003 to 2005. Vegetation dynamics in the treatments were evaluated by quantifying herbaceous canopy cover, density, annual yield, and perennial grass seed yield. Seed production was greater in the ungrazed burn treatments than in all burn–grazed treatments; however, these differences did not affect community recovery after fire. Other herbaceous response variables (cover, density, composition, and annual yield), bare ground, and soil surface litter did not differ among grazed and ungrazed burn treatments. All burn treatments (grazed and ungrazed) had greater herbaceous cover, herbaceous standing crop, herbaceous annual yield, and grass seed production than the unburned treatment by the second or third year after fire. The results demonstrated that properly applied livestock grazing after low-severity, prescribed fire will not hinder the recovery of herbaceous plant communities in Wyoming big sagebrush steppe.     

Altered Herbivore Distribution Associated With Focal Disturbance

Natural disturbances historically created structurally diverse patterns across the landscape, and large herbivores concentrated herbivory in areas where disturbance decreased standing senesced biomass that acted as a grazing deterrent through decreased palatability and overall forage quality. However, following European settlement, many natural large-scale disturbance regimes that influence vegetation and herbivore grazing selection were altered or removed and replaced with fine-scale anthropogenic disturbances. It is unclear how fine-scale focal disturbance and alteration of vegetation structure influences livestock distribution and grazing. Therefore we used a tracked vehicle as a disturbance agent in a mesic mixed-grass prairie to assess the influence of focal anthropogenic disturbance on livestock distribution and grazing. Track vehicle disturbance decreased the height of vegetation (P < 0.05) but did not alter plant species composition (P > 0.05). Cattle fecal pat density was greater (P ≤ 0.05) in locations with track vehicle disturbance. Little bluestem tiller height was shorter (P ≤ 0.05) in tracked locations than nontracked locations in grazed treatments, but was not different in nongrazed locations the first growing season following disturbance. Fecal pat density and tiller height were not different (P > 0.05) between tracked and nontracked locations following the second growing season. Therefore, we concluded that fine-scale focal anthropogenic disturbance alters herbivore distribution and defoliation and can maintain structural heterogeneity, but the effect is ephemeral and does not create long-lasting grazing lawns.     

Bison and Cattle Grazing Impacts on Grassland Stream Morphology in the Flint Hills of Kansas

Despite a shift from yr-round bison grazing throughout the Great Plains before European settlement to extensive seasonal cattle grazing, little is known about ungulate grazing impacts on grassland streams. In this study we 1) determine whether grazing management is a significant driver of grassland stream morphology within the Flint Hills Ecoregion (Kansas, United States); 2) determine if yr-round bison grazing (the precolonial condition) and seasonal cattle grazing (the currently dominant grazing practice in the region) result in distinct stream morphology; and 3) determine if the introduction of cattle into ungrazed watersheds produces significant changes to channel morphology. We use a replicated watershed-scale study design and survey 17 streams across four grazing treatments (ungrazed, long-term bison grazed [yr-round], long-term cattle grazed [seasonal], and newly cattle grazed [seasonal]). Baseline geomorphic surveys were completed in 2010 following consistent grazing management since 1992, and resurveys were completed in 2011 and 2013 to determine short-term grazing impacts. Under the conditions of the experiment, we did not detect significant differences (P > 0.10) in channel morphology or stream bed substrate size among grazing treatments following nearly 2 decades of consistent grazing management. Cattle introduction into ungrazed watersheds resulted in modest (P < 0.05) stream widening (0.19 m, 3.9%) following two grazing seasons. Bison grazed watersheds also experienced modest (P < 0.05) stream widening (0.20 m, 5.1%) during the resurvey period. Stream widening from 2010 to 2013 within newly cattle-grazed and long-term bison-grazed treatments indicates that cattle and bison are capable of producing moderate alterations to grassland stream morphology over short time periods. However, longer time periods containing more diverse hydrologic conditions may be necessary to generate larger geomorphic changes between surveys. Although we detected modest changes to stream morphology in response to grazing over short time periods, overall, stream morphology does not vary among grazing treatments in the study area.     

Stocking Rate and Riparian Vegetation Effects on Physical Characteristics of Riparian Zones of Midwestern Pastures

Grazing at high stocking rates may increase sediment and nutrient loading of pasture streams through transport in precipitation runoff and bank erosion. A 3-yr (2007–2009) grazing study was conducted on 13 cool-season grass pastures to quantify effects of stocking rate and botanical composition on forage sward height, proportions of bare and manure-covered ground, and bank erosion adjacent to streams. Pastures ranged from 2 ha to 107 ha with stream reaches of 306 m to 1 778 m that drained watersheds of 253 ha to 5 660 ha. Bare and manure-covered ground were measured at a 15.2-m distance perpendicular to the stream at 30.5-m intervals at up to 30 locations on each side of the stream by the line transect method in May, July, September, and November of each year. At the midpoint of the 15.2-m line, forage sward height was measured with a falling plate meter (4.8 kg · m^-2) and plant species identified. In November 2006, fiberglass pins (1.6 × 76.2 cm) were driven 73.7 cm into the stream bank at 1-m intervals from the streambed to the top of the bank along 10 equidistant transect locations on each side of the stream to measure bank erosion during spring, summer, and fall of each year. Increasing pasture stocking rates increased manure-covered ground and decreased sward height, but did not affect proportions of bare ground. The greatest, intermediate, and least net soil erosion rates occurred during the winter/early spring, late spring/early summer, and late summer/fall seasons. Stocking rates between measurements, expressed as cow-days · m^-1 stream, were not related to bank erosion. Increasing stocking rates per unit of stream length will increase manure cover and decrease forage sward height, but not affect proportions of bare ground or bank erosion rates adjacent to pasture streams. Therefore, managing stocking rates may reduce nutrient loading of pasture streams.     

Effects of Long-Term Livestock Grazing on Fuel Characteristics in Rangelands: An Example From the Sagebrush Steppe

Livestock grazing potentially has substantial influence on fuel characteristics in rangelands around the globe. However, information quantifying the impacts of grazing on rangeland fuel characteristics is limited, and the effects of grazing on fuels are important because fuel characteristics are one of the primary factors determining risk, severity, continuity, and size of wildfires. We investigated the effects of long-term (70+ yr) livestock grazing exclusion (nongrazed) and moderate levels of livestock grazing (grazed) on fuel accumulations, continuity, gaps, and heights in shrub-grassland rangelands. Livestock used the grazed treatment through 2008 and sampling occurred in mid- to late summer in 2009. Nongrazed rangelands had over twofold more herbaceous standing crop than grazed rangelands (P < 0.01). Fuel accumulations on perennial bunchgrasses were approximately threefold greater in nongrazed than grazed treatments. Continuity of fuels in nongrazed compared to grazed treatments was also greater (P < 0.05). The heights of perennial grass current year’s and previous years’ growth were 1.3-fold and 2.2-fold taller in nongrazed compared to grazed treatments (P < 0.01). The results of this study suggest that moderate livestock grazing decreases the risk of wildfires in sagebrush steppe plant communities and potentially other semi-arid and arid rangelands. These results also suggest wildfires in moderately grazed sagebrush rangelands have decreased severity, continuity, and size of the burn compared to long-term nongrazed sagebrush rangelands. Because of the impacts fuels have on fire characteristics, moderate levels of grazing probably increase the efficiency of fire suppression activities. Because of the large difference between fuel characteristics in grazed and nongrazed sagebrush rangelands, we suggest that additional management impacts on fuels and subsequently fires need to be investigated in nonforested rangelands to protect native plant communities and prioritize management needs.     

三江源区土壤侵蚀变化及驱动因素分析

采用修正通用土壤流失方程(revised universal soil loss equation, RUSLE)对三江源区1997-2012年的土壤侵蚀模数和土壤侵蚀量进行定量模拟,并对其生态工程实施前、后时空变化特征进行对比分析,采用空间叠加法分析降雨侵蚀力及植被覆盖度对土壤侵蚀状况的影响,利用模型参数控制法对气候变化和生态工程对土壤侵蚀变化的贡献率进行分析。结果表明:1)生态工程实施后,三江源区土壤侵蚀增加的趋势尚未得到遏制,多年平均年土壤侵蚀模数和侵蚀量较工程实施前增加6.5%,但局部地区土壤侵蚀状况有所好转,约占总面积的45%;2)长江流域在工程实施后的土壤侵蚀量与工程实施前基本持衡;黄河流域土壤侵蚀量增加明显,增幅超过45%;澜沧江流域土壤侵蚀量有所下降,降幅为9.8%;3)降水增强导致土壤侵蚀加剧的贡献率达到180%,植被恢复对土壤侵蚀变化的贡献率为-80%。全面遏制三江源区土壤侵蚀增加趋势,仍需持续努力。     

Grazing Impacts in Vegetated Dune Fields: Predictions From Spatial Pattern Analysis

This study deals with the changes induced by grazing on soil erosion processes in vegetated dune fields of Península Valdés, in the Patagonia region of Argentina. We performed a spatial analysis to assess erosion features’ patterns. Blowouts, used as main indicators of aeolian erosion processes, as well as dune crests, which are susceptible to erosion, were mapped on aerial photographs and images from Landsat 7 Enhanced Thematic Mapper Plus, in eight paddocks under two grazing conditions: lightly (0.4 sheep · ha⁻¹) and heavily grazed (0.8 sheep · ha⁻¹). From the mapped locations of water points, crests, and blowouts we calculated a spatial statistic (O-ring statistic), which gives the expected intensity of blowouts within the area covered by crests as function of distance away from water points. Additionally, to explore if the density of crests around water points influences the density of blowouts, we estimated the intensity of dune crests in the neighborhood of water points and compared the densities of blowouts among water points with low, medium, and high densities of crests. For the heavy grazing treatment we found highly significant (P < 0.05) aggregation of blowouts around water points with peak densities threefold higher than expected at random occurring between 90 and 210 m. However, the aggregation was only weakly significant for the light grazing treatment and occurred only at distances of about 30 m away from the water point. We found that the impact of grazing on soil stability contrasted among sites with different sensitivity to accelerated erosion. In sites with a high density of dune crests close to water points, wind erosion becomes more intense and the density of blowouts increases. A more sustainable management of these rangelands depends primarily on the conservation of the soils. Therefore, the location of water points in sites not susceptible to accelerated soil erosion would represent a better management strategy of the dune fields of Península Valdés.     

Grazing Systems for Yearling Cattle on Tallgrass Prairie

This 9-yr study tested steer gains, residual aboveground biomass (AGB) in mid-July and early October, and economic returns and risk for tallgrass prairie grazed annually under season-long stocking (SLS) at 1.62 ha · steer⁻¹ until early October or intensive early stocking (IES) at 0.81 ha · steer⁻¹ until mid-July compared to a composite grazing system. The three-pasture, three-herd “IES+ System” is a 3-yr fixed sequence of SLS, IES, and IES (0.81 ha · steer⁻¹) plus late-season grazing (LSG; 1.62 ha · steer⁻¹) until early October (IES/LSG). All grazing treatments began in late April. Average gains per steer for SLS and SLS in the IES+ System did not differ, but were significantly less than gains for steers that grazed the entire season under IES/LSG. Gains per steer in mid-July under IES alone or in combination with LSG were similar to the same repeated grazing treatments, but were significantly less than those for steers grazed season-long. Gains per hectare under SLS did not differ, but were significantly less than those for IES treatments and the IES+ System. Gain per hectare in July was similar for IES repeated annually and IES/LSG, but there was greater gain per hectare for IES-treated pastures rotated within the system. Residual grass and total aboveground biomass (AGB) in mid-July did not vary among years and was generally greater on SLS than IES. In early October, grass AGB was similar for all treatments except IES/LSG, which had less residual AGB. When pasture rent was charged per head, the IES+ System increased the 20-yr mean return per hectare by $5.98 compared to repeated use of IES, and $8.52 compared to using only SLS. Measures of economic risk were generally intermediate for the IES+ system compared to IES, which consistently had the highest risk, and SLS.     

围封与放牧措施下生物土壤结皮发育及其微生境土壤养分特征

为了探讨围封与放牧措施下生物土壤结皮发育及其与微生境土壤养分的对应关系,本研究以黄土高原天然草地两类生物土壤结皮（藻类和藓类）为研究对象,调查了生物土壤结皮发育的形态指标,并就不同措施下生物土壤结皮层及其下层土壤养分特征进行了比较分析。结果表明,两类生物土壤结皮对围封与放牧干扰的响应一致,盖度均表现为围封显著高于放牧,而厚度和生物量无显著差异。然而,围封与放牧对生物土壤结皮层及其结皮下层养分含量的影响不尽相同,围封藻结皮层及其结皮下层SOC,TN和Zn含量显著高于放牧,而TP和Na则显著低于放牧。另外,生物土壤结皮层与结皮下层养分间的关系均存在差异,其藻结皮层土壤SOC与TN,TP,Na,K,Fe,Mn和Zn均呈正相关,与Ca呈负相关;藓结皮层SOC分别与Mn和Zn呈正相关,而与Ca和Fe呈负相关。     

放牧干扰对亚高山草甸土壤动物群落影响的研究

对川西北红原县3个不同放牧强度(轻度放牧、中度放牧、重度放牧)的亚高山草甸土壤动物群落进行了调查研究,共获得土壤动物1 887个,分别隶属3门7纲15目49类群。不同放牧强度下土壤动物优势类群差异较大。受放牧干扰的亚高山土壤动物群落,其个体数量变化趋势是:重度放牧＞中度放牧＞轻度放牧。中度放牧生境的中小型土壤动物密度最大,大型土壤动物密度最低。3种生境中土壤动物的个体数和类群数均随土层加深而递减,其中中度放牧生境土壤动物表聚性最强。受放牧强度和放牧方式的干扰,中度放牧生境中土壤动物群落种类多样性、均匀性及丰富度均低于其他生境,而优势度高于其他生境。     

Livestock Grazing Impacts on Herbage and Shrub Dynamics in a Mediterranean Natural Park

Shrub encroachment can be explained by the abandonment of extensive livestock farming and changes to land use, and it is a common problem in the Mediterranean mountain pastures of Europe, with direct effects on biodiversity and landscape quality. In this paper, the effects of livestock exclusion vs. grazing on the dynamics of shrub and herbaceous vegetation were analyzed in a Spanish natural park located in a dry Mediterranean mountain area over a 5-yr period. Twelve 10 × 10 m exclosures were set up in six representative pasture areas of the park (with two replicates per location). Each year, the shrub number, volume, and biomass were measured in April, and the herbage height, biomass, and quality were measured in April and December (which represent the start and end of the vegetative growth season). A sustained increase of the shrub population and individual biomass was observed throughout the study, which was reflected in total shrub biomass per ha. Growth was greater in nongrazed exclosures (2 563 kg dry matter [DM] · ha^?1 · yr^?1), but it also happened in the grazed control areas (1 173 kg DM · ha^?1 · yr^?1), with different patterns depending on the location and shrub species. Herbage biomass did not change when grazing was maintained, but it did increase in places where grazing was excluded (291 kg DM · ha^?1 · yr^?1), mostly as a consequence of the accumulation of dead material, with a concomitant reduction in herbage quality. It was concluded that at the current stocking rates and management regimes, grazing alone is not enough to prevent the intense dynamics of shrub encroachment, and further reductions in grazing pressure should be avoided.