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1.
Riparian meadows of southern Patagonia are temporally and spatially heterogeneous habitats. They are thought to play a key role in regulating the dynamics of arid grazed steppes of the region. We conducted a 2-year study with sheep to determine the influence of two grazing conditions (deferment and spring grazing) on structural and nutritional parameters of three vegetation types apparently associated with a soil moisture gradient in a riparian meadow in southern Santa Cruz (Argentina). Spring deferment allowed forage accumulation in very moist and intermediate vegetation types by the beginning of the summer and had no detrimental impact on forage quality. Deferment had no effect on biomass accumulation of the drier vegetation type at our study site. Structural attributes of the vegetation appeared to affect spatial grazing patterns of sheep. Soil-related patchiness, rather than sheep grazing, appeared to control vegetation structure and nutritional value. The proportion of more mesic plant communities in riparian habitats of Patagonia could determine the outcome of plant–animal interactions in these systems. Practical recommendations of grazing capacity in paddocks, or more theoretical considerations of ecosystem dynamics of the Patagonian steppes need to explicitly consider the internal patchiness of riparian habitats. 相似文献
2.
We analyzed the structural differences associated with grazing exclusion on vegetation by studying 24 grazed-ungrazed paired sites in Patagonia (Argentina). Our objectives were to evaluate the effect of grazing on plant species composition, plant diversity, the relative abundance of plant functional types, and the vertical and horizontal structure of the plant community. This study across the shrub-grass steppes of Patagonia highlights some important characteristics of the vegetation response to a major disturbance factor we show that shrub encroachment is not a generalized response of the steppe to grazing. Grazed areas presented lower richness and diversity than exclosures. Physiognomic changes (as describe by relative abundance of plant functional types) were less important than those observed at the species level. These results shown that the status of the shrub-grass steppes of the Occidental district of Patagonia can not be characterized in terms of contrasting physiognomic states as in the case of the grass steppes of the Subandean district. Forbs and a few mesophytic and xerophytic grasses are the key elements to identify the condition of the steppe. Monitoring programs should focus on the relative abundance of these components. 相似文献
3.
In grazed semiarid ecosystems, considerable spatial variability in soil infiltration exists as a result of vegetation and soil patchiness. Despite widespread recognition that important interactions and feedbacks occur between vegetation, runoff and erosion, currently there is only limited quantitative information on the control mechanisms that lead to differences in infiltration from different vegetation types. In this paper, we determine (i) the relationship between vegetation and soil surface characteristics and (ii) the soil infiltration rate by using rainfall simulations on runoff plots (0.60 × 1.67 m) in three plant communities of northeastern Patagonia: grass (GS), degraded grass with scattered shrubs (DGS), and degraded shrub steppes (DSS). Our results clearly indicate that vegetation and soil infiltration are closely coupled. Total infiltration was significantly higher in the GS (69.6 mm) compared with the DGS and DSS (42.9 and 28.5 mm, respectively). In the GS, soil infiltration rate declined more slowly than the others communities, reaching a terminal infiltration rate significantly greater (57.7 mm) than those of DGS and DSS (25.7 and 12.9 mm, respectively). The high rate of water losses via overland-flow may limit the possibilities for grass seedling emergence and establishment and favor the persistent dominance of shrubs. 相似文献
4.
In this study the spatial patterns and dynamics of vegetation patches along a grazing gradient in the steppe ofLarrea divaricataandStipaspp. in NE Patagonia (Argentina) are described. A general effect of grazing is the reduction of total plant cover resulting from the decrease in cover of perennial grasses (Stipa speciosa, Poa ligularis, Stipa tenuis) and some tall shrubs (Chuquiraga hystrix, Bougainvillea spinosa, Lycium chilense). Dwarf shrubs (Nassauvia fueguianaandJunellia seriphioides) increase their cover under medium and/or high grazing pressures. Plant species are spatially grouped into patches which alternate with areas of bare soil. Eleven types of vegetation patch differing in the dominant plant functional type or species, floristic richness and size were identified with different relative frequency along the grazing gradient. Based on these results, it is postulated that grazing forces the replacement of large patches dominated by tall shrubs with high species richness, byLarrea divaricatapatches or small dwarf shrub patches with low species richness and the extinction of grass patches. This results from: (1) disruption of local balances of species deletions and additions; (2) fragmentation of large patches; and (3) formation of new vegetation patches. These changes lead to differing plant spatial organization and heterogeneity along the grazing gradient which may be described by characteristic arrays of vegetation patches. 相似文献
5.
J.M. Paruelo S. Pütz G. Weber M. Bertiller R.A. Golluscio M.R. Aguiar T. Wiegand 《Journal of Arid Environments》2008,72(12):2211-2231
We built a grid-based spatial explicit stochastic model that simulates grazing events and basic processes like seedling establishment, growth or mortality of the dominant species in the grass steppes of Patagonia. After evaluating the model with field data, we performed simulation experiments aimed to explore the interaction of precipitation and grazing regimes on vegetation dynamics. Grazing generated a reduction in tussock density which results in a decline in aboveground net primary production (ANPP). Both response variables presented a non-linear behavior including high temporal variability and delay effects, which may prolong for decades. There was a clear threshold in the response of the variables to stock density, though changes become evident only when a highly selective grazing scenario was used. Under high stock density conditions, precipitation use efficiency (PUE) was 82% lower than the values for non-grazed runs. The inter-annual variability of precipitation was more important than the grazing regime in explaining differences in tussock density. Simulation results highlight important issues regarding rangeland management: grazing regime might be as important as stocking density as a degradation agent, temporal lags might obscure degradation processes for decades, the definition of monitoring variables need to consider their response time constants. 相似文献
6.
The Southern Kalahari Duneveld is an arid savannah, which has been used for permanent stock farming since around 1900. Farming with high numbers of animals has led to changes in the vegetation which have been associated with excessive grazing and trampling. To determine whether altered seed dispersal could be an alternative or additional explanation, we investigated how replacing antelope with sheep affects seed dispersal. We determined the dispersal potentials of plant species by various vectors using standardized experiments and studied the variation of dispersal potential along a grazing intensity gradient and among different habitats.Wind dispersal potential was low, but diaspores of many plant species were well adapted to dispersal by water. Biotic long-distance dispersal potentials were relatively high and varied between sheep and antelope: the species of plants dispersed differed, as did the number of seeds transported and the time of year that seeds were dispersed in.The importance of zoochory and the differences found between antelope and sheep dispersal suggest that dispersal is affected by a change of animal type. Thus, sheep can not replace antelope as seed dispersers. Therefore, sustainable grazing management or restoration efforts should benefit from seed input of sensitive species where antelope are rare. 相似文献
7.
策勒绿洲外围不同地下水埋深下主要优势植物的分布和群落特征 总被引：7，自引：3，他引：4
对塔克拉玛干沙漠南缘策勒绿洲外围地下水埋深与主要优势植物分布和群落特点的研究结果表明,植物的分布和群落特征受地下水埋深变化的影响。但群落的组成成分和生物多样性受到地貌特征和地下水埋深的双重影响。群落的差异性和间断性大于群落的连续性和相似性。群落分布随地下水埋深的变化与其他荒漠地区在大的空间演替规律上相一致,但在群落类型、特别是低水位地段群落演替序列上有明显不同。随着地下水埋深的变化,植物群落依次为:低水位的骆驼刺群落,较低水位的柽柳群落,中低水位的柽柳、胡杨群落,较高水位的芦苇群落,高水位的苦豆子群落,其中地下水埋深6.0 m以上地段是植物群落类型、种类组成和生物多样性都较为丰富的地区。绿洲外围植物中柽柳具有最大的环境资源利用比率,对环境的适应性最强。 相似文献
8.
To understand the effects of grazing activities and climate change on sandy grassland ecosystems in northern China, a livestock field grazing and enclosure experiment was conducted from 1992 to 2006 in Horqin Sand Land, Inner Mongolia. The results showed that sustained heavy grazing resulted in serious degradation of the vegetation; moderate grazing can maintain vegetation stabilization; and light grazing can promote rapid restoration of degraded vegetation. The livestock productivity was the highest in the moderate grazing grassland, and sustained heavy grazing resulted in rapid decrease of the livestock productivity. Heavy grazing can cause a retrogressive succession of grassland vegetation, whereas moderate and light grazing may promote progressive succession of plant species. The effects of changing climate on succession processes were not significant in the short term; a warm-humid climate is favorable to restoration of degraded vegetation, whereas a sustained warm-drought climate may result in degradation of grassland vegetation. Heavy livestock grazing should be stopped for the sustainable use of grassland; the proper grazing intensity for sandy grassland is two to three sheep or sheep equivalents per hectare in Inner Mongolia. 相似文献
9.
Patagonia grasslands are subjected to two main disturbances, fire and grazing, but little information is available about its effects on vegetation. We studied post-fire survival and resprouting ability of two dominant grass species, Stipa speciosa and Festuca pallescens, for four years; evaluated the effect of early post-fire defoliation on both species; and tested whether competition is important in post-fire recovery in San Ramón Ranch, NW Patagonia (Argentina). To simulate grazing, a clipping treatment was applied at the beginning and end of growing seasons. Survival rates were high (>60%) and, after three years, biomass of both species in the burned area was similar to the unburned area. Competition seems to play an important role in the early post-fire recovery of both species, particularly in the case of F. pallescens that increased 87% the biomass production without competition. Fire may improve forage quality by eliminating the standing dead material, but early post-fire grazing might endanger the persistence of F. pallescens. We suggest requiring a resting period before livestock introduction and controlling grazing intensity. 相似文献
10.
11.
P.L. Sassi P.A. Taraborelli C.E. Borghi R.A. Ojeda 《Journal of Arid Environments》2009,73(4-5):537-541
Plant assemblages' dynamics have been studied to evaluate the influence of different environmental factors. The aim of our work was to assess the effect of a disturbance in the form of livestock grazing on annual plants, in a South American desert. We tested the exclusion of cattle grazing by comparing the composition of annual plants in three major vegetation types within a MaB Reserve in the Monte Desert of Argentina, with those of an adjacent grazed field. Sampling was conducted in two consecutive years that differed in precipitation. We established three sampling sites within each vegetation type at the reserve and the grazed field. Transects were set to assess plant cover, abundance, and richness. Theoretically, changes in diversity are explained by changes in one of its components: species richness. Species richness of annual plants was not different between the grazed and ungrazed sites. However, plant cover and diversity were lower in grazed sites, whereas abundance increased. Owing to a strong species-specific effect, we propose that it is evenness the main parameter involved in diversity dynamics in the heterogeneous vegetation mosaic of the Monte desert. Finally, vegetation types (spatial heterogeneity) and rainfall regime (temporal heterogeneity) greatly interacted with grazing effects. We endorse the idea that rapid responses of annual plant assemblages to changes in rainfall conditions coupled with herbivore control, might result in a restoration pathway for degraded arid landscapes. 相似文献
12.
N. Van Rooyen G. J. Bredenkamp G. K. Theron J. du P. Bothma E. A. N. Le Riche 《Journal of Arid Environments》1994,26(4)
Distance from artificial watering points was not significantly related to plant species composition in six habitats studied in the Kalahari Gemsbok National Park. Soil moisture had a significant effect on plant species composition of the riverbed habitat. Grazing intensity contributed significantly to trends in the vegetation composition of the river terrace and shrub savanna habitats, but grazing intensity was not associated with distance from artificial watering points. However, the analyses failed to show any significant influence of grazing intensity and distance from watering points on the plant species composition of the pan, savanna dune street and dune crest habitats. Therefore, although changes in plant species composition occurred in the six different habitats in the Kalahari Gemsbok National Park, they were not related to the presence of artificial watering points. 相似文献
13.
J.C. Guevara E.G. Grünwaldt O.R. Estevez A.J. Bisigato L.J. Blanco F.N. Biurrun C.A. Ferrando C.C. Chirino E. Morici B. Fernández L.I. Allegretti C.B. Passera 《Journal of Arid Environments》2009,73(2):228-237
This article reviews and analyzes the available information on range and livestock production in the Monte Desert. Cow–calf operations, goats for meat, and sheep for wool are the dominant production systems under continuous grazing. Rest-rotational grazing systems improved the efficiency of the current cow–calf production. Forage resources are primarily composed of perennial grasses and woody species. Rain-use efficiency for the total vegetation ranged from 3.9 to 4.8 kg DM ha?1 year?1 mm?1. Carrying capacity showed a broad range: 18.7, 4.5–64.5, and 21.6–89.3 ha AU?1 in the north, central, and south portions of the Monte, respectively. Mean crude protein (CP) content of grasses varied from 8.4 to 10.3 (wet season) and 7.1–3.7% DM (dry season) in the central west and Patagonia, respectively. Grasses predominated in the cattle diet, while the sheep diet was highly diverse because they ate most of the available plant species, and there was no unanimity as to the fact that goats are strictly browsers. Livestock diseases have lower prevalence indices than those recorded in other areas of the country. The high variability in carrying capacity values could be attributed to differences in rangeland condition and to the different methods used for its estimation. The CP levels in forage could meet cattle requirements provided that a proper-stocking rate were used. The most promising species for land rehabilitation are Opuntia, Atriplex spp., Eragrostis curvula and Cenchrus ciliaris. Priorities for future research should include topics such as assessment of the carrying capacity for most of the areas and nutrient content of the components of livestock diet, the livestock intake values, the economic feasibility of the use of complementary feeds and the development of seeding technology for valuable forage resources as Trichloris crinita, among others. 相似文献
14.
Sheep herd sustainability in Patagonia is limited by reproductive efficiency. As the herds graze on natural vegetation year round, efficiency is constrained by biophysical factors. Our aim was to characterize the environmental controls over effective lambing rate in northwest Patagonia. We related remotely sensed data [surface temperatures and Normal Difference Vegetation Index (NDVI), a surrogate for forage availability] and climatic information to lambing rates. A spatial model explained 63% of the variability in mean lambing rate, based on mean annual NDVI and mean spring surface temperature. The relative inter-annual variability in lambing rates was of similar magnitude to that of annual NDVI. Both lambing rate and NDVI were less variable than climatic factors. Two regional temporal models explained approximately 25% of the variability, based on NDVI values representative of ewes’ nutrition during late gestation and lambing. These models predicted, with 65% and 76% accuracy respectively, the occurrence of better-than-average and worse-than-average yearly lambing rates. These results provided insights into the factors limiting reproduction, and they form the basis for important decision tools for ranch managers. Prediction of lambing rates based on remotely sensed data will reduce the uncertainty and risk in sheep production, increasing the economic sustainability of sheep production. 相似文献
15.
内蒙伊盟牧场利用与沙漠化及其防治 总被引：1，自引：0，他引：1
本世纪七十年代以来, 沙漠化问题在世界范围内引起了广泛的注意。沙漠化对社会生产和人类的生活与生产是一个严重的威胁。内蒙古自治区伊克昭盟是一个沙漠化比较严重的地区, 迄一九七七年, 全盟流沙面积达27, 666平方公里, 比解放初扩大1.3倍。伊盟牧业在国民经济中比重很大, 牧区约占全盟面积72%。在沙漠化成因中, 牧场的利用不合理是一个主要的方面。因此, 研究牧场利用与土地沙漠化的关系, 总结牧民合理利用牧场的经验, 并提出防治沙漠化的途径, 对于解决这个地区的沙漠化问题, 是十分重要的。 相似文献
16.
Anne Elisabeth Bjune 《Polar research》2000,19(2):183-192
Analysis of the pollen and spore content of 40 fresh samples of faeces collected from Svalbard reindeer during 1994/5 and 1996 shows variations among and within seasons, with distinct species/pollen types dominating each season. Winter samples were characterized by high amounts of Salix and moss spores , while spring samples contained decreasing amounts of the plants grazed in winter and increasing values of species grazed in summer, such as Oxyria digyna and Pedicualaris spp. Summer samples had a large quantity of pollen and a great number of pollen types. Autumn samples indicated that grasses are the most important forage species in this period. The seasonally varying proportions of pollen types are related to such factors as plant phenology, abudance, palatability and nutritional quality, as well as the prefereence of reindeers for grazing in low, wet areas in the summer but on higher, wind-blown ridges during the winter. The results mostly confrim other studies of Svalbard reindeer diet and grazing behaviour. The advantages and limitations of the pollen analysis method for such investigations are discussed. 相似文献
17.
The vegetation of riparian habitats is often distinct from that of the surrounding landscape, thus representing unique habitat for a variety of biota. Although highly mobile, birds often exhibit distinct species assemblages associated with habitat. Therefore, degradation or removal of riparian habitat, particularly in arid environments, may threaten bird diversity. Along the Vaal River, South Africa, mining and agriculture have reduced natural riparian habitat to ca. 9% of its former extent in the Northern Cape Province. We surveyed bird assemblages within intact riparian, savanna and bush-thickened habitats along the Vaal River to ascertain their importance to bird diversity. Avian abundance and species richness did not differ between the three habitats. Species composition of riparian bird assemblages was significantly different to that of savanna and bush-thickened habitats, however, which were not significantly different from each other, and more species were characteristic of riparian habitat (17 species) than bush-thickened (seven species) or savanna (one species) habitats. Of three species reaching the south-western limit of their African distribution, all occurred in riparian habitat. Thus, despite its fragmented nature, the riparian vegetation on the Vaal still supports an important component of avian diversity, and a landscape-level approach is required to manage this relatively rich, arid-land, river. 相似文献
18.
《Journal of Arid Environments》1999,41(1):27-35
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
19.
How species diversity–productivity relationships respond to temporal dynamics and land use is still not clear in semi-arid grassland
ecosystems. We analyzed seasonal changes of the relationships between vegetation cover, plant density, species richness, and aboveground
biomass in grasslands under grazing and exclosure in the Horqin Sandy Land of northern China. Our results showed that in
grazed and fenced grassland, vegetation cover, richness, and biomass were lower in April than in August, whereas plant density
showed a reverse trend. Vegetation cover during the growing season and biomass in June and August were higher in fenced grassland
than in grazed grassland, whereas plant density in April and June was lower in fenced grassland than in grazed grassland. A negative
relationship between species richness and biomass was found in August in fenced grassland, and in grazed grassland the relationship
between plant density and biomass changed from positive in April to negative in August. The relationship between the density of the
dominant plant species and the total biomass also varied with seasonal changes and land use (grazing and exclosure). These results
suggest that long-term grazing, seasonal changes, and their interaction significantly influence vegetation cover, plant density, and biomass
in grasslands. Plant species competition in fenced grassland results in seasonal changes of the relationship between species richness
and biomass. Long-term grazing also affects seasonal changes of the density and biomass of dominant plant species, which further
affects the seasonal relationship between plant density and biomass in grasslands. Our study demonstrates the importance of temporal
dynamics and land use in understanding the relationship between species richness and ecosystem function. 相似文献
20.
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