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Species diversity has declined in ecosystems worldwide as a result of habitat fragmentation, eutrophication, and land-use change. If such decline is to be halted ecological mechanisms that restore or maintain biodiversity are needed. Two long-term field experiments were performed in native grassland to assess the effects of fire, nitrogen addition, and grazing or mowing on plant species diversity. In one experiment, richness declined on burned and fertilized treatments, whereas mowing maintained diversity under these conditions. In the second experiment, loss of species diversity due to frequent burning was reversed by bison, a keystone herbivore in North American grasslands. Thus, mowing or the reestablishment of grazing in anthropogenically stressed grasslands enhanced biodiversity

1. Abandonment and eutrophication are major threats to traditional pastoral landscapes and their wildlife in Europe. Social and economical developments have rendered traditional pastoralism impracticable. More knowledge is needed about the effects of grazing with free-ranging herbivores, which is increasingly used as a substitute for the traditional herding system. 2. We studied the effects of free-ranging cattle on the recovery of Calluna heather, tree encroachment and plant species richness in six habitats in a grass-rich Dutch heathland during a 10-year period. The habitats differed in soil fertility, initial heather-grass ratio and developmental stage of Calluna. 3. Despite preferential grass defoliation, cattle grazing neither reduced grass cover in grass heath nor prevented grass invasion in heather. Grass invasion failed only in a nitrogen-poor turf-stripped Calluna heath. Grazing induced a substantial Calluna recovery in grass heaths on podzolic soils, but its recovery failed in grass heath on a phosphorus-rich medieval arable field. As a consequence, the grass-heather mosaics generated by free-ranging cattle were restricted to habitats of intermediate soil fertility. 4. Grazing did not prevent encroachment by pine and birch. Removal by the site manager prevented conversion of 10-20% of the open heathland to forest. 5. During the first 5 years, grazing induced a significant increase in species richness in all habitats. During the second 5 years, species richness stabilized in grass heath and heather-grass mosaics and it declined in the pioneer Calluna heaths. 6. We found indications of various nutrient-mediated grazing effects on the competitive balance between grass and woody pioneers. These suggest that nutrientmediated feedback might be an important explanatory mechanism for the described vegetation mosaic cycling in heathlands. 7. Free-ranging grazing did not remove the high atmospheric nutrient inputs of the whole area. Substantial amounts of nutrients were redistributed from the grass lawns to the forest. 8. Free-ranging grazing combined with tree cutting appeared to be a suitable management regime for the maintenance of species-rich open heathlands with dynamic grass-heather mosaics. 9. Without tree cutting, free-ranging grazing would have created dynamic tree-grass-heather mosaics in open heathland. Wood-pasture landscapes are fundamentally different from the open heather-dominated heathlands produced by the traditional sedentary farming system. Integrated grazing of heathland, nutrient-rich farmland and forests accelerates this change. Alternative grazing regimes are discussed.

Large mammalian herbivores not only depend on plant communities for their existence but cause major changes in plant community composition and structure. These changes have direct consequences for ecosystem processes, but recent studies of ungulate-ecosystem relations show widely divergent ungulate effects in different ecosystems. We reviewed studies of ungulate effects on plant community composition to gain insight into potential mechanisms of ungulate-induced changes in both community composition and ecosystem processes. Our analysis of these studies is based on the premise that the effect ungulates exert on plant communities depends on the balance between (1) feeding selectivity of herbivores (i.e., degree to which different plant species or ecotypes experience different levels of tissue loss), and (2) differences among plant species in their ability to recover from tissue loss. A large number of studies clearly show that selective ungulate herbivory leads to the dominance of unpalatable, chemically defended plant species in communities. However, many studies have also demonstrated that intensive long-term herbivory does not lead to the invasion of unpalatable species into the community, and can even increase the dominance of highly palatable species. Our review indicates that high levels of nutrient inputs or recycling and an intermittent temporal pattern of herbivory (often due to migration) are key factors increasing the regrowth capacity of palatable species and hence maintaining their dominance in plant communities supporting abundant herbivores. Key factors limiting ungulate foraging selectivity, again limiting herbivore-induced dominance of slow-growing, unpalatable species, include herding behavior, early growing season and postfire herbivory, asynchronous phenology of palatable versus unpalatable species, and low relative abundance of unpalatable species. Our review indicates differences among ecosystems in the role played by ungulate herbivory result from the relative strength of these factors enhancing plant tolerance to herbivory and limiting foraging selectivity. Anthropogenic changes in these factors (e.g., alteration of migration patterns) therefore have the potential to significantly alter the effects of ungulates on plant communities and ecosystem processes.

To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient-rich and non-enriched or nutrient-poor ecosystems. All 19 comparisons from non-enriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems. Although nutrient-rich ecosystems seemed to produce more variable responses to grazers than did nutrient-poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient-rich environments. We suggest that species richness declines with high grazing in nutrient-poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient-rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient-rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.

Rangeland grazing management strategies have been developed in an effort to sustain efficient use of forage resources by livestock. However, the effects of grazing on the redistribution and cycling of carbon (C) and nitrogen (N) within the plant-soil system are not well understood. We examined the plant-soil C and N balances of a mixed-grass rangeland under three livestock stocking rates using an area that had not been grazed by domestic livestock for more than 40 years. We established nongrazed exclosures and pastures subjected to continuous season-long grazing at either a light stocking rate (20 steer-days/ha) or a heavy stocking rate (59 steer-days/ha, ∼50% utilization of annual production). Twelve years of grazing under these stocking rates did not change the total masses of C and N in the plant-soil (0-60 cm) system but did change the distribution of C and N among the system components, primarily via a significant increase in the masses of C and N in the root zone (0-30 cm) of the soil profile. The mass of soil C (0-60 cm) under heavy grazing was comparable to that of the light grazing treatment. Grazing at the heavy stocking rate resulted in a decrease in peak standing crop (PSC) of aboveground live phytomass, an increase in blue grama (Bouteloua gracilis [H.B.K.] Lag. Ex Steud.), and a decrease in western wheatgrass (Pascopyrum smithii [Rydb.] A. Love) compared to the light grazing treatment. The dominant species under light grazing was western wheatgrass, whereas in the nongrazed exclosures, forbs were dominant and appeared to have increased at the expense of western wheatgrass. The observed increase of soil C and N in the surface soil where roots dominate indicates a greater opportunity for nutrient availability and cycling, and hence enhanced grazing quality.

To assess the impacts of land-use changes on plant-available water (PAM and evapotranspiration (ET), volumetric water content (VWC) was measured to 8 m beneath three, adjacent ecosystems for four years (1991-1994). Estimates of PAW, ET, and deep drainage were generated for mature evergreen forest, adjacent pasture, and capoeira (second-growth forest on abandoned pasture land).

Rangelands are vast, making up one quarter of the United States and forty percent of the Earth's ice-free land. And while contemporary science has revealed a great deal about the environmental impacts associated with intensive livestock production—from greenhouse gas emissions to land and water degradation—far less is known about the historic role science has played in rangeland management and politics. Steeped in US soil, this first history of rangeland science looks to the origins of rangeland ecology in the late nineteenth-century American West, exploring the larger political and economic forces that—together with scientific study—produced legacies focused on immediate economic success rather than long-term ecological well being. During the late 1880s and early 1890s, a variety of forces—from the Homestead Act of 1862 to the extermination of bison, foreign investment, and lack of government regulation—promoted free-for-all access to and development of the western range, with disastrous environmental consequences. To address the crisis, government agencies turned to scientists, but as Nathan F. Sayre shows, range science grew in a politically fraught landscape. Neither the scientists nor the public agencies could escape the influences of bureaucrats and ranchers who demanded results, and the ideas that became scientific orthodoxy—from fire suppression and predator control to fencing and carrying capacities—contained flaws and blind spots that plague public debates about rangelands to this day. Looking at the global history of rangeland science through the Cold War and beyond, The Politics of Scale identifies the sources of past conflicts and mistakes and helps us to see a more promising path forward, one in which rangeland science is guided less by capital and the state and more by communities working in collaboration with scientists.

Recent studies of nitrogen (N) cycling in arctic tundra have indicated that inorganic N supplied to plants by mineralization is not sufficient to meet the annual requirement of N by many tundra species. Whereas N mineralization is slow in tundra soils and concentrations of inorganic N are low, these soils have large stocks of both structural and soluble organic N. In light of these observations, kinetics of absorption of three amino acids (glycine, aspartic acid, and glutamic acid) were measured in dominant vascular plant species of the four major ecosystems types in arctic Alaska and compared with concentrations of free amino acids in soils. Absorption rates were measured on roots using ^1^4C-labeled substrates. Concentrations of free amino acids in soil were measured on water-extracted samples by high pressure liquid chromatography. All species had higher capacity (V\"m\"a\"x) for ammonium uptake (measured using methylamine as an ammonium analogue) than for any amino acid. However, at concentrations observed in the field, uptake rates estimated for amino acids were similar to (glycine) or less than (aspartic and glutamic acids) that for ammonium. On the basis of these comparisons, uptake rates of the three amino acids together may account for between 10 and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than the more slowly growing evergreen shrubs, suggesting that new growth created a sink that strongly influenced capacity for amino acid uptake. In general, ectomycorrhizal species had higher amino acid uptake than did non-mycorrhizal species. In species that were sampled from more than one community, amino acid uptake rates were highest in the community where a given amino acid was most abundant in the soil. The results indicate that, in arctic tundra, plants short-circuit the mineralization step of decomposition by directly absorbing amino acids. This implies that in the organic soils of these tundra systems (1) inorganic nitrogen is an inadequate measure of plant-available soil nitrogen, (2) mineralization rates underestimate nitrogen supply rates to plants, (3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource, and (4) by short-circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.

Alpine meadow and shrub are the main pasture types on the Tibetan Plateau, and they cover about 35% of the total land area. In order to understand the structural and functional aspects of the alpine ecosystem and to promote a sustainable animal production system, the Haibei Alpine Meadow Research Station was established in 1976. A series of intensive studies on ecosystem structure and function, including the energy flow and nutrient cycling of the ecosystem, were the main tasks during the first 10 years. Meanwhile, studies with 5 different grazing intensities on both summer and winter pasture have been conducted. In the early years of the 1990s, the research station started to focus its research work on global warming, biodiversity and sustainable animal production systems in pastoral areas. Various methods for improving degraded pasturelands have been developed in the region.

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were quantified over a growing season in a reconstructed tallgrass prairie and an ungrazed cool-season pasture. In both sites, hyphal lengths increased throughout the growing season. Peak external hyphal lengths were 111 m cm**3 of soil in the prairie and 81 m cm**3 of soil in the pasture. These hyphal lengths calculate to external hyphal dry weights of 457 micromol cm**3 and 339 micromol cm**3 of soil for prairie and pasture communities, respectively. The relationships among external hyphal length, root characteristics, soil P and soil moisture were also determined. Measures of gross root morphology [e.g., specific root length (SRL) and root mass] have a strong association with external hyphal length. Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured reduction in external hyphal length. Recovery for both the pasture and prairie occurred not by increasing SRL, but rather by increasing external hyphal length. The study suggests that growth is coordinated between VAM hyphae and root morphology, which in turn, are constrained by plant community composition and soil nutrient and moisture conditions