Ungrazed Plots Research Articles

Spatial variation in the demography and population dynamics of a perennial shrub (Atriplex vesicaria) under sheep grazing in semi‐arid Australian rangelands

Abstract Atriplex vesicaria Heward ex Benth. (Chenopodiaceae) is a widespread perennial shrub in southern Australia's chenopod rangelands but is sensitive to grazing. A detailed investigation of the demography and population dynamics of A. vesicaria under sheep grazing was conducted over 6.5 years at a range of sites across a typical paddock to assess the long‐term effects of grazing on the species and elucidate the mechanisms of population change under grazing. The effects of rainfall on recruitment and mortality were also examined. Six‐monthly censuses of all A. vesicaria individuals were conducted in permanent grazed and ungrazed plots at sites located across an 1100‐ha paddock. Grazing increased adult shrub mortality close to water and reduced recruitment over a broader area of the paddock, but seedling survival did not appear to be affected by grazing. As a result of these changes, the population declined on grazed plots up to 2200 m from water during the study, but the decline was greatest closer to water. The population was most dynamic at the sites furthest from the water point where it was unaffected by grazing because of the greater recruitment and mortality of young plants, but because these processes balanced out over time, population density was effectively unchanged by the end of the study. Although statistical models indicated that six‐monthly rainfall did not explain temporal variation in recruitment or mortality, rainfall nevertheless has a central role in both processes. In particular, longer periods of favourable rainfall and drought appear to have an important influence on recruitment and mortality, respectively, with heavy grazing during a drought period increasing mortality. Occasional shortages of seed or rains occurring during the warmer months when seed germination is limited possibly explain poor recruitment at sites unaffected by grazing following good rainfall.

Microbial growth rate measurements reveal that land-use abandonment promotes a fungal dominance of SOM decomposition in grazed Mediterranean ecosystems

The present study investigated the effects of land-use abandonment on the soil decomposer community of two grazed Mediterranean ecosystems (an annual grassland with scattered holm oaks and a low-density shrubland). To test the influence of grazing abandonment, a set of plots within each site were fenced and kept undisturbed during 4–5 years, during which above-ground plant community structure was monitored. After that, soil samples were collected from grazed and abandoned plots corresponding to the three different soil conditions: away from (“grass”) and below tree canopies (“oak”) within the annual grassland, and from the shrubland (“shrub”). Soil samples were split into two different layers (0–5 and 5–15 cm) and then analyzed for saprotrophic fungal (acetate into ergosterol incorporation) and bacterial (leucine incorporation) growth rates. Ergosterol content (as a fungal biomass estimator) and a standard set of soil chemistry variables were also measured. After 5 years of grazing exclusion, saprotrophic fungal growth rate clearly increased in both grass and oak surface layers whereas bacterial growth rate was not altered. This translated into significantly higher fungal-to-bacterial (F/B) growth rate ratios within the ungrazed plots. Similar trends were observed for the shrub soils after 4 years of exclusion. On the contrary, abandonment of grazing had negligible effects on the ergosterol content, as well as on the soil chemical variables (soil organic carbon, total N, C/N ratio, and pH), in all the three soil conditions assessed. These results indicated a shift toward a more fungal-dominated decomposer activity in soils following cessation of grazing and highlighted the sensitivity of the microbial growth rate parameters to changes associated with land use. Moreover, there were evidences of a faster fungal biomass turnover in the ungrazed plots, which would reflect an accelerated, though not bigger, fungal channel in soil organic matter mineralization.

Seasonal patterns of belowground biomass and productivity in mountain grasslands in the Pyrenees

Given the importance of root dynamics for soil C storage, the aim of this study was to analyze first the seasonal dynamics of belowground productivity and then the short-term effects of grazing exclosure on root dynamics in mountain grasslands. Soil coring and root ingrowth cores were used to assess belowground biomass (BGB) and productivity in grazed and ungrazed (grazing exclosures) plots in two mountain grasslands. Annual belowground production ranged from 472 to 590 g m−2, representing from 14 to 22% of the maximum root biomass measured over the year. Spring was the most productive season, accounting for more than 50% of total annual production, indicating that factors besides temperature may affect seasonal root dynamics. Although belowground production was much higher in the top 5 cm compared to deeper, the relative productivity rate (production-to-BGB ratio) and renewal time was higher at the subsurface (5–15 cm) layer. The contribution of the subsurface layer to total belowground production increased in spring, possibly due to occasional freezing events at the uppermost layer in the early growing season. The stronger seasonality in subsurface relative productivity rates may reflect depth-dependent changes in root characteristics and lifespan. Excluding grazing increased belowground productivity in summer, but its effects on BGB showed great variability between sites.

Effects of Long-Term Livestock Grazing and Habitat on Understory Vegetation

The herbaceous understory stratum contains most of the plant diversity in ponderosa pine (Pinus ponderosa P. & C. Lawson var. scopulorum Engelm.) forests of the American Southwest and provides critical food and habitat for many wildlife species. During the last century, this stratum has been affected by livestock grazing and by increased dominance of overstory trees. We sampled a unique grazing exclosure to examine the relative importance of long-term livestock grazing (grazed or ungrazed) and habitat (park or tree) on the understory community. We sampled 3 plots of 192 contiguous quadrats (each quadrat 0.5 m2) in each of the 4 treatment combinations, for a total of 2304 quadrats. Species-area curves were generated by aggregating quadrats into nonoverlapping areas at grain sizes of 0.5 to 576 m2. The effects of habitat and grazing on species density were evident at very different scales. Species density was higher in park than tree plots at scales ≤32 m2 but did not differ between habitats at larger scales. Species density differed minimally between grazed and ungrazed treatments at small grains, but grazed plots contained more species than ungrazed plots at larger grains. Grazing treatments differed at smaller grains (to 4–8 m2) than did habitats (to 32 m2), with respect to density of native species and graminoids. Grazed plots had more exotic species than ungrazed plots at all grain sizes, though few exotics were present. Twenty-two species were identified as indicator species associated with habitats and/or grazing treatments. Evaluations of plant community response to treatments would be improved by accounting for the grain at which data have been collected and analyzed and by identifying indicator species associated with various treatments. These data would enable more-informed conservation and management decisions.

Food selection by barnacle geese (Branta leucopsis) in an Arctic pre-breeding area

Detailed patterns of food selection by pre-breeding barnacle geese (Branta leucopsis) were investigated in Vårsolbukta, western Spitsbergen, Svalbard, in moss-dominated vegetation. This habitat is favoured by geese during the early Arctic spring when grass abundance is low. Grass is more profitable food than moss in terms of nutrient content and digestibility, and a five-fold higher proportion of grass in geese faeces compared with other vegetation indicated that geese selected grass in spite of its low availability compared with moss. As profitability may also depend on various properties that enhance searching efficiency, we studied the effects of grass tiller size, density and patchiness on goose selectivity in an experiment comparing ungrazed vegetation with vegetation where geese were allowed to feed for controlled periods of time. Large (two-leaved) tillers provided more biomass than small (one-leaved) tillers. The abundance of ungrazed large tillers relative to small tillers was lower in the grazed plots than in the ungrazed control plots, indicating that the geese preferred large tillers. Grass tiller density or spatial tiller distribution did not affect the degree of selectivity of geese for grass. Thus, we suggest that the feeding strategy of the geese in the early Arctic spring is primarily driven by the degree of tiller conspicuousness, as determined by tiller size. Furthermore, we suggest that an intense time allocation to feeding and an enhanced quality of grasses compared with mosses enabled geese to profit from feeding on the scarce grasses.

Assessment of Grazing Effect on Two Sub-Shrubs (Astragalus schottianus and Thymus sipyleus) Dominated Mountain Bozoğlan Grasslands in the Semi-Arid Central-Southern Anatolian Region of Turkey

In the central-southern Anatolian region of Turkey, vegetation of Mountain Bozoğlan grasslands has severely deteriorated, and two sub-shrubs (Astragalus schottianus and Thymus sipyleus) have become dominant species. We hypothesized that the spatial distribution of these dominant shrubs is closely associated with a long history of grazing, and these species might cause alteration of the grassland plant community. Therefore, our objectives were to determine: (1) the patterns in grassland vegetation; (2) how the dominant species contribute to these patterns; and (3) the differences of patterns between ungrazed and protected grassland areas. Plant cover in ungrazed plots was 6% higher than that of grazed plots. Total sub-shrub cover did not differ significantly, but grazing considerably reduced forbs (5% less) and grass cover (5% less). Though A. schottianus and T. sipyleus did not change between the treatments, Festuca valesiaca had greater cover in ungrazed areas (6%) than in that of grazed plots (3%). Grazing effect, soil properties and bare ground appeared to account for most of the variation on the established gradients. Even after 23 years, grazing and protection had little influence on pattern formation, and grazing have expanded dwarf-shrubs at the expense of forbs and grasses. Grazing neither increased nor decreased spatial heterogeneity, so the patterns in both treatments are similar to one another, and relatively stable assemblages of species were likely derived by high grazing pressure experienced before the exclosure establishment.

Refuge effects of Juncus effusus in grazed, subtropical wetland plant communities

Unpalatable plant species often act as biotic refuges by protecting neighboring plants from herbivores. This positive interaction can increase functional diversity in grazed ecosystems by protecting species sensitive to grazing. While many studies investigate pair-wise interactions between benefactors and beneficiaries, few show that these interactions result in community composition effects. We studied the effect of an unpalatable plant, Juncus effusus, on wetland plant communities in grazed and ungrazed plots. We tested the following predictions: (1) Juncus would increase plant functional diversity in grazed wetlands; (2) Juncus would have significant effects on community composition; and (3) the effects of Juncus on other species would change across the grazing gradient. We found that Juncus preserved functional diversity in grazed wetland communities by protecting species that decrease with grazing pressure. In multivariate analyses, grazing was the strongest driver of species composition but we found significant effects of Juncus on both vegetation change and species composition in grazed plots. Interactions with Juncus were facilitative in grazed plots and competitive in ungrazed plots, but did not vary along the grazing gradient (0.15–1.7 cows/ha). These results indicate that Juncus has significant community composition effects in grazed wetland plant communities. Understanding the effects of plant interactions such as these at the community level is essential to applying plant interactions to restoration or management.

Spring Grazing Impacts on the Vegetation of Reed Canarygrass–Invaded Wetlands

The Rainwater Basin region in Nebraska is critically important stopover habitat for spring waterfowl migrations, but the ability of these sites to produce sufficient food for migrating waterfowl is endangered by the invasion of reed canarygrass (Phalaris arundinacea L.). This species produces thick litter layers and abundant aboveground biomass, reducing germination and seedling survival of the annual plant species responsible for much of the seed production in the area. Cattle grazing often is used as a management tool in the Rainwater Basin to slow or reverse reed canarygrass invasion and to improve growing conditions for more desirable plant species. However, there has been little research on the impact of grazing on these factors. We studied the impacts of one-time, early-season (between April and June) cattle grazing on the abundance of reed canarygrass, bare ground, and litter. We hypothesized that cattle grazing would result in reduced reed canarygrass by the end of the 2-yr study, and that grazing would increase the abundance of bare ground and decrease the abundance of litter. Because grazing was expected to improve conditions for seed germination, we expected to find higher species richness in grazed areas. We found that grazing did not reduce the abundance of reed canarygrass, but the application of early-season grazing for two consecutive years did reduce litter and increase bare ground. Litter abundance decreased by 7.5% in ungrazed plots and litter increased by 8.6% in grazed plots. Bare ground in grazed plots increased 10.7% in grazed plots but decreased 1.2% in ungrazed plots. Species richness was not affected by grazing during this study. We concluded that grazing, as utilized in this study, is not sufficient to reduce reed canarygrass abundance, but can be used to mitigate some of the negative impacts of reed canarygrass invasion.

Dictyostelid cellular slime molds associated with grasslands of the central and western United States

Dictyostelid cellular slime molds (dictyostelids) associated with grassland ecosystems of the central and western United States were investigated at nine sites that included examples of the three major ecological types of grasslands (tall grass, mixed grass and short grass) generally recognized for the region. Samples of soil/humus collected from each site were examined with the Cavender method of isolating dictyostelids. For each of those six sites with well developed gallery forests present, an additional set of forest soil/humus samples was collected. A more intensive sampling effort was carried out at one site (Konza LTER) to assess the possible effects of burning and grazing on dictyostelid diversity and density. Twelve species of dictyostelids were recovered from grassland sites, whereas gallery forest sites yielded only nine species. Four cosmopolitan species (Dictyostelium giganteum, D. mucoroides, D. sphaerocephalum and Polysphondylium pallidum) were represented by the greatest densities of clones, with D. sphaerocephalum particularly common. The general pattern across all sites was that both species richness and density of dictyostelids decreased with decreasing precipitation. Samples collected from ungrazed grassland plots yielded higher numbers of both species and clones as compared to grazed plots, and the general pattern was for both values to increase as the interval between fires increased. For numbers of clones this correlation was statistically significant.

Plant and soil surface responses to a combination of shrub removal and grazing in a shrub-encroached woodland

Shrub encroachment into open woodland is a widespread phenomenon in semi-arid woodlands worldwide. Encroachment or woody thickening, is thought to result from overgrazing, changes in fire regimes and increased atmospheric carbon dioxide concentrations. Eighteen years after one-off shrub removal by ploughing we assessed the effects of four different land management systems resulting from two levels each of grazing (grazed, ungrazed) with and without ploughing, on the cover of landscape units, soil surface condition, diversity of understorey plants and density of shrubs. We recorded 2–7 times more patches under conventional conservation (unploughed-ungrazed) than the others treatments, and plant cover and diversity were greater on the two conservation (ungrazed) plots, irrespective of ploughing. Soils under shrubs and log mounds had greater indices of infiltration, stability and nutrients. Shrub density under the active pastoral (ploughed-grazed) treatment was two and a half times greater than that in other treatments, but results were not significant. The effects of different treatments on shrubs were largely species-specific. Overall, our results suggest that ploughing does not provide long-term control of encroaching shrubs.

California annual grass invaders: the drivers or passengers of change?

The dominance of invasive species is often assumed to reflect their competitive superiority over displaced native species. However, invasive species may be abundant because of their greater tolerance to anthropogenic impacts accompanying their introduction. Thus, invasive species can either be the drivers or passengers of change.We distinguish between these two possibilities in California grasslands currently dominated by Mediterranean annuals (exotics) and subjected to livestock grazing since European settlement. We focused on native annual grasses and forbs, an understudied species-rich component of the California flora, and Mediterranean annual grasses, currently dominant and among the first non-native plants introduced to the area.We established a field experiment with fenced and unfenced blocks in a cattle pasture. We measured concentrations of limiting resources (nitrogen, phosphorus, light and soil moisture) in monoculture plots as an index of competitive ability (i.e. R*). We then quantified grazing impacts on biomass and seed production in grazed vs. ungrazed monoculture plots. Finally, we measured biomass and seed production of each species competing in mixture plots, in the presence and absence of grazers.We found that native and exotic species did not differ in R* indices of competitive ability, i.e. concentrations of limiting resources in ungrazed native monoculture plots did not differ from concentrations in ungrazed exotic monoculture plots. By contrast, exotic annuals suffered less from grazing than native annuals, perhaps reflecting their longer evolutionary history with cattle grazing. Consistent with these results, native and exotic annuals were equally abundant in ungrazed mixtures, but exotic species overwhelmingly dominated grazed mixtures.Species able to draw down nitrogen and light to lower levels in monocultures (i.e. those with lower R* values) dominated biomass and seeds in mixed plots without grazers. However, R* did not predict the relative abundance of species in grazed plots. Moreover, the relative abundance of species in mixtures did not correlate with grazing impacts on their monocultures, implying that grazing alters inter-specific competitive dynamics.Synthesis. We demonstrate that the displacement of native annuals by Mediterranean annual grasses in California may largely have been driven by cattle grazing.

Stocking rate effects on metabolizable energy intake and grazing behaviour of Tan sheep in steppe grassland on the Loess Plateau of Northwest China

SUMMARYA simulated grazing system was set up in Huanxian County, Gansu province, on the Loess Plateau of Northwest China, involving the purchase of 18-month-old wether lambs in June of each year atc. 20 kg body weight (BW) and sale 6 months later atc. 35 kg BW. Three stocking rate (SR) treatments of 2·7, 5·3 and 8·7 wether lambs/ha were evaluated on geographically separated warm season (WS) and cold season (CS) paddocksc. 1 km apart; 3 years' data are reported (2004–2006). The metabolizable energy (ME) yield of the grazing system, calculated from the weight of animals fed and their weight gain, averaged 1·7, 3·3 and 4·7 GJ/ha/year for 2004, 2005 and 2006, respectively, in the WS and 0·9, 1·9, and 2·7 GJ/ha/year, respectively, in the CS for SR of 2·7, 5·3 and 8·7 animals/ha. Detailed grazing behaviour records were kept in order to elucidate intake dynamics. In these grazing systems, bite weight was typicallyc. 0·04 g/bite, lower than for temperate grazing systems at comparable herbage mass. A hypothesis for further study is proposed that this may relate to the distribution of a similar herbage mass over a greater sward height range in steppe vegetation than in temperate grass pasture. Sheep increased their bite rate (bites/min) and the number of steps/min at higher SR to compensate, such that intake/animal was reduced by not more than 10% with a threefold increase in SR. At higher SR, herbage ground cover on grazed plots was still lower than on ungrazed plots, 1 year after a 90-day summer grazing period or a 48-day winter grazing period. The significance of the findings for management of these systems is briefly discussed.

The effects of grazing, fire, nitrogen and water availability on nutritional quality of grass in semi-arid savanna, South Africa

The impacts of fire and grazing management on grass nutritional quality in semi-arid savannas may depend on inherent variation in rainfall and soil nutrient status. We examined the effects of grazing, fire, nitrogen addition, and watering treatments on the nutritional value of grass in a field experiment in the Northern Cape, South Africa. Surprisingly, water addition had no effect on grass quality and biomass in this dry savanna, perhaps because of the above-average rainfall during this study. Nitrogen addition resulted in increased levels of crude protein and biomass, as well as crude protein and gross energy in the early and late wet seasons, respectively. Grazing alone generally had little effect on grass quality, although grass in grazed plots had higher levels of crude protein (CP) in the late wet season and phosphorus (P) in the dry season. Grass biomass was greater in plots protected from grazing. There was more CP and P mass per unit area in fenced (ungrazed) plots during the wet season. Fire interacted with addition of nitrogen and grazing to increase grass quality. Soil nitrogen availability appears to be the most important factor affecting nutrient quality of grasses in the wet season in this semi-arid ecosystem.

Grazing intensity affected spatial patterns of vegetation and soil fertility in a desert steppe

Spatial heterogeneities of vegetation and soil can strongly affect ecological functions of ecosystems, particularly for arid and semi-arid ecosystems where vegetation has a patchy distribution and livestock grazing is one of the major land use types. However, little is known about the impact of grazing on spatial patterns of vegetation and soil, even though grazing has variously been shown to create, maintain or destroy those patterns. We studied how grazing intensity affected the spatial patterns of vegetation and soil fertility at scales ranging from 0.1 to 18.7 m in a desert steppe in Inner Mongolia, China. Increasing grazing intensity decreased the range of spatial autocorrelation of plant aboveground biomass at the fine scale (<2 m), indicating that vegetation patches were more fragmented under higher grazing pressure. Spatial heterogeneity of soil water content and organic C concentration decreased along the gradient of increasing grazing intensity at the fine scale. Light grazing increased soil NH 4 + availability and its overall variability. Soil NO 3 − only had significant cross-correlation with aboveground biomass in un-grazed plots at the fine scale, suggesting that the spatial correlation between plant and soil fertility was affected by grazing. Spatial patterns of studied variables did not respond to grazing intensities at a coarse scale (1–18 m). Our results demonstrated that the grazing intensity altered fine scale processes in this desert steppe and caused divergent responses of spatial distribution of vegetation and soil fertility.

Effects of grazing on soil physical properties and soil erodibility in semiarid grassland of the Northern Loess Plateau (China)

Grazing animals provide a livelihood for farmers, but they may also produce adverse environmental effects. We investigated whether grazing leads to deterioration of soil physical properties that subsequently increases topsoil erodibility. We sampled three sites (an ungrazed grassland, a continuously grazed grassland, and a track trampled by stock) on the northern Loess Plateau of China. The bulk density, water content, proportion of stable aggregates, infiltration rate, and resistance to scouring were determined for each soil sample. The results showed that the track had the highest soil bulk density and the lowest soil water content, proportion of stable aggregates, infiltration rate, and ability to resist scouring. The ungrazed plots had the best results for these parameters, in terms of reduced erosion. Soil bulk density and the proportion of stable aggregates differed significantly with depth beneath the track. However, the effect of depth on water content, infiltration rate, and the soil resistance to scouring was not significant at any sampling site. The ability of the soil to resist scouring was negatively correlated with the soil's bulk density and positively correlated with the soil's water content, infiltration rate, and proportion of stable aggregates. Thus, soil physical properties played an important role in determining soil erodibility. Grazing and trampling by livestock therefore appear to cause deterioration of soil physical properties and to increase soil erodibility.

Effects of Livestock Grazing on Grasshopper Abundance on a Native Rangeland in Montana

Livestock grazing can affect habitat quality for grasshoppers through effects on food and oviposition site availability, microclimate, and other factors. Because of this, some authors have suggested that grazing programs can be used to help manage pest grasshopper populations. In a 6-yr study, we controlled access of cattle to replicated experimental plots on an Agropyron spicatum/Poa sandbergii pasture to create consistent year-to-year differences in postgrazing plant cover, with resultant affects on microclimate. After sampling grasshoppers multiple times after grazing treatments each summer, we found evidence of between-treatment differences in grasshopper abundance for the entire assemblage during 4 of the 6 yr. Some species, including Melanoplus sanguinipes (perhaps the worse rangeland grasshopper pest in the western United States), tended to be more abundant on ungrazed plots, whereas Melanoplus gladstoni often had greater densities on heavily-grazed plots. The effect of grazing on grasshopper densities in this study was lower in magnitude and less consistent among years than in a study we conducted simultaneously at a nearby site where the vegetation was dominated by the exotic species crested wheatgrass (Agropyron cristatum). Our results generally support proposals that grazing could be used to reduce pest grasshopper densities, although the effectiveness of a particular grazing scheme may vary among sites, years, and grasshopper and vegetation assemblages.

THE EFFECTS OF EXCLOSURE ON PLANTS IN THE SEMI-ARID RANGELAND OF NORTH KHORASAN PROVINCE, IRAN

Abstract Over the last fifty years, most of the semi-arid rangeland in Iran has been converted to cropland without an equivalent reduction in grazing animals. This shift has led to heavy grazing pressure on rangeland vegetation. The present study has been conducted in Sisab Research Station in the North Khorasan Province to evaluate effects of long-term grazing exclosures on biodiversity of range plants. The study was initiated in May 2008 using three transects within the exclosure and another three transects near exclosure, each with 100m length in the vicinity of exclosure has been established to determine differences in plant composition between areas that have not been grazed in 22 years with neighboring grazed plant communities. The sampling method was randomized systematic one comprised of 10 plots, each of 1m2 in area. A total of 53 plant species were identified in the study area with the ungrazed plots containing 18 plants more than the grazed plots. The major species were Festuca ovina, Centurea depressa, Stachys turkamanica, Stipa barbata, Astragalus sp and Phelomis cancellata. Grazing impacts on forbs were more pronounced than for grasses and shrubs. Based on Jaccard’s index, there was only a 45% similarity of plant species between the two treatments. Our study led to four generalizations about the current grazing regime and long-term exclosures in the semi-arid rangeland around the study area: (1) exclosures will increase species richness, (2) heavy grazing may have removed some plant species, (3) complete protection from grazing for a prolonged period of time after a long history of grazing disturbance may not lead to an increase in desirable plant species with a concomitant improvement in range condition, and (4) research needs to be conducted to determine how these rangelands can be improved.

Short‐term impact of grazing by sheep on vegetation dynamics in a newly created salt‐marsh site

AbstractThe effects of grazing by sheep at a stocking density of c. 1 sheep ha−1 from August to April on total vegetation cover, diversity of plant species, above‐ground plant biomass and concentration of crude protein, neutral‐detergent fibre and acid‐detergent fibre of salt‐marsh vegetation was assessed over a 2‐year period on a newly created salt‐marsh area on the Belgian coast. Assessments were made in both grazed and ungrazed areas in four elevation zones: low, intermediate, high and floodmark zone. The cover of dominant salt‐marsh species (Salicornia europaea, Limonium vulgare and Elymus athericus), plant diversity, plant biomass and chemical composition of herbage were compared between grazed and ungrazed plots, and between 2005 (initial state) and 2007 (after a 2‐year grazing period) using paired t‐tests. Grazing by sheep maintained plant diversity in the high zone, whereas diversity decreased in the ungrazed plots. Grazing had no apparent effect on total vegetation cover and composition of the vegetation. Concentration of crude protein in herbage increased and that of neutral‐detergent fibre decreased in the high and intermediate zones. Expansion of the dominant species of the high zone, E. athericus, was not affected by the stocking density used in the study.

Effects of large herbivore exclusion on southern Mongolian desert steppes

Large areas of Mongolia have been subject to semi-nomadic pastoralism for centuries, but recently increasing livestock numbers have raised concerns about possible degradation effects. We assessed the impact of grazing in southern Mongolian desert steppes (mean precipitation <150 mm) over eight years in order to capture the high interannual variation, as well as possible medium-term trends in the management regime. We assessed whether the impact of varying precipitation levels (coefficient of variation >30%) exceeds the effects of grazing on vegetation composition, species richness, plant flowering activity, biomass production, and soil nutrient content. The data presented here originate from a set of nine fenced exclosures established in 1999. Plant community composition and species richness showed interannual changes over years of varying precipitation levels: mean species numbers on ungrazed plots ranged between 8 and 19 per 100 m 2. Grazing effects were apparent but light, as grazed plots had, on average, 1–2 species fewer than ungrazed plots. There was also no evidence of directed succession under grazing exclusion. Mean numbers of flowering species on ungrazed plots ranged between 2 and 14 over the years while numbers on grazed plots were slightly lower (range 1–12). In one moist year, mean biomass production in previously ungrazed plots was higher than in previously grazed plots (ca. 800 kg/ha vs. 250 kg/ha), but differences in a subsequent drier year were negligible. This was paralleled by soil phosphorus content, which tended to be higher inside the exclosures. The overriding effects of precipitation variability are in line with the non-equilibrium theory of rangeland science and imply that the risk of direct degradation is low in southern Mongolian drylands. However, small but statistically significant differences between grazed and ungrazed sites suggest that herbivore impact is apparent and can alter plant reproduction and, ultimately, soil conditions and productivity.

Woodland regeneration on grazed former arable land: A question of tolerance, defence or protection?

Ecological restoration of native woodlands and wooded pastures on former agricultural land is an important topic in modern conservation practice. The introduction of large herbivores is increasingly used to achieve these aims. We investigated how grazing, resistance traits of plants (concerning herbivory) and associational resistance interact and affect the establishment pattern of woody species on abandoned arable land (N-Belgium, W-Europe). In these early successional tree assemblages, we tested whether grazing increased or decreased spatial heterogeneity, which is supposed to be a crucial factor for biodiversity. With repeated measurements, 2-3 and 5-7 years after the cessation of agricultural use, we sampled 87 grazed and 56 ungrazed plots (314 m 2) in 14 sampling areas (former arable land) on nutrient rich, (sandy) loam soils. We recorded established tree frequencies, related to grazing, time, resistance traits and unpalatable/spiny vegetation cover in the herb and low shrub layer. We investigated horizontal and vertical heterogeneity using variances in establishment frequencies and variances in frequencies of trees that were able to grow beyond the browse line, respectively. We found massive colonisation of grazing tolerant and resistant woody species in early successional stages. Grazing decreased frequencies and height of the most abundant tolerant species (mainly Salix caprea L.). After 5-7 years, frequencies of defensive and tolerant species were equal, but the former (mainly Betula pendula Roth) were able to grow beyond the browse line. When the cover of unpalatable/spiny vegetation was high enough (>60% of plot size), it also provided suitable nurse sites for tolerant species to grow out. In early assemblages, grazing increased horizontal and vertical heterogeneity, resulting in intermediate successional stages. In the long-term, the mechanism of associational resistance will also allow non-resistant and tolerant species to grow beyond the browse line and promote forest succession and the order of species establishment and replacement.