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1.
In the semi-arid steppe rangelands of Central Turkey, Festuca valesiaca and Thymus sipyleus ssp rosulans have become the dominant species on degraded pastures. We hypothesized that decreases in species richness and abundance are correlated with increasing prevalence of these two species. Therefore, our objectives were to determine whether there are patterns in examined vegetation; how dominant species contribute to these patterns; and how patterns differ between grazed and ungrazed vegetation. We determined that protection from grazing increased species richness. Grazing significantly changed composition through decreasing total plant, forb, grass and F. valesiaca covers, while substantially increasing T. sipyleus cover. Topography, soil and grazing appear to impact the dominance of plant communities where F. valesiaca and T. sipyleus prevail. These two dominant species had a significant effect in shaping vegetation patterns. Based on regression analysis, alterations in species richness with changes in cover of forbs and shrubs were evident, and spatial heterogeneity of F. valesiaca and T. sipyleus indicated unstable vegetative patterns in heavily grazed pastures and successional changes in protected pastures. Our study results identify F. valesiaca and T. sipyleus as indicator species of vegetation suppression in condition assessments of degraded steppe rangelands.  相似文献   

2.
Esophageally fistulated goats were released in a microphyll desert scrub habitat with 14.6% or 46.2% aerial cover of Atriplex canescens (induced by removal of all shrubs except A. canescens, with livestock grazing exclusion for 10 years) to determine the seasonal forage species selected by the goats. The shrub removal strongly influenced the goats’ feeding habits. The goats grazing the Atriplex-dominated site consumed 4.5 times more A. canescens than the goats on the untreated (control) pasture during all seasons. Shrubs were used heavily during all seasons by goats in both pastures, with higher percentages (75.5–82.8%) in the diets of goats grazing the control pasture than in the diets of goats grazing the Atriplex-dominated area (62.5–68.5%). No differences were found between seasons. The goats in the Atriplex-dominated pasture ate more grass than the goats grazing the control area and used more perennial graminoids during the summer (15.0%) and spring (18.3%) than the goats on the control area (6.0–7.0%). Forbs were an important component of diets. It was concluded that the successful revegetation of the treated pasture affected the goats’ feeding strategy, with an increase in the use of A. canescens, forbs and grasses in all seasons.  相似文献   

3.
Research on grazing effects usually compares the same sites through time or grazed and ungrazed sites over the same time period. Both approaches are complicated in arid environments where grazing can have a long undocumented history and landscapes can be spatially heterogenous. This work employs both approaches simultaneously by comparing grazed and ungrazed samples through both time and space using fossil plant macrofossils and pollen from packrat middens. A series of 27 middens, spanning from 995 yr BP to the present, were collected from Glen Canyon in southeastern Utah, USA. These middens detail vegetation change just prior to, and following, the historical introduction of domesticated grazers and also compares assemblages from nearby ungrazable mesas. Pre-grazing middens, and modern middens from ungrazed areas, record more native grasses, native herbs, and native shrubs such as Rhus trilobata, Amelanchier utahensis, and Shepherdia rotundifolia than modern middens from grazed areas. Ordinations demonstrate that site-to-site variability is more important than any temporal changes, making selection of comparable grazed versus ungrazed study treatments difficult. But within similar sites, the changes through time show that grazing lowered the number of taxa recorded, and lessened the pre-existing site differences, homogenizing the resultant plant associations in this desert grassland.  相似文献   

4.
To examine the influence of buffelgrass land conversion and pasture management on native species diversity and regeneration patterns, we describe community attributes and population structure in four different active pastures in thornscrub vegetation from eastern Sonora, Mexico. We compare a relatively undisturbed thornscrub community with a contiguous five years old active pasture, to identify species able to regenerate under current management practices. Buffelgrass conversion has a significant influence on species diversity and other community attributes. Active pastures have lower species diversity, crown cover and basal area of native species than the studied native thornscrub. An active pasture subjected to heavy grazing and without management of the thorny legume, Acacia cochliacantha, showed the lower species diversity and an almost monospecific stand of this legume. It is likely that the reduction in species diversity is caused by the conversion process, inadequate management and regeneration barriers that limit seedling establishment in active pastures. Our data recorded eight native species (13%) that were able to regenerate in active pastures. However, the great majority of native species were unable to regenerate under pastures. This study shows that under current management, active buffelgrass pastures maintain only a small fraction of native species with regenerating populations in the thornscrub.  相似文献   

5.
Little is known about the effects of grazing on vegetation composition on the Arabian Peninsula. The aim of this study therefore was to analyse the vegetation response to environmental conditions of open woodlands along an altitudinal and a grazing gradient in the Jabal al Akhdar mountain range of Oman. The species composition, vegetation structure, grazing damage and several environmental variables were investigated for 62 samples using a nested plot design. Classification analysis and a Canonical Variate Analysis (CVA) were used to define vegetation types and to identify underlying environmental gradients. The relationship between environmental variables and diversity was analysed using correlation coefficients and a main-effects ANOVA. The plant species richness followed a unimodal distribution along the altitudinal gradient with the highest number of species at the intermediate altitudinal belt. The cluster analysis led to five vegetation groups: The Sideroxylon mascatense–Dodonaea viscosa group on grazed and the Olea europaea–Fingerhuthia africana group on ungrazed plateau sites at 2000 m a.s.l., the Ziziphus spina-christi–Nerium oleander group at wadi sites and the Moringa peregrina–Pteropyrum scoparium group at 1200 m a.s.l, and the Acacia gerrardii–Leucas inflata group at 1700 m a.s.l. The CVA indicated a clear distinction of the groups obtained by the agglomerative cluster analysis. The landform, altitude and grazing intensity were found to be the most important variables distinguishing between clusters. Overgrazing of the studied rangeland is an increasing environmental problem, whereas the plant composition at ungrazed sites pointed to a relatively fast and high regeneration potential of the local vegetation.  相似文献   

6.
Eleven plant communities of the central Tibetan ecotone (31°20′-33°00′N/89°00′-92°10′E) between the Kobresia pygmaea grazing pastures of the eastern highlands and the open short grass steppe with cushion plants (“Alpine Steppe”) of the north-western highlands have been described on the basis of 189 plot-based floristically complete vegetation records. In parallel, remote sensing techniques distinguish four main land-use cover types. Our hypotheses are: (1) The plant communities of the central Tibetan highlands are grazing-adapted and resilient to degradation. (2) In contrast to grazing resilient plant functional types, the turf cover of the K. pygmaea pastures and the Kobresia schoenoides wetlands is degradable through desiccation, periglacial processes, soil-dwelling small mammals and livestock. Five grazing-related plant functional traits are introduced. Grazing tolerance is the prevalent functional type. Species with no specific protection against grazing make up not more than 6% of the total cover. Unpalatable plants cover up to 8%. Only the azonal K. schoenoides swamps - the indispensable winter grazing reserve - have been widely degraded and depleted by 75%, being replaced by Carex sagaensis grazing pastures. It can be foreseen that governmental policy of sedentarisation of nomads will lead to reduced grazing mobility and degradation of winter grazing reserves.  相似文献   

7.
We evaluated the net outcome of the interaction between the shrub Retama sphaerocarpa, our target plant, and different herbaceous neighbours in response to changes in the magnitude and frequency of rainfall events during three years. The experiment was conducted in natural and anthropogenic grasslands dominated by a perennial stress-tolerator and ruderal annual species, respectively. In spite of the neutral or positive effects of neighbours on water availability, neighbouring plants reduced the performance of Retama juveniles, suggesting competition for resources other than water. The negative effects of grasses on the photochemical efficiency of Retama juveniles decreased with higher water availabilities or heavier irrigation pulses, depending on the grassland studied; however, these effects did not extent to the survival and growth of Retama juveniles. Our findings show the prevalence of competitive interactions among the studied plants, regardless of the water availability and its temporal pattern. These results suggest that positive interactions may not prevail under harsher conditions when shade-intolerant species are involved. This study could be used to further refine our predictions of how plant–plant interactions will respond to changes in rainfall, either natural or increased by the ongoing climatic change, in ecosystems where grass–shrubs interactions are prevalent.  相似文献   

8.
Responses of herbaceous and suffrutescent species to fire, grazing, and presence of Prosopis glandulosa were examined in a Chihuahuan desert grassland in south-central New Mexico. Treatments were assigned randomly to eight 12×8 m plots within each of two blocks. Following fires in June 1995, unfenced plots were exposed to livestock grazing over 4 years. Plots were established that either included or excluded P. glandulosa. Perennial grass cover, primarilyBouteloua eriopoda , decreased by 13% in burned plots but increased 5% in unburned areas. Conversely, perennial forb cover was 4% greater after fire. Perennial grass frequency decreased 30% more and perennial forb frequency increased 10% more following burning. Further, increases in evenness after fire resulted in a 225% increase in species diversity. Grazing also resulted in a decrease in perennial grass cover while frequency decreased 22% more in grazed than ungrazed plots. Only frequency and not cover of perennial forbs and annual grasses increased more following grazing. Presence of P. glandulosa had no differential effect on responses of non-shrub species. Fires were conducted during near drought conditions while grazing occurred during years of precipitation equivalent to the long-term average. Precipitation immediately following fire may be critical for recovery of B. eriopoda -dominated desert grasslands; relationships between fire and post-fire precipitation patterns require future investigation.  相似文献   

9.
This study explored the impacts of sustained heavy grazing in six vegetation types across an altitudinal and rainfall gradient in the Kamiesberg mountain range in Namaqualand. The study was carried out across the fence separating the Leliefontein communal area and surrounding privately owned farms. The communal area has been stocked over decades at approximately twice the government recommended stocking rate, while adjacent privately owned farms have generally adhered to recommended rates. Plant community data were collected from 66, 0.1 ha modified Whittaker plots and analysed for diversity and compositional changes. Consideration of community-wide responses through NMDS ordination showed that heavy grazing did not result in the dominance of a few wide-spread, weedy species in communal areas. Species richness at the 0.1 ha scale was also not affected by different land use practices. However, there was a significant compositional shift away from large woody and succulent shrubs, and an associated increase in dwarf shrubs and herbaceous perennial plants on the communal areas. This shift was only evident on the sandy lowland habitats, while a reduction in perennial grass was recorded in the rocky upland habitats on the communal areas. Compositional shifts towards smaller and more ephemeral species in the communal area are indicative of a system more closely dependent on rainfall. This has implications for people's livelihoods in the region, particularly in light of predicted climate change.  相似文献   

10.
Starting in 2005, we examined differences in vegetation for three consecutive years across an airport fence that separated heavily grazed areas from areas in which grazing had been excluded for 24 years in Mandalgobi, Mongolia. We performed repeated-measures analysis separately on two community types (dominated by Allium polyrrhizum and Achnatherum splendens, respectively) to compare the effects of fencing and year on the cover of different plant functional types. There was a significant fence × year interaction for grass cover in the Allium type (but not the Achnatherum type), due to greater cover of grasses inside the fence only when rainfall was sufficient during the growing season. The effect of grazing exclusion on perennial forb cover was confounded by a significant fence × year interaction in both types. In 2007, perennial forbs were found outside the fence, but had almost disappeared inside the fence, resulting in this interaction. Annual forbs only had much greater cover values inside the fence than outside in 2006, also resulting in a significant fence × year interaction in both community types. This study thus suggests that the high rainfall variability in arid and semi-arid rangelands may modify the effects of long-term exclosure on vegetation.  相似文献   

11.
In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha−1), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha−1) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h−1 for 30 min applied on 1 -m2 plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h−1, followed by recent pasture (8.2 mm h−1), long-established pasture (5.9 mm h−1) and open woodland (4.2 mm h−1). Total sediment production in recent pasture was 11.6 g m−2, followed by woody encroachment (9.0 g m−2), long-established pasture (7.3 g m−2) and open woodland (4.3 g m−2). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h−1 and 1.3 g m−2) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h−1and 6.5 g m−2, respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.  相似文献   

12.
The eddy covariance technique was used to measure evapotranspiration (ET) at four different grazing intensity sites to investigate the grazing effects on ET in the semiarid steppe ecosystems of Inner Mongolia. By reducing available energy, and decreasing soil water content (SWC), grazing decreased ET on a seasonal scale compared with the site ungrazed since 1979 (UG79). The most important climatic factor controlling ET on daily scale shifted from SWC to Net radiation (Rn) when grazing intensity increased. SWC, Rn and air temperature (or vapor pressure deficit) can explain 59%–71% of the variation in daily ET. On the other hand, leaf area index (LAI) affected ET slightly at UG79 under the commonly limited soil water conditions. Even no effect of LAI at the heavily grazed site was detected. This suggests that the direct effect of grazing reducing LAI on ET is not significant in this semiarid steppe ecosystem. Soil evaporation compensates for most of the loss in transpiration due to reduced LAI.  相似文献   

13.
Grassland of Inner Mongolia, the main grassland region of China and part of the Eurasia Steppe that stretches from East China to Hungary, is among the most seriously degraded grasslands in China. Previous studies on causes of grassland degradation in this region focused primarily on anthropogenic activities, such as rapid development, ranching, agricultural activities, and mining for mineral resources. Few studies have examined plant interactions during the processes of grassland degradation. In this study, using the thermal-desorption cold trap/gas chromatography/mass spectrometer technique (TCT/GC/MS) we examined the effects of volatile organic compounds (VOCs) of Artemisia frigida on the seed germination and seedling growth of 4 common grass species, Melitotus suaveolens, Sorghum sudanense, Elymus dahuricus and Agropyron cristatum, in the Inner Mongolian pastures. We found that the major constituents of the VOCs from natural A. frigida were 1,8-cineole, camphene, (E)-3-hexen-1-ol acetate, α-terpineol, β-Terpineol, (R)-(−)-p-Menth-1-en-4-ol, p-cymene, and camphor, whose relative content accounted for 96.0% of the 22 compounds identified for natural A. frigida. The content and amount of these VOCs varied when A. frigida was damaged artificially. The VOCs from A. frigida significantly decreased the seed germination and seedling growth of all the tested plant species. Our finding that artificially damaged plants of A. frigida released different types and amount of VOCs from undamaged ones under laboratory conditions indicate that grazing activity on natural pastures may influence the type and amount of VOCs released from A. frigida. However, more research is needed under field conditions to draw a conclusion if grazing activity has similar impacts on release of VOCs as under laboratory conditions.  相似文献   

14.
The Brazilian faxinal is a traditional agrosilvopastoral system, which combines extensive grazing, subsistence cropping and low impact forest extraction, managed within a communal property system. Today it is at serious risk of disappearing due to increasing grazing pressure and logging activities that are leading to environmental problems such as forest fragmentation and soil degradation. The objectives of the study were to: a) evaluate soil physical degradation levels in different grazing areas within the faxinal system; and b) correlate soil degradation, fodder quantity and livestock dynamics in different pasture areas according to the household's location. Landscape mapping, vegetation surveys, and livestock grazing dynamics were assessed. In addition, soil physical parameters were evaluated for each land use (e.g., resistance, bulk density, porosity, moisture, and water infiltration rates). Results showed that livestock grazing and household locations are the main driving forces that lead to forest fragmentation. The household location creates three interrelated gradients in the communal grazing land: 1) soil physical degradation decreases from the pasture areas to araucaria forests; 2) fodder quantity decreases from pastures to araucaria forests; and 3) the livestock grazing intensity decreases from pastures to araucaria forests. In addition, some management practices to mitigate land degradation, including improvement of pastures, installation of removable troughs for drinking water in the upland areas and restriction of livestock from the riparian zone, have been proposed.  相似文献   

15.
The effect of both a non-prescribed summer fire and grazing at high stocking rate following fire on plant community composition, the frequency of occurrence of bare soil, grasses and shrubs, species diversity and biomass of herbaceous forage were evaluated for three growing seasons after fire. Changes in community composition occurred as a consequence of both fire and grazing. Communities were dominated by unpalatable shrubs and grasses in unburned sites as a result of a long history of overgrazing. Fire contributed to a conversion of those shrublands to communities with a more favourable balance between woody and herbaceous species. The frequency of palatable grasses and herbaceous forage biomass increased by a factor ofc. 3 at the expense of woody vegetation. Grazing after fire had significant positive (i.e. decrease in undesirable grasses) and negative (i.e. increase in bare soil) effects.  相似文献   

16.
The arid rangelands of Syria cover over half of the nation's landmass. Punctuating this landscape are broad, dry basins, or wadis, and gentle landscape depressions that exhibit localized elevated vegetation productivity and unique edaphic and hydrologic properties. Historically, continuous heavy grazing and aggressive agricultural activities resulted in excessive ecological degradation within these sensitive environments. Information is needed to determine the influence of livestock grazing on plant communities in landscape depressions and the impact that this has on ecosystem resilience. The purpose of this research is to evaluate the effect of short-term sheep grazing on vegetation characteristics and plant community structure within depressions, and to provide recommendations for improved grazing management. Study plots were randomly located within paired topographic depressions located in northwestern Syria. Vegetation samples were collected along transects including plant biomass, plant density, herbaceous cover, and species diversity. In grazed plots, plant biomass was 49 g DM/m2 compared to 234.4 g DM/m2 in protected plots. Average plant density was 65 plants/m2 in grazed plots compared to 1013 plants/m2 in protected plots (P = 0.001). Herbaceous cover was 175% higher on protected sites compared to grazed plots. Average diversity (Shannon–Wiener index value) was 0.8 in grazed plots compared to 2.3 in protected plots. These results suggest that plant community structure will be impacted from short-term grazing and that a site's ability to positively respond to disturbance over time may be limited. We conclude that carefully planned grazing management should result in greater plant productivity and diversity.  相似文献   

17.
The study was conducted to assess the status and trends of soil erosion and relate the perceptions of farmers on cattle productivity and botanical indicators to measured ecological conditions of rangelands in three altitude regions of southwest Ethiopia. A total of 342 farmers were interviewed. In addition, the ecological condition of rangelands was assessed. Severe soil erosion, ranked as the primary restriction to free-ranging livestock, occurred predominantly in the lower altitude region (LAR) (P < 0.05). More farmers in LAR witnessed an inadequacy of palatable plant biomass, grazable pasture as well as increased gully formation and expansion, which are strong indicators of soil erosion (P < 0.001). In addition to a decrease in grass cover and productivity of cattle, botanical composition, species richness and grazing capacity of herbaceous plants, less fodder trees and shrubs were observed (P < 0.05). There was a corresponding increase in the percentage of bare ground and soil erosion status along the degradation gradients (P < 0.05). The reported shift in botanical composition, and especially encroachment of invading plant species, can be attributed to soil erosion (P < 0.001). The results suggest that erosion is associated with reduced availability of feed resources and is related to altitude variation.  相似文献   

18.
Arthropods living in the canopies of two woody shrub species (a sub-shrub (Gutierrezia sarothrae) and a large shrub (Prosopis glandulosa)) and perennial grasses plus associated herbaceous species, were sampled on 18, 0.5 hectare plots in a Chihuahuan Desert grassland for five consecutive years. Mesquite shrubs were removed from nine plots, six plots were grazed by yearling cattle in August and six plots were grazed in February for the last 3 years of the 5 year study. Arthropod species richness ranged between 154 and 353 on grasses, from 120 to 266 on G. sarothrae, and from 69 to 116 on P. glandulosa. There was a significant relationship between the number of families of insects on grass and G. sarothrae and growing season rainfall but species richness was not a function of growing season rainfall on any of the plants. Several of the arthropod families that were the most species rich in this grassland were found on all of the plants sampled, i.e. Salticid spiders, Bruchid and Curculionid beetles, Cicadellid and Psyllid homopterans, and ants (Formicidae). There were more species rich families that were shared by grasses and the sub-shrub G. sarothrae than with mesquite. The absence of a relationship between growing season rainfall and species richness was attributed to variation in life history characteristics of arthropods and to the non-linear responses of annual and perennial desert grassland plants to rainfall.There were no significant differences in insect family or species richness on any of the plant types as a result of removal of mesquite (P. glandulosa) from selected plots. Intense, short duration (24 h) grazing by livestock during in late summer resulted in reduced species richness in the grass-herb vegetation layer but had no effect on insect species richness on snakeweed or mesquite shrubs. Livestock grazing in winter had no effect on insect species richness on any of the vegetation sampled.  相似文献   

19.
We tested the hypothesis that herbivores contribute to feedbacks maintaining arid ecosystems in a degraded state. We studied small mammal herbivory on a subshrub, broom snakeweed (Gutierrezia sarothrae), and perennial grasses at three sites: (1) ungrazed black grama (Bouteloua eriopoda) grassland; (2) grassland degraded by intense short-duration grazing; and (3) mesquite (Prosopis glandulosa) coppice dunes. Snakeweed was browsed by herbivores primarily during dry winter months.The average percent of G. sarothrae standing crop biomass removed by browsing was 9.2 in ungrazed grassland, 7.4 in intensely grazed grassland, and 4.1 in the dunes. In ungrazed grassland, an average of 12% of grass cover was harvested by herbivores; in the intensely grazed plots – 80%. Herbivore exclusion plots showed that jackrabbits (Lepus californicus) were the primary browsers on snakeweed and rodents on grasses and G. sarothrae inflorescences. Rodent removal of G. sarothrae inflorescences allows wind dispersal of seeds in disturbed and desertified areas, thereby increasing abundance of this poisonous shrub. Grass-tiller cutting by rodents provides a strong feedback that may be responsible for keeping the grass cover low on the intensely grazed areas. Jackrabbit pruning has little effect on G. sarothrae abundance at any stage of desertification.  相似文献   

20.
Climate and grazing are the main drivers of plant community composition and species richness in arid environments. This study aimed to examine the vegetation response to a spatial precipitation gradient, interannual rainfall variability, and grazing pressure in Mongolia. To examine the effect of a spatial precipitation gradient, we compared species richness among six sites. To investigate the effects of interannual rainfall variability and grazing pressure, we compared species richness for 2 years at two sites, in desert-steppe and steppe areas. The regional gradient in annual precipitation showed positive and negative relationships with grass richness and shrub richness, respectively, although total species richness did not vary significantly. The proportions of the different functional groups were affected by grazing pressure and rainfall variability in both zones. In the desert-steppe zone, species richness was lower in the drier year but did not vary with grazing pressure. In the steppe zone, species richness varied significantly with grazing pressure but did not vary between years. Precipitation would be more important than grazing pressure on vegetation changes in drier areas with high rainfall variability.  相似文献   

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