Manipulation of vegetation communities on the Abu Dhabi rangelands. I. The effects of irrigation and release from longterm grazing

The effect on vegetation communities of release from grazing by camels and goats has been investigated in the Baynunah region of Abu Dhabi emirate, in The United Arab Emirates, by the study of an exclosure established 11 years previously. Also the effect of sprinkle irrigation (in the absence of grazing) on the rangeland vegetation was investigated. Perennial species richness was significantly lower outside the exclosure compared with inside on both sand and gravel substrata. Annual species richness however was not significantly different. Perrenial percentage covers were lower outside the exclosure compared with inside, especially on sand substrata. The perennial grass Stipagrostis plumosa (L) showed the greatest difference in percentage cover in this respect. The species richness of annuals and perennials was not significantly different between irrigated and non-irrigated areas within the exclosure. The perennial percentage cover was much greater on sprinkle irrigated sand and gravel substrata. The perennial which benefited the most was Zygophyllum hamiense, Scweinf. Annual percentage cover was lower in irrigated quadrats. Suggestions are made about the effect of intensive grazing on the vegetation communities at Baynunah and on the effectiveness of irrigation as a tool for increasing the quality of the rangeland for livestock and wildlife.     

Response of native and non-native ruderals to natural and human disturbance

The ruderal strategy is widely shared among non-native plants, providing a general explanation for the commonly observed positive effects of disturbance on invasions. How native ruderals respond to disturbance and how their abundance compares to that of non-native ruderals remains, however, poorly understood. Similarly, little is known about the role that disturbance type plays in the coexistence between native and non-native ruderals. We proposed that natural disturbance favors native over non-native ruderals, whereas novel anthropogenic disturbance favors non-natives over natives. To assess our general hypothesis, we conducted extensive field samplings in which we measured relative abundance, richness, and diversity of native and non-native ruderals in sites with natural and anthropogenic disturbance in central Argentina, a system where the ruderal strategy is common to a large number of native and non-native species. We found that natives dominated ruderal communities growing in recently burned grasslands, whereas non-natives dominated in roadsides. Additionally, the richness and diversity of native ruderal species were much greater than those of non-natives in sites with fire and in sites with grazing, but species richness and diversity did not differ between groups in roadsides. Because vegetation evolved with fire in our system and, in contrast, the construction and maintenance of roads is recent in it, these results support our hypothesis. Our work indicates that the ruderal strategy does not seem to suffice to explain why disturbance facilitates invasions. According to our data, species origin interacts with disturbance type to determine dominance in communities with coexisting native and non-native ruderals.     

Effects of different clipping intensities on above- and below-ground production in simulated herbaceous plant communities

We studied the effect of clipping on above- and below-ground production in different plant communities through a factorial experiment. We designed five pasture systems with different species composition, perennials/annuals ratio and soil water availability, recreating different altitudinal locations, and simulated a gradient of grazing intensity by clipping with different heights and frequencies. Response patterns of above- and below-ground production were similar, increasing with the higher clipping frequency and decreasing with altitude. These results suggest that high grazing intensity stimulate above-ground production, but only in certain situations of species composition, density, diversity, perennials/annuals ratio and water availability. This stimulus, however, is unsustainable over time, and the lower clipping frequencies are those that favour the maintenance of production.     

Impact of Precipitation Patterns on Biomass and Species Richness of Annuals in a Dry Steppe

Annuals are an important component part of plant communities in arid and semiarid grassland ecosystems. Although it is well known that precipitation has a significant impact on productivity and species richness of community or perennials, nevertheless, due to lack of measurements, especially long-term experiment data, there is little information on how quantity and patterns of precipitation affect similar attributes of annuals. This study addresses this knowledge gap by analyzing how quantity and temporal patterns of precipitation affect aboveground biomass, interannual variation aboveground biomass, relative aboveground biomass, and species richness of annuals using a 29-year dataset from a dry steppe site at the Inner Mongolia Grassland Ecosystem Research Station. Results showed that aboveground biomass and relative aboveground biomass of annuals increased with increasing precipitation. The coefficient of variation in aboveground biomass of annuals decreased significantly with increasing annual and growing-season precipitation. Species richness of annuals increased significantly with increasing annual precipitation and growing-season precipitation. Overall, this study highlights the importance of precipitation for aboveground biomass and species richness of annuals.     

Loss of a large grazer impacts savanna grassland plant communities similarly in North America and South Africa

Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance.     

Species–area relationships in Mediterranean-climate plant communities

Aim To determine the best‐fit model of species–area relationships for Mediterranean‐type plant communities and evaluate how community structure affects these species–area models. Location Data were collected from California shrublands and woodlands and compared with literature reports for other Mediterranean‐climate regions. Methods The number of species was recorded from 1, 100 and 1000 m² nested plots. Best fit to the power model or exponential model was determined by comparing adjusted r² values from the least squares regression, pattern of residuals, homoscedasticity across scales, and semi‐log slopes at 1–100 m² and 100–1000 m². Dominance–diversity curves were tested for fit to the lognormal model, MacArthur's broken stick model, and the geometric and harmonic series. Results Early successional Western Australia and California shrublands represented the extremes and provide an interesting contrast as the exponential model was the best fit for the former, and the power model for the latter, despite similar total species richness. We hypothesize that structural differences in these communities account for the different species–area curves and are tied to patterns of dominance, equitability and life form distribution. Dominance–diversity relationships for Western Australian heathlands exhibited a close fit to MacArthur's broken stick model, indicating more equitable distribution of species. In contrast, Californian shrublands, both postfire and mature stands, were best fit by the geometric model indicating strong dominance and many minor subordinate species. These regions differ in life form distribution, with annuals being a major component of diversity in early successional Californian shrublands although they are largely lacking in mature stands. Both young and old Australian heathlands are dominated by perennials, and annuals are largely absent. Inherent in all of these ecosystems is cyclical disequilibrium caused by periodic fires. The potential for community reassembly is greater in Californian shrublands where only a quarter of the flora resprout, whereas three quarters resprout in Australian heathlands. Other Californian vegetation types sampled include coniferous forests, oak savannas and desert scrub, and demonstrate that different community structures may lead to a similar species–area relationship. Dominance–diversity relationships for coniferous forests closely follow a geometric model whereas associated oak savannas show a close fit to the lognormal model. However, for both communities, species–area curves fit a power model. The primary driver appears to be the presence of annuals. Desert scrub communities illustrate dramatic changes in both species diversity and dominance–diversity relationships in high and low rainfall years, because of the disappearance of annuals in drought years. Main conclusions Species–area curves for immature shrublands in California and the majority of Mediterranean plant communities fit a power function model. Exceptions that fit the exponential model are not because of sampling error or scaling effects, rather structural differences in these communities provide plausible explanations. The exponential species–area model may arise in more than one way. In the highly diverse Australian heathlands it results from a rapid increase in species richness at small scales. In mature California shrublands it results from very depauperate richness at the community scale. In both instances the exponential model is tied to a preponderance of perennials and paucity of annuals. For communities fit by a power model, coefficients z and log c exhibit a number of significant correlations with other diversity parameters, suggesting that they have some predictive value in ecological communities.     

Succession of bee communities on fallows

Wild bee communities were studied on one- to five-year-old set-aside fields with naturally developed vegetation (n = 20). and old orchard meadows (n = 4) to analyse effects of secondary succession on species diversity, resource use and associated life history traits. General theory predicts a steady increase of species richness with age of succession. In contrast, we found a first maximum in species richness of bees on two-year-old set-aside fields and a second on old meadows. Successional changes of bee communities were related to changes of vegetation. The transition from pioneer successional stages, dominated by annuals, to early successional stages, dominated by perennials, resulted in the highest species richness of flowering plants in the second year within the first five years of succession. Species richness of flowering plants was the best predictor variable for species richness of bees, whereas the cover of flowering plants correlated with the abundance of bees. Annual plants were visited more often and perennials less often than expected from their flower cover. Halictidae tended to prefer flowers of annuals, whereas Megachilidae. Apidae and Anthophoridae significantly preferred perennials. In departure from successional theory, body size, proportion of specialised bees and proportion of parasitic bees did not significantly increase with successional age, but number of generations and the proportion of soil-nesting bees decreased with successional age. Comparison of different management types showed that set-aside fields with naturally developed vegetation supported much more specialised and endangered bee species than set-aside fields sown with Phacelia tanacetifolia.     

Variations in species and functional plant diversity along climatic and grazing gradients

Different components of biodiversity may vary independently of each other along environmental gradients giving insights into the mechanisms that regulate species coexistence. In particular, the functional diversity (FD) or the presence of rare or endemic species in natural assemblages do not necessarily increase with species diversity. We studied if different components of plant species diversity (species richness, Simpson diversity, evenness) varied similarly to FD (measured as a generalization of the Simpson index) and rarity along grazing intensity and climatic gradients. Plots under different sheep grazing regimes (high and low intensity, abandonment) were surveyed in five locations along a climatic gradient in north-eastern Spain, from semi-arid lowland to moist upland locations. Variation in species diversity, functional diversity and rarity followed different patterns. Species diversity was lowest in water-stressed environments (arid locations and southern aspects) and increased with grazing more makedly in humid locations. The FD was comparable between the most species-poor and species-rich locations and decreased with grazing in the moistest location, i.e. where species diversity markedly increased. The FD did not show a strong correlation with species richness nor with the Simpson index and less specious communities could show the highest functional diversity. The rarest species in the region were more frequently found in the abandoned areas, which held the lowest species diversity. Consequently, the mechanisms that enhance the diversity of species do not necessarily support a functional differentiation among those species or the maintenance of rare species in a region. We hypothesize that the degree of dependence of functional diversity on species diversity might be mostly related to the amplitude of the species' traits pool and on how species partition the niche space available.     

Perennial polypores as indicators of annual and red-listed polypores

Many polypores are specialized in their requirements for substrate and environment, and they have been suggested to indicate the continuity of coarse woody debris or naturalness of a forest stand. However, the use of polypores as indicators of conservation value is restricted by the temporally limited appearance of annual fruit bodies. We studied whether the species richness of perennial polypores (perennials) can be used to predict the species richness of annual or annual red-listed polypores (annuals). Our data included 1471 separate datasets (sample plots or larger inventoried areas) in different parts of Finland and Russian Karelia, ranging from the southern to northern boreal zone. At the large scale (the whole area) the number of perennials explained about 70% of the variation in the number of annuals, and about 67% in the number of red-listed annuals. A minimum set of 40–60 perennial occurrences gave a reliable estimate on the species richness of annuals, and 60–80 occurrences on the species richness of red-listed annuals. The richness of perennials predicted the richness of annuals and, in particular, richness of red-listed annuals, better than the size of inventoried area. According to our results, perennial polypores can be used as a surrogate for overall polypore species richness in natural and seminatural boreal forests, but the predictive power is weaker in managed forests. In addition, the relationship between the perennial and annual species seems to differ in different vegetation zones, management types and forest types. Due to this variation direct application of the indicator values derived from different vegetation zones and management or forest types are not recommended. Since perennials are easier to identify than annuals, detectable throughout the year, and have much smaller year-to-year variation, their use as an indicator group seems to offer advantages regarding the timing and cost-efficiency of inventories.     

Effects of livestock grazing on soil,plant functional diversity,and ecological traits vary between regions with different climates in northeastern Iran

Understanding the responses of vegetation characteristics and soil properties to grazing in different precipitation regimes is useful for the management of rangelands, especially in the arid regions. In northeastern Iran, we studied the responses of vegetation to livestock grazing in three regions with different climates: arid, semiarid, and subhumid. In each region, we selected 6–7 pairwise sampling areas of high versus low grazing intensity and six traits of the present species were recorded on 1 m² plots—five grazed and five ungrazed in each area. The overall fertility was compared using the dissimilarity analysis, and linear mixed‐effect models were used to compare the individual fertility parameters, functional diversity indices, and species traits between the plots with high and low grazing intensity and between the climatic regions. Both climate and grazing, as well as their interaction, affected fertility parameters, functional diversity indices, and the representation of species traits. Grazing reduced functional evenness, height of the community, the representation of annuals, but increased the community leaf area. In the subhumid region, grazing also reduced functional richness. Further, grazing decreased the share of annual species in the semiarid region and seed mass in the arid region. Larger leaf area and seed mass, smaller height and lower share of annuals were associated with intensive grazing. Species with large LA and seed mass, lower height and perennials can be therefore presumed to tolerate trampling and benefit from high nutrient levels, associated with intensive grazing. By providing a detailed view on the impacts of overgrazing, this study highlights the importance of protection from grazing as an effective management tool for maintaining the pastoral ecosystems. In general, the composition of plant traits across the pastures of northeastern Iran was more affected by intensive grazing than by the differences in climate.     

Alien plant species coexist over time with native ones in Chilean Mediterranean grasslands

Aims Alien species are commonly considered as harmful weeds capable of decreasing native biodiversity and threatening ecosystems. Despite this assumption, little is known about the long-term patterns of the native–alien relationships associated with human disturbed managed landscapes. This study aims to elucidate the community dynamics associated with a successional gradient in Chilean Mediterranean grasslands, considering both native and alien species.Methods Species richness (natives and aliens separately) and life-form (annuals and perennials) were recorded in four Chilean post-agricultural grazed grasslands each covering a broad successional gradient (from 1 to 40 years since crop abandonment). A detrended correspondence analysis (DCA), mixed model effects analyses and correlation tests were conducted to assess how this temporal gradient influenced natives and aliens through community dynamics.Important findings Our results show different life-form patterns between natives and aliens over time. Aliens were mainly represented by annuals (especially ruderals and weeds), which were established at the beginning of succession. Annual aliens also predominated at mid-successional stages, but in old grasslands native species were slightly more representative than alien ones within the community. In the late successional states, positive or no correlations at all between alien and native species richness suggested the absence of competition between both species groups, as a result of different strategies in occupation of the space. Community dynamics over time constitute a net gain in biodiversity, increasing natives and maintaining a general alien pool, allowing the coexistence of both. Biotic interactions including facilitation and/or tolerance processes might be occurring in Chilean post-agricultural grasslands, a fact that contradicts the accepted idea of the alien species as contenders.     

Can heavy grazing on communal land elevate plant species richness levels in the Grassland Biome of South Africa?

A common perception, particularly in South Africa, is that heavily and continuously grazed communal land leads to degradation and loss of plant diversity when compared to commercial rangeland farming or conservation areas. We focus on whether this applies to the Grassland Biome of South Africa and whether the opposite can occur, namely, an increase in plant species richness under heavy grazing. A study of a contrast between a communal area of the former Ciskei and a neighbouring nature reserve showed that intense utilization under communal use led to a significant increase in plant species richness. However, this increase was scale-dependent with the greatest significant difference occurring at sample plot scale (50 m²) but converging at the broader scale of the whole study site. Species that increased with heavy grazing included those from arid Karroid areas as well as some from more mesic grassland and savanna areas. The contribution of beta diversity to gamma diversity across the grazing contrast was relatively low which reflects the relatively high proportion of species shared between treatments. Total plant canopy cover declined sharply with heavy grazing. In terms of plant canopy cover, grazing favoured annual over perennial plants, prostrate over erect plants, and stoloniferous over tussock plant architecture. This pattern was not supported when expressed in terms of number of species belonging to these grazing response groups or traits.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

Arid vegetation in disequilibrium with livestock grazing: Evidence from long‐term exclosures

In recent decades, the conventional equilibrium paradigm for explaining rangeland vegetation dynamics has been challenged. Proponents of an alternative non‐equilibrium paradigm argue that in variable rangeland environments, external climatic events are critical to vegetation dynamics and there is little opportunity for plant–herbivore interactions to reach equilibrium. Understanding which paradigm more effectively describes an ecosystem has important consequences for management. In particular, some authors have argued that a focus on reducing stocking rates in non‐equilibrium systems may be futile, and management should be opportunistic in response to unpredictable rainfall events. We measured herbaceous biomass and plant species richness and abundance at five 14‐year exclosures on Innamincka Regional Reserve. Four were situated in the dunefields land system, and one on the Cooper Creek floodplain. We did not detect any significant differences between grazed and ungrazed treatments in total species richness or abundance, life form richness or abundance, or herbaceous biomass. Only one species, Portulaca oleracea, showed differences in abundance between treatments at more than one site, but the direction of these differences was not consistent. These results suggest that the non‐equilibrium paradigm more accurately describes vegetation dynamics in the dunefields and floodplains of north‐eastern South Australia. It is possible that some species had been lost from the study area prior to the establishment of the exclosures, thereby precluding recovery with protection from grazing; however, a regional analysis of the flora reveals little evidence of this. We argue that the dominance of ephemeral species confers resilience by limiting the development of strong feedbacks between grazing intensity and vegetation dynamics. Current grazing practices seem consistent with the conservation of plant species diversity across the dunefields and floodplains. Future studies should focus on the impacts of cattle grazing on areas of the landscape dominated by palatable perennials, as well as the small number of rare and potentially grazing‐sensitive species identified.     

Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size

Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xeric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure.     

The importance of monkey beetle (Scarabaeidae: Hopliini) pollination for Aizoaceae and Asteraceae in grazed and ungrazed areas at Paulshoek, Succulent Karoo, South Africa

The relative importance of monkey beetles (Hopliini, Scarabeidae) as pollinators of Asteraceae and Aizoaceae in the Succulent Karoo as well as the influence of livestock grazing on their abundance and diversity was investigated. Hopliine beetles proved to be the, or among the, most abundant flower visitors of 12 investigated plant species. However, during single flower observations at three Aizoaceae species, bees (Apoidea), bee flies (Bombyliidae) and pollen wasps (Masaridae) were the most frequent flower visitors. However, monkey beetles carried the highest Asteraceae and Aizoaceae pollen loads, and are therefore considered to play a vital role in the pollination of these two families. Abundance, species richness and diversity of Hopliini did not appear to be heavily affected by livestock grazing. Annual variation in the composition of monkey beetle populations was more dramatic. Still, some species showed higher abundances on heavily grazed rangeland while others only occurred under low grazing pressure. It is presumed that changes in the composition of the vegetation, especially the observed decrease of perennial plants in favour of annuals and geophytes (Todd and Hoffman 1999) could in turn affect the composition of monkey beetle assemblages.     

The underappreciated role of life history in mediating the functional consequences of biodiversity change

Biodiversity is changing on both global and local scales, motivating research to understand the consequences of these changes for how communities and ecosystems function. Here, we explore the role of life history strategies in mediating biodiversity and ecosystem functioning. In particular, we evaluate how the composition, biomass (% cover), and richness of perennial (persistence ≥ 1 year) and ephemeral (persistence < 1 year) species change along a gradient of increasing seaweed species richness on a rocky shoreline. We show that the majority of biomass is comprised of perennial species, especially where overall richness is low, whereas the majority of species are ephemeral, especially where overall richness is high. We then present and discuss the results of an 18‐month field manipulation quantifying the factorial effects of tidal elevation, wave exposure, herbivore removals, thermal and desiccation stress amelioration, and nutrient additions on perennial versus ephemeral species. In particular, the diversity of ephemeral species was strongly affected, relative to perennial species, by tidal elevation, wave exposure, and herbivory; herbivores reduced diversity of ephemeral species relative to perennials. Relative to perennial cover, ephemeral cover was greater higher on the shore, in more wave‐exposed habitats, and where herbivores were removed, plots were unscreened, and/or nutrients were added. Thus, perennials and ephemerals responded differently to environmental context and experimental manipulation. We compared nitrate uptake and photosynthesis rates of ephemeral and perennial species and found that maximum nitrate uptake and photosynthesis rates of ephemerals were twice as high as those of perennials. These results highlight the disproportionate roles that ephemeral species play in mediating ecosystem‐level processes. In combination with our comparisons of the diversity and cover of perennial and ephemeral species along a biodiversity gradient, these results demonstrate the utility of incorporating life history traits into our efforts to understand the functional consequences of biodiversity change.     

The vertical distribution of below-ground biomass in grassland communities in relation to grazing regime and habitat characteristics

Abstract. 40 sites, representing different pasture types in Northwest Spain, were sampled in respect of their floristic composition, distribution of above and below-ground biomass and environmental and physical variables. Five plant community types were identified by classification techniques of plant species composition. These communities were then characterized in terms of the percentage of ground covered by herbaceous and shrub vegetation, stones, rocks and gaps as well as their topographic location and characteristics of the shallow soil (pH, organic matter, nitrogen and calcium content). Bio-mass was assessed in terms of above-ground structures, surface crowns and three below-ground layers to a depth of 10 cm. Three types of grazing regime were distinguished: Concentrated Intense Grazing in early spring (CIG), Extended Intense Grazing throughout the spring (EIG), and Non-Intense Grazing (NIG). Grazing regime showed the highest association with plant community type and three broad categories were identified: xeric stressed pastures, which nevertheless received CIG, mesic pastures with EIG, and three kinds of NIG mesic pastures. The xeric communities had the highest proportion of aboveground biomass, as a consequence of their greater proportion of woody perennials. These xeric communities displayed a more gradual reduction in below-ground biomass with depth than mesic pastures, a likely consequence of the low water content in the upper soil layers. The mesic communities had a high concentration of below-ground biomass in the upper layers when they were intensely grazed. However, when grazing was low (i.e. NIG situations), these communities had greater variability in biomass profiles than any of the other pasture types. Possible causes of the patterns in biomass distribution of the intensely grazed pastures are discussed.     

Long-term grazing impacts on vegetation diversity,composition, and exotic species presence across an aridity gradient in northern temperate grasslands

Little is known about the specific role of exotic species on measures of grassland plant diversity, including how this may vary with climatic conditions or large mammal herbivory. This study examined vegetation responses to long-term livestock grazing, including plant richness and diversity, as well as the contribution of exotic species to these metrics, across a network of 107 northern temperate grasslands in Alberta, Canada, spanning a broad aridity gradient. Exposure to grazing modestly increased plant richness, but did not alter Shannon’s diversity, Simpson’s diversity, or evenness, suggesting stability in floral diversity relative to grazing. However, grazing did increase grass cover while reducing shrub cover, the latter of which was only apparent in mesic grasslands. Unlike total plant diversity, exotic species richness and cover, together with exotic plant contributions to diversity, varied jointly with grazing and aridity. While long-term grazing increased exotic species, this response was most apparent in wetter areas, and non-grazed grasslands remained more resistant to the presence of exotics. Several exotic species were positive indicators of grazing in wetter grasslands, and coincided with lower native species cover, indicating grazing may be facilitating a shift from native to exotic vegetation under these conditions. Overall, our results indicate that while long-term grazing has altered the composition and cover of certain functional groups, including favoring exotics and minimizing woody vegetation in mesic areas, overall changes to plant diversity were limited. Additionally, these findings suggest that semi-arid northern temperate grasslands remain relatively resistant to grazing effects, including their susceptibility to exotic plant encroachment. These results improve our understanding of how ongoing grazing exposure may impact grassland diversity, including efforts to conserve native vegetation, as well as the important role of climate in altering fundamental grassland responses to grazing.     

Restoring biopedturbation in grasslands with anthropogenic focal disturbance

Grassland ecosystems evolved with natural disturbance events on multiple spatial scales in which focal, fine-scale soil disturbance by animals often was imbedded within large-scale grazing disturbance. The resulting plant communities adapted to both broad-scale and fine-scale disturbance that resulted in species-rich plant communities. These natural disturbance regimes have been largely replaced by anthropogenic disturbance. While we generally understand grassland response to modern grazing practices, we know much less about plant community response to soil disturbance imbedded within non-focal grazing. Therefore, we used a tracked vehicle to focally disturb soil in a North American mesic mixed prairie that was either undisturbed prairie or prairie with a recent history of disturbance from either grazing or haying. Successional trajectory and recovery time following focal soil disturbance was similar between grazed and hayed plant communities. Species composition did not differ (P < 0.05) between grazed or hayed prairie and the respective undisturbed prairie. Plant species richness and bare ground increased (P < 0.05) following focal soil disturbance in both grazed and hayed communities, but focal soil disturbance combined with either grazing or haying did not change either plant species richness or bare ground more than (P > 0.05) focal soil disturbance alone. Also, the effect of focal soil disturbance was shortlived with recovery in two growing seasons. Our results suggest that anthropogenic focal soil disturbance is a reasonable mechanism to restore soil disturbance to the grassland ecosystem.