Long-term impacts of livestock grazing and browsing in the Succulent Karoo: a 20-year study of vegetation change under different grazing regimes in Namaqualand§

This study used a fence-line contrast approach to investigate the long-term impact of high grazing pressure on the vegetation at a site in Namaqualand, South Africa. Forty pairs of permanently marked plots were surveyed in 1996, 2006 and 2016. The main objective was to investigate changes in the vegetation structure and species composition between the near-continuously grazed communal rangelands and the relatively lightly grazed commercial rangelands over the 20-year period. The results showed a decline in total vegetation cover in both commercial and communal rangelands in 2016 relative to the two earlier sampling periods. This can be attributed to the low rainfall in 2016 and was due largely to a reduction in annual plant cover, especially on the communal rangeland. Perennial shrub species provide a fodder bank that can be utilised by livestock in times of drought and can buffer short-term deficits in forage supply. However, the annuals that dominate the vegetation of the communal rangeland do not form such fodder banks and consequently do not have the same multi-year buffering capacity as perennial shrubs. This provides the mechanism whereby long-term continuous grazing decreases resilience to rainfall fluctuations and increases livestock variability, thereby promoting non-equilibrium-type dynamics in the system.     

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.     

放牧强度对内蒙古典型草原群落结构和家畜生产性能的影响

认识放牧对草地植物群落结构和家畜生产性能的影响是修复放牧退化草地和制定可持续放牧体系的理论基础。本研究在内蒙古锡林郭勒典型草原开展了基于草地放牧留茬高度的适应性放牧实验,连续4年监测了无牧(NG)、轻牧(LG)、中牧(MG)和重牧(HG)条件下草地植物群落的变化和家畜生产性能,分析其对放牧强度的响应。结果表明：与NG相比,HG显著降低了优势种的相对生物量,提高了伴生种的相对生物量(P<0.05);LG和MG处理下主要提高了优势种的生产力,而在HG处理主要提高了伴生种的生产力;在所研究的典型草原采用适应性放牧方式下,中度放牧可获得最佳的草地植物生产和家畜生产性能。研究为制定合理的草原放牧制度、促进草地群落结构优化和草畜平衡发展提供了理论依据。     

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.     

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.     

山地牧道与植被空间异质性研究评述

山地牧道和沿牧道放牧改变植被空间异质性。综合国内外研究进展,将山地牧道的涵义扩展为家畜在山地垂直空间移动的固定路线、放牧往返小道、相对固定的采食路径以及践踏小径等,并总结了山地牧道的形成与分布规律,重点评述了山地牧道与植被空间异质性相互作用机制。山地牧道作为一种线状廊道,为物种扩散提供通道、生境,分割斑块景观,影响植被边界动态,在不同尺度上影响植被空间异质性;家畜沿牧道的选择采食与植被空间异质性相互影响,家畜还通过践踏和排泄改变植被空间异质性;家畜采食、践踏和排泄综合作用于土壤,通过影响土壤异质性间接影响植被的空间分布。山地牧道在动物行为学、放牧生态学研究中具有理论和实践价值,可以为监测植被退化、控制载畜量提供依据。     

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.     

Patterns of plant selection by grazing cattle in two savanna grasslands: A plant's eye view

The amount of grazing experienced by individual tufts of the most abundant perennial grasses of two Transvaal savannas was monitored at regular intervals for two growing seasons. In general, Setaria incrassata and Themeda triandra were heavily grazed, Bothriochloa insculpla and Aristida bipartita were lightly grazed, and Heteropogon conlortus and Digitaria eriantha were intermediate. Tufts with a greater amount of moribund material were less heavily grazed. The smallest tufts of all species were the most lightly grazed, although the most heavily‐grazed tufts were on occasion intermediate in size rather than being the largest. A tuft had an increased likelihood of being heavily grazed if it had previously been heavily grazed, both within a growing season and for two successive growing seasons. A spatial pattern of grazing was evident, as individual species were less heavily grazed at different locations within the paddocks. The influence of species identity on the amount a tuft was grazed was not absolute but rather quantitative and conditional on the level of other variables. Tuft size and the amount a tuft had been previously grazed generally had a greater influence on the amount a tuft was grazed during an individual grazing event than species identity, location or moribund material. Species selection is possibly, in part, a consequence of selection for plant structure which optimizes intake.     

Grazing Intensity on Vegetation Dynamics of a Typical Steppe in Northeast Inner Mongolia

Vegetation features radiating from residential areas in response to livestock grazing were quantified for an arid steppe rangeland in the Keshiketeng Banner, Chifeng Prefecture, in northeastern Inner Mongolia in 2004 and 2006. The aim of this study was to estimate grazing impacts on the vegetation dynamics of these historical grazed ecosystems. Grazing intensities were classified as reference area (RA), light (LG), moderate (MG), and heavy (HG) according to the vegetation utilization across the study area. Rangelands were studied along a grazing gradient, where characteristics of plant communities, heights of dominant species, aboveground vertical structures, and belowground biomass were investigated. Along this grazing gradient, vegetation changed from the original dominant plant species Leymus chinensis (Trin.) Tzvel. to a semi-subshrub species Artemisia frigida Willd. when moving from the reference area (RA) to the region around the settlement. Canopy coverage, aboveground productivity, and the number of perennial species declined as one moved toward the residential area. Heights of five dominant species, except for Stipa grandis P. Smirn., declined with increased grazing intensity. Aboveground vertical structure in the RA treatment showed more resilience than the other treatments. There was no difference in root biomass in the top 1 m of soil (P > 0.05) between the RA treatment and the area immediately around settlement (HG treatment). Generally, we found that the intensity of grazing disturbance did not exceed the tolerance of the rangeland ecosystem within LG treatment. However, vegetative conditions in HG treatment became worse with increased grazing pressure. Rangelands in this arid steppe are under tremendous threat due to excessive forage utilization, which cannot be considered a sustainable practice.     

不同放牧制度对土壤种子库的影响——以青藏高原东缘高寒草甸为例

以青藏高原玛曲地区高寒草甸为研究对象,通过对暖季牧场与冷季牧场不同放牧强度的比较分析,探讨了可萌发土壤种子库的密度、垂直分布规律及其与地上植被的关系。结果表明,1)暖季牧场土壤种子库中,轻度放牧有12种,种子密度为2 307.66粒·m~(-2),重度放牧有6种,种子密度为1 065.08粒·m~(-2),禁牧有5种,种子密度为828.38粒·m~(-2),轻度放牧的物种丰富度及密度都显著大于重度放牧草地(P0.05);冷季牧场轻度放牧有12种,种子密度为3 786.88粒·m~(-2),重度有8种,种子密度为5 621.15粒·m~(-2),禁牧有5种,种子密度为532.53粒·m~(-2),轻度放牧种子密度小于重度放牧。2)无论是暖季牧场还是冷季牧场轻度放牧土壤种子库的多样性指数、均匀度指数、优势度指数和丰富度指数均大于重度放牧,土壤种子库与地上植被的相似性在不同的放牧强度下都小于0.3,其中不放牧时的相似性最小,轻度放牧时的相似性最高。而暖季牧场和冷季牧场的轻度放牧和重度放牧两种放牧强度之间的相似性系数都最大,分别为0.63和0.67。3)土壤种子库在不同处理间随着土层的加深数量都呈现迅速减少的趋势,大部分集中在0-5cm。     

Livestock Grazing Impacts on Desert Vegetation,Khirthar National Park,Pakistan

The impact of livestock grazing on desert vegetation in Khirthar National Park, Pakistan, was investigated by comparing dry and wet season plant species composition, richness, cover, and a grazing index for quadrats outside (“open”) and inside (“exclosed”) native mammal breeding enclosures that had excluded livestock for 6 years. A total of 93 plant species were recorded in the dry season, 88 species in exclosed quadrats and 50 in open quadrats. While only 5 species were unique to open quadrats, 43 species were found only in the exclosed quadrats. Species richness was higher in the exclosures because of the presence of more grass and herb species, while grazing was higher in the open. After rain, species richness and cover were significantly higher than in the dry season because of the growth of summer ephemeral herbs and grasses, but richness was no longer different between the exclosure and open treatments. Although some herbaceous species may have been adversely affected by livestock grazing, overall species richness suggests strong ecosystem resilience to grazing, with levels no different after seasonal rains regardless of grazing level. Many grass and herb species absent from open sites during the dry season reappeared after rain, which suggests that livestock grazing may eliminate them as the dry season proceeds, but that a soil seed or bud bank persists.     

基于物种周转揭示放牧对荒漠草原植物多样性的影响

放牧引起草原生态系统物种更替,进而调控群落植物多样性。然而,放牧过程中物种更替与植物多样性改变间具有怎样的联系,目前缺乏系统的研究。本试验在内蒙古荒漠草原放牧实验平台进行,对群落植物多样性进行了调查。研究发现：随放牧强度的增加,群落物种总数下降了28%～52%且物种组成发生了明显的分异;随放牧强度的增加,群落科的数目和多年生杂类草的物种数逐渐减少,一年生植物的物种数增加,同时群落Simpson指数和Shannon-Weiner指数等显著降低(P<0.05);随放牧强度的增加,物种损失率和周转率显著提高(P<0.05),但物种获得率无显著变化;放牧荒漠草原物种损失率和周转率与植物多样性指数间呈显著负相关关系(P<0.05)。本研究从物种周转的角度揭示了放牧荒漠草原植物多样性降低的驱动机制,为优化荒漠草原管理和利用提供科学支撑。     

Abundance and guild structure of grasshoppers (Orthoptera: Acridoidea) in communally grazed and protected savanna

This study was conducted to determine how savanna grass sward modifications caused by heavy grazing pressure influenced the abundance and guild structure of grasshoppers. Heavily grazed communal land was compared with a lightly grazed area and a mowed airstrip, in adjacent protected land, in the Mpumalanga lowveld, South Africa. Plant species composition, height, aerial cover and greenness of grass in the herbaceous stratum were measured in representative sites. Total grasshopper abundance and relative abundance of grasshopper species were also assessed in each site. Grasshoppers were assigned to feeding and habitat functional groups for comparison among the three areas. The heavily grazed area, characterised by short vegetation and low aerial cover, high greenness of grass, and high frequency of forbs, was inhabited by grasshopper species associated with bare ground or short and/or sparse grass, that were non-graminivorous or soft grass feeders. The lightly grazed area, characterised by tall vegetation and high aerial cover, low greenness of grass, and low frequency of forbs, was inhabited by grasshopper species associated with long and/or thick grass, that were mixed feeders or tough grass feeders. The mowed area, characterised by short vegetation and low aerial cover, low greenness of grass, and low frequency of forbs, exhibited lower grasshopper abundance, species richness, and diversity than either of the grazed areas.     

围封年限对典型草原植被与土壤特征的影响

针对典型草原退化草地恢复与合理利用问题,选取生长季（4-9月）围封收获干草,其他时间轻度放牧利用的天然草地为研究对象,同时选取自由放牧草地为对照,进行不同围封年限草原植物群落与土壤特征的比较研究。结果表明,1）与自由放牧草地相比,重度退化草地采用生长季围封恢复措施后群落地上现存量、盖度、密度、根系生物量、地表凋落物现存量及土壤养分含量显著增加,土壤容重、紧实度及＞0.25 mm的粗颗粒含量显著降低,群落结构优化,土壤环境改善, 植被与土壤间形成一个相互作用的良性循环系统,退化草地正向演替。2）草地在围封恢复过程中若连续多年刈割利用,容易导致生产性能降低,群落盖度与密度下降,草群矮化,土壤养分含量下降,草地发生2次逆行演替。3）季节性围封的管理方式既可保证退化草地在一定程度上得到恢复,也能达到充分利用草地资源的目的。季节性围封在我国牧区是可行方法之一,但适宜的围封季节及围封后的合理利用问题（如合理的割草制度等）有待进一步研究。     

宁夏干草原不同放牧方式对植物群落经济类群的影响

该研究紧密结合宁夏中部地区草原的实际,系统地开展了不同放牧方式下草地植物特性、草地植物群落结构、牧草生物学与生态学特性及放牧家畜的生产性能等方面的研究。结果表明:实行6区的划区轮牧方式是科学利用该类草地的最佳方式。轮牧比对照草地生产力提高73.84%。羊只体重轮牧组分别比对照(传统舍饲)、连续放牧和季节放牧组提高4.7kg、0.35kg和1.6kg。     

宁夏干草原不同放牧方式对植物群落经济类群的影响

该研究紧密结合宁夏中部地区草原的实际，系统地开展了不同放牧方式下草地植物特性、草地植物群落结构、牧草生物学与生态学特性及放牧家畜的生产性能等方面的研究。结果表明：实行6区的划区轮牧方式是科学利用该类草地的最佳方式。轮牧比对照草地生产力提高73．84％。羊只体重轮牧组分别比对照（传统舍饲）、连续放牧和季节放牧组提高4．7kg、0．35kg和1．6kg。     

Diverse Management Strategies Produce Similar Ecological Outcomes on Ranches in Western Great Plains: Social-Ecological Assessment

Experiments investigating grazing systems have often excluded ranch-scale decision making, which has limited our understanding of the processes and consequences of adaptive management. We conducted interviews and vegetation monitoring on 17 ranches in eastern Colorado and eastern Wyoming to investigate rancher decision-making processes and the associated ecological consequences. Management variables investigated were grazing strategy, grazing intensity, planning style, and operation type. Ecological attributes included the relative abundance of plant functional groups and categories of ground cover. We examined the environmental and management correlates of plant species and functional group composition using nonmetric multidimensional scaling and linear mixed models. After accounting for environmental variation across the study region, species composition did not differ between grazing management strategy and planning style. Operation type was significantly correlated with plant community composition. Integrated cow-calf plus yearling operations had greater annual and less key perennial cool-season grass species cover relative to cow-calf − only operations. Integrated cow-calf plus yearling ranches were able to more rapidly restock following drought compared with cow-calf operations. Differences in types of livestock operations contributed to variability in plant species composition across the landscape that may support diverse native faunal species in these rangeland ecosystems. Three broad themes emerged from the interviews: 1) long-term goals, 2) flexibility, and 3) adaptive learning. Stocking-rate decisions appear to be slow, path-dependent choices that are shaped by broader social, economic, and political dynamics. Ranchers described having greater flexibility in altering grazing strategies than ranch-level, long-term, annual stocking rates. These results reflect the complexity of the social-ecological systems ranchers navigate in their adaptive decision-making processes. Ranch decision-making process diversity within these environments precludes development of a single “best” strategy to manage livestock grazing.     

绵羊放牧绢蒿荒漠草地植物群落多样性及其环境解释

对天山北坡放牧条件下的绢蒿草地进行野外调查,以探讨植物群落物种组成、多样性特征及其与环境因子之间的关系。结果表明:1)绵羊放牧过程中形成了明显的放牧强度梯度,草地利用率适中。通过除趋势对应分析 (DCA)将放牧区内的植物群落划分为8个类型,各类型植物群落的物种多样性有显著差异,且群落间存在异质性;2)采用除趋势对应典范分析法(DCCA)中的前项选择,对所获取的9个环境因子进行筛选,最终土壤有机质含量、粪便量、放牧强度、凋落物生物量、海拔、坡向等6个环境因子成为决定物种分布和群落结构组成的主导因素。DCCA排序结果反映出了实际的生态意义,有机质、放牧强度、粪便量在第一排序轴有明显的梯度变化,而坡向和凋落物则在第2轴上呈现出明显梯度变化,物种和群落也沿着上述环境要素的梯度变化而依次分布,很好地解释了放牧条件下绢蒿荒漠草地物种、群落与环境条件之间的关系。     

放牧牛羊对云贵地区禾草/白三叶草地群落稳定性的影响

禾草/白三叶(Trifolium repens)草地是中国南方喀斯特地区主要刈牧地之一,探究不同家畜放牧下禾草/白三叶草地植被构成和群落稳定性,可一定程度揭示其植被演替特征。本研究定量分析了多年放牧牛(grazing cattle pasture,GC)、放牧羊(grazing sheep pasture,GS)的禾草/白三叶草地群落特征、物种多样性、生物量构成及群落稳定性。结果显示:1)草地死物质量及植物物种数为GC>GS,植物物种Simpson指数为GS>GC;2)草地禾草[多年生黑麦草(Lolium perenne)、鸭茅(Dactylis glomerata)及原生禾草]、原生双子叶(马鞭草科、车前科、酢浆草科和十字花科)及适口性中等植物地上生物量组分为GC>GS,播种白三叶地上生物量组分为GS>GC;且GC草地地上生物量以禾草为主(77.20%),GS草地地上生物量以禾草(51.79%)和豆科(28.02%)为主;3)草地土壤K含量为GC>GS;4)GC草地植物种Raunkiaer频度直方图呈反J型,群落演替度(37.08)比GS草地(45.45)的低。研究认为,GC草地群落处于相对稳定状态,而GS草地群落处于更高演替阶段,牛放牧更利于禾草/白三叶草地群落稳定性和物种多样性的维持。因此,放牧时可实行牛、羊混牧制,或牛、羊轮牧制,以提高禾草/白三叶草地的稳定性和生产力。