Grazing effects on plant functional group diversity in Mediterranean shrublands

Grazing is one of the prevalent human activities that even today are taking place inside protected areas with direct or indirect effects on ecosystems. In this study we analyzed the effects of grazing on plant species diversity, plant functional group (PFG) diversity and community composition of shrublands. We analyzed plant diversity data from 582 sampling plots located in 66 protected areas of the Greek Natura 2000 network, containing in total 1102 plant species and subspecies. We also classified a priori all plant species in seven PFGs: annual forbs, annual grasses/sedges, legumes, perennial forbs, perennial grasses/sedges, small shrubs and tall shrubs. For each site, grazing intensity was estimated in four classes (no grazing, low, medium and high grazing intensity). We found that, at the spatial and temporal scale of this study, as grazing intensity increased, so did total species richness. However, each PFG displayed a different response to grazing. Short-lived species (annual grasses or forbs and legumes) benefited from grazing and their species richness and proportion in the community increased with grazing. Perennial grasses and forbs species richness increased with grazing intensity, but their dominance decreased, since their proportion in the community declined. Short shrub species richness remained unaffected by grazing, while tall shrub diversity decreased. Finally, in sites without grazing the spatial pattern of species richness of the different PFGs was not congruent with each other, while in grazed sites they were significantly positively correlated (with the exception of tall shrubs). This finding may imply that grazing is a selective pressure organizing the community structure, and imposing a certain contribution of each PFG. So, in Mediterranean shrublands in protected areas with a long historical record of grazing, it seems that grazing promotes species diversity and its continuation on a portion of the landscape may be a necessary part of an effective management plan.     

Influence of grazing and fire frequency on small-scale plant community structure and resource variability in native tallgrass prairie

In grasslands worldwide, grazing by ungulates and periodic fires are important forces affecting resource availability and plant community structure. It is not clear, however, whether changes in community structure are the direct effects of the disturbance (i.e. fire and grazing) or are mediated indirectly through changes in resource abundance and availability. In North American tallgrass prairies, fire and grazing often have disparate effects on plant resources and plant diversity, yet, little is known about the individual and interactive effects of fire and grazing on resource variability and how that variability relates to heterogeneity in plant community structure, particularly at small scales. We conducted a field study to determine the interactive effects of different long-term fire regimes (annual vs four-year fire frequency) and grazing by native ungulates ( Bos bison ) on small-scale plant community structure and resource variability (N and light) in native tallgrass prairie. Grazing enhanced light and nitrogen availability, but did not affect small-scale resource variability. In addition, grazing reduced the dominance of C₄ grasses which enhanced species richness, diversity and community heterogeneity. In contrast, annual fire increased community dominance and reduced species richness and diversity, particularly in the absence of grazing, but had no effect on community heterogeneity, resource availability and resource variability. Variability in the abundance of resources showed no relationship with community heterogeneity at the scale measured in this study, however we found a relationship between community dominance and heterogeneity. Therefore, we conclude that grazing generated small-scale community heterogeneity in this mesic grassland by directly affecting plant community dominance, rather than indirectly through changes in resource variability.     

Plant species diversity and grazing in the Scandinavian mountains - patterns and processes at different spatial scales

There is a long tradition of grazing by semi‐domestic reindeer and sheep in alpine and sub‐alpine Scandinavian habitats, but present management regimes are questioned from a conservation point of view. In this review we discuss plant diversity patterns in the Scandinavian mountains in a global, regional and local perspective. The main objective was to identify processes that influence diversity at different spatial scales with a particular focus on grazing. In a global perspective the species pool of the Scandinavian mountains is limited. partly reflecting the general latitudinal decline of species but also historical and ecological factors operating after the latest glaciation. At the local scale, both productivity and disturbance are primary factors structuring diversity, but abiotic factors such as soil pH, snow distribution and temperature are also important. Although evidence is scarce, grazing favours local species richness in productive habitats, whereas species richness decreases with grazing when productivity is low. Regional patterns of plant diversity is set by, 1) the species pool. 2) the heterogeneity and fragmentation of communities, and 3) local diversity of each plant community. We suggest that local shifts in community composition depend both on the local grazing frequency and the return‐time of the plant community after a grazing session. In addition, an increasing number of grazing‐modified local patches homogenises the vegetation and is likely to reduce the regional plant diversity. The time scale of local shifts in community composition depends on plant colonisation and persistence, From a mechanistic point of view, diversity patterns at a regional scale also depend on the regional dynamics of single species. Colonisation is usually a slow and irregular process in alpine environments, whereas the capacity for extended local persistence is generally high. Although the poor knowledge of plant regional dynamics restricts our understanding of how grazing influences plant diversity, we conclude that grazing is a key process for maintaining biodiversity in the Scandinavian mountains.     

放牧强度对短花针茅荒漠草原甲虫群落分布格局的影响

放牧对脆弱的荒漠草原生态系统有着重要影响,且随放牧强度及持续时间不同而变化。鞘翅目昆虫是环境监测与生物多样性变化的指示生物。利用巴氏罐诱法对短花针茅荒漠草原不同放牧强度草地的甲虫群落组成和多样性进行调查,探究放牧对荒漠草原甲虫群落的影响。结果表明:(1)步甲科、金龟科为短花针茅荒漠草原甲虫群落优势类群,埋葬甲科、芫菁科、拟步甲科和花金龟科为常见类群。(2)放牧强度增加不利于维持更多的捕食性甲虫;对照和轻度放牧样地可维持更多的腐食性甲虫。(3)甲虫数量随放牧强度增加而递减;群落多样性以重度放牧草地最大,轻度放牧草地最小;群落优势度为对照、中度、重度显著高于轻度放牧草地。各甲虫类群在不同放牧强度草地出现时间、高峰期均不同。(4)对照、轻度、重度放牧样地的甲虫优势类群群落结构不同于其他生境,但均与中度放牧样地存在相似性。轻度、中度、重度放牧样地的甲虫稀有类群群落结构不同于其他生境,但均与对照样地存在相似性。(5)甲虫群落个体数与植物群落物种丰富度、盖度、植物平均高度、生物量呈显著正相关。Shannon-Wiener多样性指数、Margalef丰富度指数均与植物群落物种丰富度、生物量显著负相关...     

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.     

放牧对短花针茅荒漠草原植物多样性的影响

放牧干扰对草原植物多样性影响机制是放牧生态学研究的核心问题。以内蒙古锡林郭勒盟苏尼特右旗的短花针茅荒漠草原的长期放牧控制实验为平台,系统研究了放牧调控下植物多样性随组织尺度转换的影响,为荒漠草原植物多样性尺度推绎提供理论基础。结果显示:1)现存草地物种数未放牧最高,适度放牧次之,重度放牧最低,差异体现在多年生杂类草和一年生草本2个功能群上,且各功能群的权重基本不受放牧强度影响;2)群落尺度,放牧强度没有显著影响丰富度指数,未放牧小区的植物Simpson生态优势度指数、Shannon-Wiener物种多样性指数与Pielou均匀度指数大于适度放牧小区,显著大于重度放牧小区(P0.05);功能群尺度,多年生禾草与一年生草本的多样性指数对放牧无显著响应,多年生杂类草的多样性指数未放牧小区最高,适度放牧小区次之。3)Godron群落稳定性指数显示,适度放牧的小区稳定性高于未放牧小区和重度放牧小区。研究表明,放牧强度的上升使短花针茅荒漠草原不同组织尺度植物多样性降低,但群落稳定性结果显示适度放牧的草地表现出了更高的稳定性,植物多样性与稳定性的权衡将是合理制定区域科学放牧强度的重要途径。     

The relationship between the diversity of arbuscular mycorrhizal fungi and grazing in a meadow steppe

Aims

To study the relationship between changes in soil properties and plant community characters produced by grazing in a meadow steppe grassland and the composition and diversity of spore-producing arbuscular mycorrhizal fungi (AMF).

Methods

A field survey was carried out in a meadow steppe area with a gradient of grazing pressures (a site with four grazing intensities and a reserve closed to grazing). The AMF community composition (characterized by spore abundance) and diversity, the vegetation characters and soil properties were measured, and root colonization by AMF was assessed.

Results

AMF diversity (richness and evenness) was higher under light to moderate grazing pressure and declined under intense grazing pressures. Results of multiple regressions indicated that soil electrical conductivity was highly associated with AMF diversity. The variation in AMF diversity was partially associated to the density of tillers of the dominant grass (Leymus chinensis), the above and below-ground biomass and the richness of the plant community.

Conclusions

We propose that the relationship between plants and AMF is altered by environmental stress (salinity) which is in turn influenced by animal grazing. Direct and indirect interactions between vegetation, soil properties, and AMF community need to be elucidated to improve our ability to manage these communities.     

Intraspecific variation overrides origin effects in impacts of litter-derived secondary compounds on larval amphibians

Vertebrate herbivores can be key determinants of grassland plant species richness, although the magnitude of their effects can largely depend on ecosystem and herbivore characteristics. It has been demonstrated that the combined effect of primary productivity and body size is critical when assessing the impact of herbivores on plant richness of perennial-dominated grasslands; however, the interaction of site productivity and herbivore size as determinants of plant richness in annual-dominated pastures remains unknown. We experimentally partitioned primary productivity and herbivore body size (sheep and wild rabbits) to study the effect of herbivores on the plant species richness of a Mediterranean semiarid annual plant community in central Spain over six years. We also analyzed the effect of grazing and productivity on the evenness and species composition of the plant community, and green cover, litter, and plant height. We found that plant richness was higher where the large herbivore was present at high-productivity sites but barely changed at low productivity. The small herbivore did not affect species richness at either productivity site despite its large effects on species composition. We propose that adaptations to resource scarcity and herbivory prevented plant richness changes at low-productivity sites, whereas litter accumulation in the absence of herbivores decreased plant richness at high productivity. Our results are consistent with predictions arising from a long history of grazing and highlight the importance of both large and small herbivores to the maintenance of plant diversity of Mediterranean annual-dominated pastures.     

放牧干扰下高寒草甸物种、生活型和功能群多样性与生物量的关系

为明晰放牧强度对高寒草甸物种、生活型以及功能群多样性和生物量的时间累积作用以及对多样性与生物量之间相互关系的影响。选择青藏高原东北缘高寒草甸,在6个放牧强度样地连续4 a(2012—2015)进行物种、生活型、功能群多样性和生物量调查。采用重复测量方差分析和线性回归法分析放牧强度和放牧年份对物种、生活型、功能群多样性以及多样性与生物量之间的相关关系的影响。结果表明:(1)放牧强度和放牧年份均对生物量产生显著影响。随放牧强度增加,生物量显著降低。但随放牧年份延长,不同放牧强度区生物量无一致的变化规律。(2)放牧年份对物种、生活型和功能群丰富度、均匀度和优势度的影响均达到显著水平。但放牧强度和放牧年份只对物种丰富度产生交互作用,放牧强度对物种丰富度的影响具有时间的累积效应。(3)放牧干扰下不同层次多样性,仅物种丰富度与所有多样性指数显著相关,物种丰富度可以作为物种多样性测度的代表性指标。(4)多样性与生物量关系的研究,对丰富度而言,仅低放牧强度样地(Plot2)中物种丰富度随生物量增加而显著降低,其余放牧强度样地的物种、生活型和功能群丰富度均与生物量无关。对均匀度而言,高放牧强度样地(Plot4、Plot5、Plot6)生活型均匀度随生物量的增加而显著降低。对优势度而言,高放牧强度样地生活型优势度随生物量的增加而显著增加。生活型多样性可作为放牧干扰下生物量变化快速预测的有效指标。     

Effects of grazing intensity on pollinator abundance and diversity,and on pollination services

1. Pollinating insects provide important ecosystem services and are influenced by the intensity of grazing. Based on the Intermediate Disturbance Hypothesis (IDH), pollinator diversity is expected to peak at intermediate grazing intensities. However, this hump‐shaped relationship is rarely found. 2. The effect of grazing intensity was tested on flower cover, on the abundance and richness of bees, hoverflies and bee flies, and on pollination services to early‐flowering bee‐pollinated Asphodelus ramosus L. For that, we used data on 11 plant–pollinator phryganic communities from Lesvos Island (Greece) widely differing in grazing intensities. 3. Flower abundance and richness showed hump‐shaped relationships with grazing intensity. Grazing affected the abundance and richness of bees and hoverflies directly and also indirectly, through changes in the flower community. Grazing influenced directly the richness but not the abundance of bee flies. Overall, pollinator abundance and richness showed hump‐shaped relationships with grazing intensity, but variations in strength (hoverfly abundance) and direction (bee community) of the effect appeared along the season. Early in the season, grazing increased bee abundance but decreased richness, resulting in increased pollen limitation in A. ramosus. 4. The effects of grazing on pollinators vary with the intensity of the disturbance, generally supporting the IDH, and the timing of land‐use activities may influence pollination services. Management strategies should include moderate grazing levels to preserve overall diversity in this area, however, the conservation of particular early bee or bee‐pollinated species may benefit from reduced grazing in early spring.     

放牧干扰下高寒草甸物种多样性指数评价与选择

在青藏高原东北缘高寒草甸设置6个放牧强度样地,连续4年研究10个多样性指数(Richness和Abundance 2个实测指数,优势度指数、均匀度指数、丰富度指数和综合指数各2个)对放牧强度和年限影响植物群落的解释能力.结果表明: 相对于重要值,利用多度计算的多样性指数对放牧干扰更敏感.优势度指数(Berger-Parker、Dominance)与放牧强度和年限均无关,不能将放牧干扰对群落优势种的影响有效区分.均匀度指数(Equitability、Evenness)均与放牧强度无关,但Evenness指数与放牧年限呈显著负相关,不受偶见种影响且与物种多度的变异系数呈显著正相关,在基于时间尺度的均匀度比较中可以选择Evenness指数.丰富度指数(Menhinick、Margalef)均与放牧年限无关,但Margalef指数与放牧强度呈显著正相关,且不受偶见种影响.综合指数(Shannon、Simpson)均与放牧强度无关,但Shannon指数与物种丰富度和多度呈显著正相关,且随放牧年限增加而显著升高,不受偶见种影响,Shannon指数可用于在长时间尺度下比较物种多样性变化.在所有多样性指数中,只有实测物种丰富度和多度均与放牧强度呈显著负相关,与放牧年限呈显著正相关,且不受偶见种影响,故实测物种丰富度和多度相结合可作为放牧干扰下多样性比较的首选指标.此外,多样性指数选择须考虑放牧干扰的强度与时间特征、多样性组分和研究目的.     

放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应

为了揭示高寒小嵩草草甸群落在放牧扰动下,探讨土壤养分供给水平的变化对生态系统初级生产力和多样性影响,为高寒草地的退化演替机理研究提供依据,以野外样地调查和室内分析法研究了放牧扰动下高寒草甸植物多样性、生产力对土壤养分条件变化的响应.结果表明,放牧干扰不仅改变了高寒小蒿草草甸群落土壤根系和蕴育土壤根系的"载体"量及根土比例,改变了植物群落的结构和功能,而且使土壤的物理和化学特性发生了明显的改变.随着放牧强度的增加,蕴育土壤根系的基质量逐渐减少,根土比特别是0～10 cm土层的根土比例增加;"载体"量减少导致大部分地下根系由于营养供给水平的降低而死亡,归还土壤中有机质的数量逐渐减少,加之地上部分持续利用,土壤养分也在不断消耗,土壤基质量的减少和土壤资源持续供给能力的下降,草地发生逆向演替(退化),表现在:物种数减少、多样性下降、能量的分配转向地下等;土壤性状上的某些改变(土壤容重、土壤湿度等),也会引起植被组成、物种多样性变化;放牧主要通过影响土壤环境及其养分含量来改变草地群落生物量(地上、地下);土壤表面的适度干扰和原有植物的适度破坏为新成员提供了小生境,从而允许新的植物侵入群落,并提高了植物的丰富度.但是,在受到强度干扰时,草地植物群落的主要物种的优势地位发生明显的替代变化.     

Habitat type plays a greater role than livestock grazing in structuring shrubsteppe plant–pollinator communities

Livestock grazing is a widespread grassland disturbance and can negatively impact biodiversity. Pollinators constitute a vital component of grassland ecosystems, but the impact of grazing on pollinator diversity has seldom been evaluated in North America. We assessed vegetation structure, and pollinator and flowering plant abundance, richness, diversity, and community composition in four pairs of spring-grazed/ungrazed sites in south-central British Columbia, Canada. We also investigated whether pollinator or floral communities differed between the two threatened shrubsteppe habitat types we sampled—antelope-brush and big sagebrush shrubsteppe. Pan-trapping surveys captured 5907 bees, flies, beetles, wasps and butterflies constituting 253 species. We found that the percent cover of shrubs and bare soil increased with grazing, while the height of grasses and forbs decreased. In contrast, pollinator and flowering plant abundance, richness, diversity, and community composition were not significantly affected by grazing. Flowering plant and pollinator community composition did differ significantly between shrubsteppe habitats. Our results indicate that grasslands in North America, when managed responsibly, can maintain pollinator and flowering plant diversity under grazing pressure. The continued effort of land managers to balance ecological integrity and economic viability will be important for the conservation of grassland pollination systems.     

Grazer exclusion alters plant spatial organization at multiple scales,increasing diversity

Grazing is one of the most important factors influencing community structure and productivity in natural grasslands. Understanding why and how grazing pressure changes species diversity is essential for the preservation and restoration of biodiversity in grasslands. We use heavily grazed subalpine meadows in the Qinghai‐Tibetan Plateau to test the hypothesis that grazer exclusion alters plant diversity by changing inter‐ and intraspecific species distributions. Using recently developed spatial analyses combined with detailed ramet mapping of entire plant communities (91 species), we show striking differences between grazed and fenced areas that emerged at scales of just one meter. Species richness was similar at very small scales (0.0625 m²), but at larger scales diversity in grazed areas fell below 75% of corresponding fenced areas. These differences were explained by differences in spatial distributions; intra‐ and interspecific associations changed from aggregated at small scales to overdispersed in the fenced plots, but were consistently aggregated in the grazed ones. We conclude that grazing enhanced inter‐ and intraspecific aggregations and maintained high diversity at small scales, but caused decreased turnover in species at larger scales, resulting in lower species richness. Our study provides strong support to the theoretical prediction that inter‐ and intraspecific aggregation produces local spatial patterns that scale‐up to affect species diversity in a community. It also demonstrates that the impacts of grazing can manifest through this mechanism, lowering diversity by reducing spatial turnover in species. Finally, it highlights the ecological and physiological plant processes that are likely responding to grazing and thereby altering aggregation patterns, providing new insights for monitoring, and mediating the impacts of grazing.     

Grazing increases functional richness but not functional divergence in Tibetan alpine meadow plant communities

Plant community diversity and ecosystem function are conditioned by competition among co-occurring species for multiple resources. Previous studies suggest that removal of standing biomass by grazing decreases competition for light, but coincident grazing effects on competition for soil nutrients remain largely unknown in Tibetan rangelands where grazing tends to deplete soil phosphorus availability. We measured five functional traits indicative of plant productivity and stoichiometry leaf carbon concentration (LCC), leaf nitrogen concentration (LNC), leaf phosphorus concentration (LPC), specific leaf area (SLA), leaf dry matter content (LDMC) for component species of plant communities in grazed and ungrazed plots in five Tibetan alpine meadows. We examined the diversity of traits singly Rao index of functional diversity (FDrao) and in aggregate functional richness (FRic), functional divergence (FDiv), and functional evenness (FEve) in response to grazing. We tested whether foliar trait diversity increases with nutrient competition but decreases with light competition when competitive exclusion is reduced by grazing. The FDrao of LPC significantly increased under grazing, but FDrao for LCC, LNC and SLA tended to decrease. The FDrao of LDMC increased at the drier site but decreased at the wettest site. There was a strong negative association between increase in FDrao of LPC and decrease in soil nutrients, especially soil phosphorus availability. The FRic for all five traits together increased with species diversity following grazing, but neither FDiv nor FEve differed significantly between grazed and ungrazed plots at most sites. Grazing in Tibetan alpine meadows tends to increase competition for soil phosphorus while decreasing competition for light, resulting in an increase in the functional richness in grazed plant communities without any significant changes in the overall functional diversity of foliar traits. Our study highlights the potential importance of grazing mediated competition for multiple resources in alpine meadow ecosystems.     

How do sheep affect plant communities and arthropod populations in temperate grasslands?

Grasslands being used in sheep farming systems are managed under a variety of agricultural production, recreational and conservational objectives. Although sheep grazing is rarely considered the best method for delivering conservation objectives in seminatural temperate grasslands, the literature does not provide unequivocal evidence on the impact of sheep grazing on pasture biodiversity. Our aim was therefore to review evidence of the impacts of stocking rate, grazing period and soil fertility on plant communities and arthropod populations in both mesotrophic grasslands typical of agriculturally improved areas and in native plant communities. We therefore conducted a literature search of articles published up to the end of the year 2010 using ‘sheep’ and ‘grazing’ as keywords, together with variables describing grassland management, plant community structure or arthropod taxa. The filtering process led to the selection of 48 articles, with 42 included in the stocking rate dataset, 9 in the grazing period dataset and 10 in the soil fertility dataset. The meta-analysis did not reveal any significant trends for plant species richness or plant community evenness along a wide stocking rate gradient. However, we found frequent shifts in functional groups or plant species abundance that could be explained by the functional properties of the plants in the community. The meta-analysis confirmed that increasing soil fertility decreased plant species richness. Despite the very limited dataset, plant species richness was significantly greater in autumn-grazed pastures than in ungrazed areas, which suggests that choosing an appropriate grazing period would be a promising option for preserving biodiversity in sheep farming systems. Qualitative review indicated that low grazing intensity had positive effects on Orthoptera, Hemiptera (especially phytophagous Auchenorrhyncha) and, despite a diverse range of feeding strategies, for the species richness of Coleoptera. Lepidoptera, which were favoured by more abundant flowering plants, also benefited from low grazing intensities. Spider abundance and species richness were higher in ungrazed than in grazed pastures. In contrast, there are insufficient published studies to draw any firm conclusions on the benefits of late grazing or stopping fertilization on insect diversity, and no grounds for including any of this information in decision support tools at this stage.     

Effects of grazing disturbance on plant diversity,community structure and direction of succession in an alpine meadow on Tibet Plateau,China

To elucidate the effects of grazing intensity and grazing time on plant diversity and community structure, as well as the successional differentiation in an alpine meadow, a controlled grazing trial, with six grazing intensities on an alpine meadow was conducted in the eastern Qilian Mountain region for four years. Using species accumulation curves, RDA ordination and variance decomposition, we analyzed the changes in proportion of dominant species, richness, abundance, as well as the life forms of plant communities under grazing disturbance. Both the grazing intensity and grazing time had a significant effect on these dominant species, richness, abundance, as well as the life forms in the plant community (P < 0.01). More detailed results showed that: (1) The richness and abundance of plant species were highest in the light grazing plot, and these increased as time passed. In the heavy grazing plot, the abundance of plant species decreased as time passed, but the richness of these species did not change significantly. (2) The abundance of Gramineae and Umbelliferae were negatively and significantly correlated with the duration of grazing treatments, whereas Plantaginaceae and Geraniaceae were positively and significantly correlated with the grazing intensity. Over time, the abundance of bunch-type plants decreased and other life forms of plants have increased. With the increase in grazing intensity, the plants' abundance with the rosette type did not change, but other life forms of plants decreased. The results of variance decomposition indicated that grazing disturbance has had greater effects on life forms and plant populations, followed by the changes in the dominant species and their abundance, with lesser effects on the richness of the species. Grazing intensity had a greater effect than the duration of the grazing treatment had. The results of PCA showed that the climax community in both the lowest and the highest grazing intensity plots had changed over time. In the sample plots with light grazing intensity, the plant community changed to an Elymus nutans + Poa crymophila community, but later changed to a Melilotoides ruthenicus + Kobresia humilis community under heavy grazing.     

Effects of grazing disturbance on plant diversity,community structure and direction of succession in an alpine meadow on Tibet Plateau,China

To elucidate the effects of grazing intensity and grazing time on plant diversity and community structure, as well as the successional differentiation in an alpine meadow, a controlled grazing trial, with six grazing intensities on an alpine meadow was conducted in the eastern Qilian Mountain region for four years. Using species accumulation curves, RDA ordination and variance decomposition, we analyzed the changes in proportion of dominant species, richness, abundance, as well as the life forms of plant communities under grazing disturbance. Both the grazing intensity and grazing time had a significant effect on these dominant species, richness, abundance, as well as the life forms in the plant community (P < 0.01). More detailed results showed that: (1) The richness and abundance of plant species were highest in the light grazing plot, and these increased as time passed. In the heavy grazing plot, the abundance of plant species decreased as time passed, but the richness of these species did not change significantly. (2) The abundance of Gramineae and Umbelliferae were negatively and significantly correlated with the duration of grazing treatments, whereas Plantaginaceae and Geraniaceae were positively and significantly correlated with the grazing intensity. Over time, the abundance of bunch-type plants decreased and other life forms of plants have increased. With the increase in grazing intensity, the plants' abundance with the rosette type did not change, but other life forms of plants decreased. The results of variance decomposition indicated that grazing disturbance has had greater effects on life forms and plant populations, followed by the changes in the dominant species and their abundance, with lesser effects on the richness of the species. Grazing intensity had a greater effect than the duration of the grazing treatment had. The results of PCA showed that the climax community in both the lowest and the highest grazing intensity plots had changed over time. In the sample plots with light grazing intensity, the plant community changed to an Elymus nutans + Poa crymophila community, but later changed to a Melilotoides ruthenicus + Kobresia humilis community under heavy grazing.     

Slow recovery in desert perennial vegetation following prolonged human disturbance

Questions: How long may it take for desert perennial vegetation to recover from prolonged human disturbance and how do different plant community variables (i.e. diversity, density and cover) change during the recovery process? Location: Sonoran Desert, Arizona, USA. Methods: Since protection from grazing from 1907 onwards, plant diversity, density and cover of perennial species were monitored intermittently on ten 10 m × 10 m permanent plots on Tumamoc Hill, Tucson, Arizona, USA. Results: The study shows an exceptionally slow recovery of perennial vegetation from prolonged heavy grazing and other human impacts. Since protection, overall species richness and habitat heterogeneity at the study site continued to increase until the 1960s when diversity, density and cover had been stabilized. During the same period, overall plant density and cover also increased. Species turnover increased gradually with time but no significant relation between any of the three community variables and precipitation or Palmer Drought Severity Index (PDSI) was detected. Conclusions: It took more than 50 yr for the perennial vegetation to recover from prolonged human disturbance. The increases in plant species richness, density, and cover of the perennial vegetation were mostly due to the increase of herbaceous species, especially palatable species. The lack of a clear relationship between environment (e.g. precipitation) and community variables suggests that site history and plant life history must be taken into account in examining the nature of vegetation recovery processes after disturbance.     

Grazing impacts on the diversity and composition of alpine rangelands in Northwest Yunnan

Aims The eastern Himalayan region of Southwest China represents the world's most biodiverse temperate region as well as a cultural hotspot undergoing rapid cultural and ecological change. This area represents the center of origin for many plant groups including horticulturally valuable species of Pedicularis, Rhododendron and Primula. Alpine meadows here also provide summer pastures for Tibetan yak herders and the source for important medicinal plants. Stocking levels for livestock here have quadrupled over the last five decades and shrubs are encroaching into many historical rangelands. Yak herders voice concerns over both shrub encroachment and shrinking grasslands. In this study, we sought to determine: (i) Are alpine rangelands in Deqin County overgrazed and degraded? (ii) What are the local impacts of grazing on plant diversity and community composition? And (iii) which environmental variables covary with these differences in species composition across the grazing gradient?Methods To examine the ecological impacts of grazing in Deqin County, northwest Yunnan and assess its long-term sustainability, we used county records to determine historical population and livestock numbers, confirmed the results of interviews by Haynes (2011) and surveyed plant species richness and cover along a grazing gradient extending away from herder huts. Along these transects, we sampled 1?m 2 quadrats at 5 m intervals, noting species present and percent cover for vascular plants, grasses, sedges, rushes, moss, lichen, exposed rock, bare ground and feces. We also measured the average and maximum plant height within each quadrat. We then computed species richness for each of the 38 transects, calculating alpha and beta diversities. We used one-way ANOVAs to compare mean species richness values and average and maximum plant height across grazing intensities. To chart changes in composition along the 100m gradient, we also plotted the percent cover of graminoid, forb, shrub and bare ground versus distance from the hut. We applied NMS ordination to relate community patterns to environmental variables and grazing intensity using distances to determine species groupings.Important findings Livestock impacts are clearly evident with proportions of grass and bare ground decreasing, shrubs increasing and forbs maintaining even cover with increasing distance from the huts. In comparison with earlier surveys of sites farther from huts, we found reduced plant cover and diversity. Plant species richness almost doubles with increasing distance from herder huts from 9.9 to 19.3 species per 1 m 2 quadrat. An ordination of species and environmental variables demonstrates that grazing strongly affects plant community com position across these plots with strong impacts on palatable plants. If herd sizes remain large and suitable areas for grazing continue to decline, the cumulative impacts of grazing appear likely to degrade the rich diversity of the region and reduce rangeland quality, threatening its ability to sustain current grazing levels.