Long-term Grazing Exclosure Research Articles

High resistance to non-native plant invasion exhibited in a maritime alpine environment

AbstractMost alpine ecosystems are subject to non-native species invasion as climate warms and human disturbance increases. Therefore, it is important to identify the main barriers and facilitators of alpine plant invasions. While there is much research in continental alpine areas, there is limited research in maritime environments, which have distinctive biological, edaphic and geomorphologic features due to the scarcity of snow in winter. Here, we examine the contribution of anthropogenic and biotic factors on non-native plant invasions in the alpine areas of lutruwita (Tasmania), Australia. Specifically, we test whether disturbances associated with roads and livestock grazing facilitate non-native plant invasion and if biotic resistance from native plants and wild grazing animals inhibits such invasion. We used floristic data from: a statewide database; data from long-term grazing exclosures, and data from paired quadrats on roadsides and natural vegetation. Our results showed that non-native plants were associated with roadside disturbance and livestock grazing, with the latter having a legacy effect of 50 years. The persistent effect of stock grazing was evident in the exclusion experiment monitored over 30 years, where non-native plants occurred sporadically in time and space. In contrast, we found that the presence of wild grazing mammals, complete vegetation cover and high native richness, restricted non-native plants, emphasizing the importance of biotic resistance in controlling non-native plant invasions. These results indicate that livestock removal and road closure could be effective in reversing non-native plant invasion in this and other areas with wild vertebrate herbivores and high plant cover.

Long-term mammal herbivory on arthropod assemblages at Kruger National Park, South Africa.

Protected savannas are essential reserves for biological diversity, including endangered arthropod species, however, extreme grazing by mammals has cascading impacts on the communities and disrupts the functioning of these ecosystems globally. The current study assessed the abundance, species richness and composition of arthropods at the long-term grazing exclosures of Kruger National Park, South Africa. Pitfall traps and active searches were used to sample arthropods at the ungrazed, moderately, and heavily grazed exclosures. We found that Hymenoptera, Coleoptera, Diptera and Araneae were the most abundant orders of arthropods sampled. The abundance of multi-taxon, Diptera and Hymenoptera was significantly different between exclosures. In contrast, Coleoptera had high numbers of morphospecies compared to Hymenoptera, Araneae and Diptera. Species richness for multi-taxon, Diptera and Hymenoptera was significantly high at the heavily grazed compared to moderately grazed and ungrazed exclosures. Up to 22.2%, 41.2%, and 44.4% of the morphospecies were unique to the ungrazed, moderately and heavily grazed exclosures, respectively. A high proportion of morphospecies shared between exclosures were Coleoptera (41.0%) and Hymenoptera (38.5%) compared to Diptera and Araneae with less than 5% recorded across exclosures. Thus, morphospecies within the least abundant orders, namely Diptera and Araneae, were represented by singletons that were unique to particular exclosures compared to the most abundant arthropod orders (e.g., Coleoptera and Hymenoptera). We conclude that long-term mammal grazing enhances species richness and niche composition together with sparse and unique arthropods in the protected savannas. Therefore, managed grazing regimes can serve as a tool for maintaining the integrity of the protected savannas.

Grazing effects on shrub-induced resource islands and herbaceous vegetation heterogeneity in sagebrush-steppe communities

Spatial heterogeneity in plant communities promotes coexistence and diversity by providing a variety of niches for different species. In shrub-steppe communities, shrubs create distinct microsites under their canopies (canopy microsites) compared to areas between their canopies (interspace microsites). This creates spatial heterogeneity in soil nutrient availability and herbaceous vegetation. Grazing can influence spatial heterogeneity, but the effect of grazing on shrub-induced spatial heterogeneity is largely unknown. We investigated the long-term (+80 yrs.) effects of moderate grazing by cattle on sagebrush ( Artemisia )-induced spatial heterogeneity in soil nutrients, herbaceous vegetation, and ground cover in sagebrush-bunchgrass steppe communities at eight sites in southeastern Oregon. Each site consisted of a long-term grazing exclosure and an adjacent grazed area. Almost all measured herbaceous vegetation (cover, density, diversity, and evenness) and ground cover variables differed between canopy and interspace microsites. Grazing did not influence the effects of microsites on most measured herbaceous vegetation characteristics and ground cover variables. Available soil nutrients were not influenced by grazing, but the majority differed between microsites. The limited effect of moderate grazing on shrub-induced spatial heterogeneity provides evidence that sagebrush exerts a strong influence on patterns of soil nutrients and herbaceous vegetation in sagebrush-bunchgrass communities. These results also demonstrate that moderate grazing by cattle can be compatible with maintaining spatial heterogeneity within shrub-steppe communities. In these ecosystems, maintaining and restoring shrubs is critical to promoting spatial heterogeneity, thereby encouraging coexistence and diversity.

Newly assimilated carbon allocation in grassland communities under different grazing enclosure times

The flow of photosynthesized carbon (C) from plants to soil and its loss from the respiration in the soil-plant system were measured via in situ 13CO2 pulse labeling pasture in overgrazed and long-term grazing exclosures (where grazing was not permitted for 5, 10, and 30 years). During a 30-day chase period, the allocation of newly assimilated C in the shoots, roots, and soil of grazing-excluded grasslands was higher than that in overgrazed grasslands. But the newly assimilated C allocated to shoots and soil respiration in overgrazed grasslands was significantly higher than that in grazing-excluded grasslands. Thirty days after labeling, 35% and 40% of the newly assimilated C in overgrazed grasslands were lost through shoots and soil respiration, respectively. The shoot 13C recovery was 32% in the 30 years 30 days after labeling, which was significantly higher than that obtained at the 5 and 10 years exclosure sites. Root and soil 13C recoveries in the 5- and 10-year exclosures were higher than those at the 30-year sites. In the grazing-excluded grasslands, the amount of 13C in soil organic C peaked 3 days after 13CO2 exposure and became relatively stable after 19 days of assimilation. The mean C residence time for non-structural C used in the soil respiration under overgrazing (10 days) was shorter than that under 5- (20 days) and 10- (17 days) or 30-year exclosures (14 days). Compared to overgrazing, grazing exclosure, especially moderate grazing exclosure, led to more allocation of newly assimilated C in the plant-soil system but fewer allocation in shoots and soil respiration.

On China’s Qinghai-Tibetan Plateau, duration of grazing exclosure alters R:S ratio, root morphology and attending root biomass

Unsustainable human actions and climate change have threatened global grasslands and lead to biodiversity loss and compromised ecosystem functions. Widely considered to be an effective approach to restore degraded grasslands, grazing exclosure (GE) likely affects biomass allocation and root morphology in the long-term, although their impacts are yet to be documented. The objective of this study was to investigate the effects of grazing exclosure durations of 5, 22, and 39 years (GE5, GE22, GE39, respectively) on root:shoot biomass allocation (R:S ratio) and root morphology — including root average diameter (RAD), root density (RD), specific root length (SRL), specific root area (SRA), and root length density (RLD) — in an alpine grassland on the Qinghai-Tibetan Plateau. Seasonal grazing (SG) land served as a control. In the 0−0.30 m soil layer, the R:S ratios of the SG (1.27) and GE5 sites (0.90) were significantly lower than that of the GE39 site (2.14). However, there was no difference between the GE22 (1.83) and GE39 sites. The R:S ratio of the 0−0.15 m soil layer showed an increasing trend with duration of grazing exclosure and had a significantly greater value under GE39 (1.89) than under GE5 (0.76). However, the R:S ratios for the 0.15−0.30 m soil layer (0.14, 0.28 and 0.25 for GE5, GE22, and GE39, respectively) showed no significant changes with grazing exclosure duration. In the 0−0.30 m soil layer across sampled sites, RAD remained constant, RD and RLD exhibited an increasing trend, while SRA and SRL displayed a slightly decreasing trend with grazing exclosure duration. Below-ground biomass (BGB) was most strongly related to RLD across all sites; to RD and RLD at the GE5 and GE22 sites, and to RAD and RLD at the GE39 site. Both R:S ratio and root morphology were mainly influenced by soil nitrate nitrogen. Although long-term grazing exclosure enhanced BGB and R:S ratio, soil nutrients declined with increasing exclosure duration. Therefore, to achieve improved ecosystem functions, grazing exclosure should be used with caution in this area or other similar regions around the world.

Long-term grazing exclosure: implications on water erosion and soil physicochemical properties (case study: Bozdaghin rangelands, North Khorasan, Iran)

Grazing exclosure (GE) is used to improve rangelands in the dry area of the world, so it is important to investigate its effects on soil physicochemical and erodibility properties. This study was conducted to evaluate the effect of long-term GE on the physicochemical and erodibility properties of soil and vegetation as compared with open grazing (OG) areas in Bozdaghi region of North Khorasan province, Iran. Soil and vegetation data were sampled from two sites in the grazing exclosure and the open grazing in early autumn in a randomized complete block design with three replications. Therefore, in each area (GE and OG), three transects of 500-m length and 200-m intervals) were set up. Along each transect, five soil samples were taken at the depths of 0-15 and 15-30 cm in a random-systematic method (15 soil samples in each area) and transferred to the laboratory. In the laboratory, some soil physicochemical properties such as saturation percentage, soil texture (clay, silt, and sand), bulk density, porosity, percentage of soil organic matter (SOM), total nitrogen (TN), exchangeable sodium, potassium, pH, and electrical conductivity (EC) were measured and Soil Erodibility Index (SEI) was calculated by using the modified clay ratio relation. After installing the transects (with a length of 50 m) in three blocks in each area, the parameters of species composition and canopy cover were investigated by the linear-contact method. Data analysis was performed by using an independent t test on Statistical Package for Social Sciences (SPSS) v. 23 software. The results indicate that the application of GE in the Bozdaghi area has a significant positive effect on soil physicochemical properties (P < 0.05). Also, the application of GE has increased the amount of SOM and TN in the surface layer of the soil by about twice as much as the OG pastures. Due to these changes, during 20 years in the GE area, the effects of GE can be positively assessed. According to the obtained results, especially the positive changes in vegetation and the soil physicochemical and erodibility properties, it is recommended to execute the GE plan in the study area.

Degraded or Just Dusty? Examining Ecological Change in Arid Lands

AbstractThe ecological history of rangelands is often presented as a tale of devastation, where fragile drylands are irreversibly degraded through inappropriate land use. However, there is confusion about how to recognize and measure degradation, especially in low-productivity environments characterized by extreme natural variability and where abrupt and comprehensive management upheavals preclude benchmarks. These issues have important consequences for rangeland management programs, which are typically founded on presumptions of substantial and ongoing degradation from former “natural” states. We explore complementary approaches to critically assess degradation: the historical record, long-term grazing exclosures, surveys for potentially rare and sensitive plant species, and assessment of water-remote areas in relation to rare plant occurrence. Employing these approaches in inland Australia, we show that prevailing paradigms have become entrenched despite being inconsistent with empirical evidence. Our methodology can be applied to drylands with abrupt changes in management and contentious ecological narratives.

Comparing the effects of continuous grazing and long term exclosure on floristic composition and plant diversity in rangeland ecosystems of Saral, Iran

Continuous grazing as the most common land us in Kurdistan rangeland is practiced wildly. Along with continuous grazing, long term exclosure has been implemented in research site in Saral. The main objective of this study was to investigate the changes in plant diversity, floristic composition and plant groups of the major life forms in response to 40 years of continuous grazing and long terrm exclosure in Saral rangelands, Iran. This research was conducted in cold semi-arid rangeland of Baharestan located in Saral’s agricultural and natural resources station (including grazed and excluded sites). In total, six transects were sampled in both grazed and excluded sites and ten plots were randomly placed per transect to measure vegetation and plant characteristics. Sampling was carried out in late May to early June (peak biomass) in 2017. T test was used to compare the means of total plant material, and Shannon’s diversity index was used based on the total cover percentage in both grazed and exclosure sites. Our results indicated that long-term grazing exclosure significantly increased canopy covers and species richness of annuals, perennial grasses, perennial forbs and shrubs. Conversely, continuous grazing caused some species to disappear namely Ferula haussknechtii and Prangos ferulacea. There was statistically a significant difference between litter and total plant material in experimental sites (p < 0.001). Apart from palatability, exclosure increased floristic diversity by 17.8%. Our findings elucidated that exclosure in degraded vegetation is an effective strategy and management policy to recover and restore missing vegetation cover.

Effects of enclosing on soil microbial community diversity and soil stoichiometric characteristics in a degraded alpine meadow

Enclosing is an effective rehabilitation measure for degraded pastures that mimics natural recovery of vegetation. To examine the interaction between biotic and abiotic in lightly degraded and enclosing grasslands is helpful for a clear understanding of the structure and function of grassland ecosystem. In this study, soil microbial community and soil stoichiometric characteristics in lightly degraded and 10-year enclosing alpine meadows were studied by high-throughput sequencing and Biolog-Eco methods. The results showed that compared with lightly degraded grassland, the concentration of soil NH4+-N in the enclosing grassland increased significantly, while total K (TK) dramatically decreased. There was no obvious variation in soil total organic carbon (TOC), total N (TN), total P (TP), NO3--N, available P (AP), available K (AK), microbial biomass C (MBC) and microbial biomass N (MBN). The soil microbial biomass C/N was significantly enhanced. The carbon metabolic capabilities of soil microbes in different soil layers of alpine mea-dow were obviously increased with the prolonged incubation time, but there was no significant difference between lightly degraded and enclosed meadows. The OTUs of soil bacteria was significantly higher than that of fungi in alpine meadow. The microbial similarity between lightly degraded and fencing grasslands was 27.0%-32.7%. Enclosing significantly increased the fungal relative richness of Ascomycota, Zygomycetes and Chytridiomycota, while simultaneously decreased the rela-tive abundance of Basidiomycetes. Compared with lightly degraded meadow, the bacterial relative richness of Acidobacteria significantly decreased in enclosing meadow. The community composition of soil fungi and bacteria greatly varied among different soil layers. There was significant difference of fungal community composition in the upper soils between lightly degraded and enclosed grassland. The soil bacterial community diversity was greatly affected by soil TN and AK, while the fungal community diversity was significantly affected by plant aboveground biomass. Soil AK produced great influence on soil microbial carbon source utilization capacity. Generally, long-term grazing exclosure had no significant effects on soil nutrients and soil microbial community diversity of lightly degraded grassland and thus would waste the pasture resources, whereas appropriate grazing could maintain the sustainable utilization of grassland.

Fire increases the reproduction of the dominant grass Brachypodium retusum and Mediterranean steppe diversity in a combined burning and grazing experiment

AbstractAimsWe assessed what the effects of fire and grazing are on vegetative growth and sexual reproduction of Brachypodium retusum, the dominant species of Mediterranean Thero‐Brachypodietea grasslands (“steppes”) and on species composition of the associated plant community.Location“La Crau”, southeastern France.MethodsWe set up a split‐plot experiment testing the effects of fire season (winter, summer) and grazing on plant cover, inflorescence number, seed set and germination of Brachypodium retusum in traditionally grazed grasslands. We further analyzed plant species composition and diversity in vegetation relevés. The same fire treatments were tested in a second experiment in long‐term grazing exclosures.ResultsBrachypodium retusum showed a rapid post‐fire recovery, but other species of the associated plant community, in particular annuals, recovered as fast or even faster. Fire increased Brachypodium retusum inflorescence production and seed set per inflorescence and this effect was stronger in the summer fire treatment. At the community level, fire significantly increased species richness, evenness and Bray–Curtis dissimilarity in the second post‐fire season and again, the summer fire effect was stronger. Grazing exclusion for two seasons had only a small effect on Brachypodium retusum and the associated plant community. The effect of both fire treatments on Brachypodium retusum was similar in long‐term exclosures. In these exclosures, fire resulted in a community shift towards the species composition of traditionally grazed steppes.ConclusionsThe concomitant positive effects of experimental burning on Brachypodium retusum reproduction and on plant diversity of Mediterranean steppe vegetation suggest that the system is adapted to fire as an important driver of community composition. Annual species surviving as seeds are as successful in post‐fire recovery as perennial resprouters. Prescribed burning may be an alternative strategy to restore community structure in abandoned (ungrazed) steppes showing a decline in typical grassland species after several years of grazing abandonment.

Grazing enhances belowground carbon allocation, microbial biomass, and soil carbon in a subtropical grassland

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant-derived SOC following microbial decomposition. Here, we used insitu isotope pulse-chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long-term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4-dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.

Effects of Feral Horse Herds on Rangeland Plant Communities across a Precipitation Gradient

Feral horses are widespread in the western United States, with the majority of feral horse herds found in the Great Basin. There is a federal mandate to manage these herds in order to maintain “ecological balance”; however, understanding of the specific effects of feral horse grazing on rangeland plant communities in this region is incomplete. To address this research gap, we utilized long-term grazing exclosures and fenceline contrasts to evaluate the impacts of feral horses on several plant community variables (diversity, richness, dominance, and biomass) and species composition. Because the effects of grazing can vary with site precipitation and productivity, we selected 5 sites from 4 different rangeland types (Great Basin Desert, Colorado Plateau, Rocky Mountain grassland, and mixed grass prairie) that spanned a mean annual precipitation gradient of 229 to 413 mm. Our results did not reveal a significant effect of feral horse grazing on plant community composition, species richness, diversity, evenness, or dominance. In contrast, total aboveground herbaceous biomass and grass biomass were significantly reduced with feral horse grazing, but these effects did not vary with mean annual precipitation. Our results suggest that, at least at the sites we studied, feral horses have affected the plant community by reducing herbaceous biomass but have not caused plant community shifts. Additional multisite studies, preferably with standardized exclosures and larger sample sizes, would increase our understanding of feral horse grazing effects and inform management of feral horse herds in the western United States.

Community responses of arthropods to a range of traditional and manipulated grazing in shortgrass steppe.

Responses of plants to grazing are better understood, and more predictable, than those of consumers in North American grasslands. In 2003, we began a large-scale, replicated experiment that examined the effects of grazing on three important arthropod groups-beetles, spiders, and grasshoppers-in shortgrass steppe of north-central Colorado. We investigated whether modifications of the intensity and seasonality of livestock grazing alter the structure and diversity of macroarthropod communities compared with traditional grazing practices. Treatments represented a gradient of grazing intensity by cattle and native herbivores: long-term grazing exclosures; moderate summer grazing (the traditional regime); intensive spring grazing; intensive summer grazing; and moderately summer-grazed pastures also inhabited by black-tailed prairie dogs (Cynomys ludovicianus Ord). Beetles and spiders were the most common groups captured, comprising 60% and 21%, respectively, of 4,378 total pitfall captures. Grasshopper counts were generally low, with 3,799 individuals observed and densities <4 m(-2). Two years after treatments were applied, vegetation structure differed among grazing treatments, responding not only to long-term grazing conditions, but also to the short-term, more-intensive grazing manipulations. In response, arthropods were, in general, relatively insensitive to these grazing-induced structural changes. However, species-level analyses of one group (Tenebrionidae) revealed both positive and negative effects of grazing treatments on beetle richness and activity-density. Importantly, these responses to grazing were more pronounced in a year when spring-summer rainfall was low, suggesting that both grazing and precipitation-which together may create the greatest heterogeneity in vegetation structure-are drivers of consumer responses in this system.

How old is the human footprint in the world's largest alpine ecosystem? A review of multiproxy records from the Tibetan Plateau from the ecologists' viewpoint

The age at which a human environment was made is strongly debated in Quaternary science. The perception of the human footprint requires an understanding of the contrast between the present cultural environment and the natural vegetation. This is especially true for high altitude ecosystems, such as the world's largest alpine ecosystem, the 450,000 km2 of golf-course-like pastures dominating the southeastern Tibetan Plateau. This ecosystem is widely considered to be natural, but only because of the unawareness of the effects of grazing management and the incorrect assessment of human signals in palaeo proxies. Here we posit the hypothesis that this ecosystem is a human-induced replacement. To test our hypothesis against defined a priori criteria, we 1) monitored vegetation in grazing exclosures; 2) produced floristically complete vegetation records; 3) compared the vegetation records with data of the nearest climate stations; 4) surveyed forest and experimental reforestation trials with endemic tree species; 5) analyzed pollen and re-evaluated published pollen diagrams; and 6) integrated these results with palaeopedological and anthracological results of previous studies. The results of long-term grazing exclosure experiments, the prevalence of grazing-adapted plant functional types, the occurrence of isolated vigorous forests in “alpine” pastures, and the successful reforestation trials in presently treeless pastures together suggest that dominant pastures replaced forests in the montane belt and tall grassland in the alpine belt. The spatial and temporal coincidence of palaeosols and archaeological sites with tree species charcoal, the decline in forest pollen during the mid-Holocene climatic optimum, and the first appearance of human indicator pollen are most parsimoniously explained by the early presence of foragers and pastoralists. The onset of pastoralism in the Tibetan Highlands is presumed to date from the 8th millennium BP.

Islands of Fertility: A Byproduct of Grazing?

Empirical work in arid shrubland systems has documented a distinct spatial pattern of soil nutrient distribution, with higher concentrations of nutrients under shrub canopies compared to bare ground interspaces between shrubs. This “fertile island” pattern is considered characteristic of arid ecosystems. However, recent work at a desertified shrubland site reported no island of fertility pattern for nitrogen inside a long-term grazing exclosure despite a strong fertile island pattern outside the exclosure, leading to a hypothesis that the pattern may be associated with grazing. In this paper we examine this hypothesis in two ways. First, we collected soil nitrogen data inside and outside two long-term livestock exclosures. Second, we examined published work on the fertile island pattern for nitrogen, phosphorus and carbon, and its association with livestock grazing. At the field sites, there was a strong fertile island pattern for soil nitrogen in the grazed area outside each exclosure but no fertile island pattern in the ungrazed area inside the fence. From the literature, we identified 49 studies, which represent 99 cases that examine differences in soil nutrients under and between shrubs (that is, tests of the fertile island pattern). In 92 cases, fertile islands were present for at least one of the nutrients. All cases where livestock grazing was present exhibited fertile island patterns for nitrogen and most did so for carbon and phosphorus. In contrast, fertile islands were less consistent where livestock grazing was absent; all nine cases that had been ungrazed for more than 30 years did not exhibit fertile islands for nitrogen. Our empirical data and literature review suggest that the fertile island pattern may be an artifact of grazing rather than an inherent property of arid shrublands. Further, we suggest that the grazing history of a site needs to be explicitly considered when documenting spatial patterns of soil nutrients in arid systems. This has significant implications for our understanding of soil nutrient dynamics in arid systems, the management of rangelands, and the potential for reversal of desertification.

Carbon sequestration and biodiversity restoration potential of semi-arid mulga lands of Australia interpreted from long-term grazing exclosures

Limited data regarding soil carbon (C) sequestration potential and biosequestration potential in arid and semi-arid environments is an impediment to appropriate policy formulation directed at greenhouse gas abatement. This paper assesses the terrestrial C biosequestration and biodiversity restoration potential of the semi-arid mulga lands of eastern Australia by measuring above and below ground C, and by making floristic biodiversity assessments in old grazing exclosures. Grazing exclusion increased water infiltration rates and water retention capacity in the soil. Exclosures also had increased herbaceous cover and decreased bare ground. Biodiversity benefits included higher species richness and increased abundance of native grasses, many of which have become locally rare under increased grazing pressure. The study indicates that in the absence of grazing, soil and above ground biomass, when combined, has potential carbon sequestration rates of between 0.92 and 1.1 t CO 2-e ha −1 year −1 over a period of approximately 40 years. The contribution to these figures from soil C sequestration is approximately 0.18 t CO 2-e ha −1 year −1, with above ground biomass contributing an additional 0.73–0.91 t CO 2-e ha −1 year −1. If 50% of eastern Australia's mulga lands (half of 25.4 million ha) were managed for C sequestration and biodiversity through the control of all herbivores, then annual sequestration rates could reach between 11.6 and 14 Mt CO 2-e year −1 which is between 2 and 2.5% of Australia's annual emissions. The potential to sequester carbon and improve biodiversity outcomes in extensive semi arid grazing lands will require significant policy shifts to encourage and reward necessary land use change.

Grazing changes topography-controlled topsoil properties and their interaction on different spatial scales in a semi-arid grassland of Inner Mongolia, P.R. China

Semiarid steppe ecosystems account for large terrestrial areas and are considered as large carbon (C) sinks. However, fundamental information on topsoil sensitivity to grazing is lacking across different spatial scales including the effects of topography. Our interdisciplinary approach considering soil chemical, physical, and vegetation properties included investigations on pit scale (square-metre scale), plot scale (hectare scale), and the scale of a landscape section (several hectares). Five different sites, representing a grazing intensity gradient, ranging from a long-term grazing exclosure to a heavily grazed site were used. On the pit scale, data about aggregate size distribution, quantity of different soil organic carbon (SOC) pools, SOC mineralisation, hydraulic conductivity and shear strength was available for topsoil samples from representative soil profiles. Spatial variability of topographical parameters, topsoil texture, bulk density, SOC, water repellency, and vegetation cover was analysed on the basis of regular, orthogonal grids in differently grazed treatments by using two different grid sizes on the plot scale and landscape section. On the pit scale, intensive grazing clearly decreased soil aggregation and the amount of fresh, litter-like particulate organic matter (POM). The weak aggregation in combination with animal trampling led to an enhanced mineralisation of SOC, higher topsoil bulk densities, lower infiltration rates, and subsequently to a higher risk of soil erosion. On the plot scale, the effects of soil structure disruption due to grazing are enhanced by the degradation of vegetation patches and resulted in a texture-controlled wettability of the soil surface. In contrast, topsoils of grazing exclosures were characterised by advantageous mechanical topsoil characteristics and SOC-controlled wettability due to higher POM contents. A combined geostatistical and General Linear Model approach identified topography as the fundamental factor creating the spatial distribution of texture fractions and related soil parameters on the scale of a landscape section. Grazing strongly interfered with the topography-controlled particle relocation processes in the landscape and showed strongest effects on the aboveground biomass production and biomass-related soil properties like SOC stocks. We conclude that interdisciplinary multi-scale analyses are essential (i) to differentiate between topography- and grazing-controlled spatial patterns of topsoil and vegetation properties, and (ii) to identify the main grazing-sensitive processes on small scales that are interacting with the spatial distribution and relocation processes on larger scales.

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.

Effects of grazing and rainfall variability on root and shoot decomposition in a semi-arid grassland

Overgrazing increasingly affects large areas of Inner Mongolian semi-arid grasslands. Consequences for ecosystem functions and, in particular, for the decomposition as a key process of ecosystem carbon (C) and nitrogen (N) cycling are still unclear. We studied the effects of grazing on shoot and root decomposition with the litter bag method in a long-term grazing exclosure (UG79), a moderate winter grazed (WG) and a long-term heavily grazed site (HG). We separated the effects of local environmental factors and litter quality as altered by grazing. Growing seasons of average and very low precipitation allowed us to study the effect of inter annual rainfall variability on decomposition. Grazing-induced differences in environmental factors of the three studied grassland sites had no effect on decay rates of shoot and root dry mass. Also differences in litter quality among the grazing sites were not reflected by root decomposition dynamics. The accelerated shoot decay at site HG could not clearly be linked to litter quality parameters. Shoot decay rates were more or less constant, even under very dry conditions. This indicates the possibility of photodegradation (solar UV-B radiation) to control aboveground decomposition in this semi-arid ecosystem. By selecting the best predictors of root decomposition from regression analysis, we found that soil water content was the best parameter explaining the dynamics. Net N immobilization was generally not detected during the decay process of shoot and root. It is likely, when root decomposition is strongly reduced in dry periods, shoot decomposition becomes relatively more important for nutrient cycling. A separate analysis of shoot and root decay dynamics is required in order to describe C and N cycling in this semi-arid grassland. The grazing impact on C and N fluxes through decomposition of plant material likely exhibits a strong interaction with seasonal rainfall pattern.

Grazing-induced spatial variability of soil bulk density and content of moisture, organic carbon and calcium carbonate in a semi-arid rangeland

Abstract The landscape of many semiarid rangelands is characterized by a two-phase, shrub–intershrub vegetation mosaic, each phase having different soil properties. However, this broad subdivision groups together types of intershrub surface cover that may also differ in their soil properties and play important roles in ecosystem functioning. In the northern Negev region of Israel, we examined the soil properties associated with flock trampling routes and rock fragment clusters, as well as those associated with the remainder of the intershrub area and shrub patches. Moisture content, organic carbon content, bulk density and calcium carbonate content of the soil were determined for the above four types of cover, inside and outside long-term grazing exclosures. Soil was sampled in the peak of the growing season and in the end of the dry season, on a north- and a south-facing hillside, and from two depths. The shrub patches exhibited the highest soil moisture and organic carbon contents, and the lowest bulk density and calcium carbonate contents. The trampling routes showed opposite trends. The rock fragment clusters and the remainder of the intershrub area did not generally differ and had intermediate values of these properties. Grazing did not have a significant effect on soil properties at the whole-plot scale, but there were highly significant interactions between grazing and type of cover. Compared with the former trampling routes in the exclosures, the active trampling routes outside them had higher bulk density and lower moisture and organic carbon contents. The intershrub area had higher moisture and organic carbon contents under grazing than in the exclosures. Grazing increased the spatial heterogeneity of the soil properties examined via the creation of a network of trampling routes on the hillsides. The routes themselves, which constituted over 20% of the landscape cover, had degraded soil properties but they led to the improvement of the properties of the remainder of the intershrub area via functionally important source–sink relationships. The study of the soil of regions in which such networks are apparent should be duly cognizant of this intershrub subdivision in addition to the widely recognized shrub–intershrub dichotomy.