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1.
A prescribed burn resulted in significant decreases in canopy cover of the grasses: Bouteloua eriopoda, Sporobolus flexuosus, and Aristida purpurea. One year post-burn, basal cover of B. eriopoda remained significantly lower in burned patches than in unburned areas but there were no differences in basal cover of the other perennial grasses. Only one species of the 14 summer annual species occurred in both burned and unburned plots. There were six species of spring annuals in burned patches but no spring annuals in the unburned grassland ten months post-burn. Fire killed 100% of the snakeweed shrubs (Gutierrezia sarothrae), 77% of the Ephedra torreyana shrubs, and 36% of the Yucca elata. All mesquite shrubs that were top-killed by fire, resprouted one month post-burn. Fire had no effect on abundance and species richness of rodents. There were fewer wolf spider, Geolycosa spp. burrows in burned areas than in unburned grassland. The area and volume of soil in termite galleries and sheeting were significantly larger in the unburned grassland than in the burned areas.  相似文献   

2.
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   

3.
Soil disturbance is a wildlife habitat management tool that retards succession and promotes early seral vegetation. Our objective was to determine responses of two invasive herbaceous species (Pennisetum ciliare and Salsola iberica) and native perennial grasses to disking on different soils. Two 10 ×40 m plots were delineated within each of 4 blocks on Ramadero loams and 4 blocks on Delmita fine sandy loams. On Delmita soils, canopy cover of P. ciliare, S. iberica, and native perennial grasses averaged across all years was not affected by disking (ANOVA, P>0.05). On Ramadero soils, P. ciliare canopy cover was similar (Tukey's, P>0.05) on control and disked plots for the first 4 years post-disturbance, but P. ciliare cover was 10-fold greater (Tukey's, P=0.02) the 5th year after disking on disked versus control plots. On Ramadero soils, S. iberica canopy cover averaged across all years was 221 times greater (ANOVA, P=0.05) on disked plots than on control plots. Disking did not affect native perennial grass canopy cover. Land managers should consider soil series when disking for wildlife management, as disking disturbance may exacerbate exotic plant ingress and establishment on certain soils.  相似文献   

4.
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   

5.
Restoration treatments are based on the largely untested notion that desired recovery of plant communities following disturbance wouldn’t occur in the absence of active intervention. We identified rate of short-term (10 year) floristic changes following removal of plant functional groups in Wyoming big sagebrush plant communities in 1999-2005 and 2008. Treatments imposed on 6 × 6 m plots were: 1) removal of all plant functional groups, 2) perennial grass removal, 3) shrub removal and 4) control. Our data suggest recovery of the shrub component on shrub removal plots could take decades. Similarly, perennial grass cover and density on perennial grass removal plots was less than half that of unaltered plots 10 years after treatment. When all functional groups were removed, cover of annual forbs, annual grasses, and shrubs returned to unaltered levels within ten years or less. Perennial forbs were unaffected (p > 0.05) by treatment. The fact that natural recovery of some components occurred within a relatively short post-disturbance time interval (i.e. <10 years) suggests that intervention may not be necessary for some functional groups. Restoring shrubs in areas dominated by perennial grasses may require targeted reductions of competing perennial grasses. Conversely, shrub dominance may limit perennial grass re-establishment.  相似文献   

6.
Restoration of palatable grasses on degraded rangelands dominated by unpalatable grasses in central Argentina is limited by low availability of seeds and safe sites for seedling establishment. The objective of our study was to determine how mechanical disturbance of unpalatable grasses (Stipa spp.) in combination with seeding of a palatable grass (Poa ligularis) influenced species composition in a degraded rangeland excluded from livestock grazing. In April 2001 10 blocks were uniformly distributed on a previously burned site dominated by unpalatable grasses, and treatments applied in 8 m×8 m experimental plots. Treatments were ‘disked and seeded’ and control (no disking, no seeding). Perennial plant cover and end-of-season standing crop, at species or species group level, were assessed in December 2002/2003 and in December 2004/2006, respectively. P. ligularis out-competed both tillers of unpalatable grasses that survived mechanical disturbance and seedlings of unpalatable grasses established after mechanical disturbance. The cover and end-of-season standing crop of unpalatable grasses was higher in the control than in the ‘disked and seeded’ treatment, whereas the cover and end-of-season standing crop of P. ligularis was higher in the latter than in the former treatment. Our results suggest that a rapid transition from a state dominated by unpalatable grasses to a state dominated by palatable grasses can be achieved by mechanical disturbance of unpalatable grasses in combination with seeding of palatable grasses in semiarid rangelands of central Argentina.  相似文献   

7.
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   

8.
Rates and impacts of atmospheric nitrogen (N) deposition are poorly understood in arid land ecosystems where soils are typically low in plant available N. To address this issue, we quantified long-term trends in N deposition and estimated its impact on plant community structure in the northern Chihuahuan desert of Central New Mexico, USA. Annual and seasonal rates of N deposition were strongly positively correlated with precipitation. When precipitation effects were removed statistically, N deposition increased at an annual rate of 0.049 kg ha−1 yr−1 between 1989 and 2004. Based on two independent fertilization studies at our desert grassland field site, continued atmospheric inputs are likely to increase grass cover, decrease legume abundance, and may favor blue grama (Bouteloua gracilis) at the expense of the current dominant species, black grama (Bouteloua eriopoda). We conclude that, although arid lands have low rates of N deposition and are primarily water limited, observed trends in N deposition rates may lead to significant changes in plant community structure.  相似文献   

9.
Encroachment of the shrub Prosopis glandulosa Torr. (honey mesquite) into semi-arid grasslands is a serious concern in the south-western United States, yet little is known about the long-term dynamics of the invasion process. We used ten high-resolution aerial and satellite images taken from 1936 to 1996 to track the population dynamics and spatial pattern of all P. glandulosa greater than 2 m in diameter on a 75 ha area in southern New Mexico.Shrub cover and patch numbers increased from 1936 to the 1970s, then stabilized at 43% cover and 83 patches ha−1. Individual patches were extremely persistent: 95% of the area occupied by shrub patches in 1936 was still occupied in 1996. Recruitment into the 2 m size class was more variable: 0·6–5·2% year−1 (mean 0·8% year−1). Patch-shape complexity increased from 1936 to 1983 as adjacent shrubs merged, and then declined as those clusters filled in and became rounder. Spatial pattern of shrubs showed a distinct trend over time: strongly clustered in 1936 at lag distances up to 250 m, then random arrangement at all scales, and by 1983 pattern was regular at lag distances greater than 100 m. There was no clear relationship with precipitation.The use of remote sensing imagery allowed us to examine one site over time, and revealed patterns in population dynamics and spatial pattern that would not have been visible otherwise. Comparison of field estimates collected in 2001 with 1996 image data suggest that the canopy cover estimates were accurate, but shrub densities were seriously underestimated in the satellite photographs, which do not show shrubs smaller than 2 m diameter. As long as limitations of the imagery are understood, these methods can be applied over a larger and more heterogeneous area to examine environmental correlates of invasion success.  相似文献   

10.
This study was designed to test hypotheses about the combined effects of short-term, seasonal grazing with seasonal drought, fire, and carbon enrichment on soil microarthropod communities in a Chihuahuan Desert grassland. The study was conducted in eighteen 0.5 ha plots following three consecutive years of treatment: six plots intensively grazed in summer, six in winter, and six not grazed. There was no difference in perennial grass cover on the summer-grazed and winter-grazed plots. Intensive seasonal grazing had no effect on the abundance and community composition of soil microarthropods. Within each plot there were six subplots: summer rain-out, winter rain-out, burned, glucose amendment, rain-out control and burn-glucose control. Fire and carbon enrichment had no significant effect on soil microarthropod abundance or community composition. The average number of microarthropods ranged from 8915 ± 1422 m−2 in the ungrazed, unburned plots to 7175 ± 1232 m−2 in the winter-grazed, unburned plots. Microarthropod densities in the glucose-amended plots were 8917 ± 4902 m−2 in the winter-grazed plots and 10,731 ± 863 m−2 in the glucose-amended, summer-grazed subplots.The prostigamatid mite, Tydeus sp., was the most abundant microarthropod taxon in all treatment plots.  相似文献   

11.
Grazing effects on patchy dryland vegetation in northern Patagonia   总被引：1，自引：0，他引：1  
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   

12.
Fire is an important driver of ecological pattern and process in grasslands worldwide, although its role in semi-arid systems is less well known. We used published studies and new experimental research to 1) provide a synthesis of existing knowledge of fire in the semi-arid grasslands of the North American Great Plains, and 2) assess the degree of similarity in semi-arid and mesic grassland responses to fire in this region. Based on published studies, burning has neutral to negative effects on aboveground productivity in semi-arid grasslands and variable effects on plant communities. To more rigorously assess fire effects, replicated experimental plots were established in ungrazed shortgrass steppe in northern Colorado and prescribed spring fire was applied in 2006 and 2007, 2006 only, or not at all. Aboveground net primary productivity decreased or remained unchanged with burning. Plant community changes included increases in perennial forbs, decreases in annual grasses and a positive response in annual forbs to the combination of fire and a wet spring in 2007. Combined, these results indicate that post-fire changes in productivity in semi-arid grasslands are neutral to negative, in contrast to positive responses in mesic grasslands, and not strongly negative as previously assumed.  相似文献   

13.
Vegetation changes in the Jornada Basin from 1858 to 1998   总被引：1，自引：1，他引：1  
Notes made by land surveyors in 1858 were utilized to estimate cover of grasses and shrubs on the Jornada Experimental Range (JER) and the Chihuahuan Desert Range Research Center (CDRRC) in the northern Chihuahuan Desert in southern New Mexico, USA. Portions of these areas have been previously assessed for historical vegetation dynamics but the entire 84,271 ha assessed in the 19th century has not been examined in total. In 1858, fair to very good grass cover occurred on 98% and 67% of the JER and CDRRC, respectively. Shrubs were present throughout both properties but 45% of the JER and 18% of the CDRRC were shrub free. Reconnaissance surveys, made to determine carrying capacity for livestock were made in 1915–1916 and 1928–1929 on the JER and in 1938 on the CDRRC, show that shrubs had made large increases in area occupied at the time of the surveys. Vegetation type maps were made of both properties in 1998. Mesquite (Prosopis glandulosa) was the primary dominant on 59% of the JER in 1998 and creosotebush (Larrea tridentata) was the primary dominant on 27% of the area. On the CDRRC mesquite and creosotebush were primary dominants on 37% and 46% of the area, respectively. Grass cover has decreased greatly with the increase in shrubs and only shrub control efforts have maintained the once abundant black grama (Bouteloua eriopoda) as a primary dominant on 1% or less of the area on both properties.  相似文献   

14.
The effect of intensive grazing and browsing of domestic stock and wildlife on the number of species and abundance of vegetation was investigated in the Hoanib River catchment, north-western Namibia. The seasonal abundance of ground cover, bare earth, canopy cover, annual grass, perennial grass and annual forbs were measured in each of the focus-study areas. In three of the focus areas where the ranges of both domestic stock and wildlife were restricted either by fencing or water availability, impact on the vegetation was greatest. The final focus area was a more ‘open range’ system that allowed for the free movement of wildlife. Under these conditions the species abundance and availability of browsing and grazing was greater than the other focus areas during both the wet and dry seasons. However, very little difference in abundance and availability of vegetation was observed between focus areas in both seasons regardless of landuse. There is generally a low abundance of perennial grasses and browse species affording the ecosystem little resistance and resilience to disturbance caused by grazing and drought.  相似文献   

15.
Small-scale vegetation dynamics were followed for ecotones and in uniform stands inArtemisia-dominated steppe vegetation under grazing and when recovering from heavy grazing. Species composition was followed annually for 5 years in 1 m2and 0·25 m2plots for (1) presence–absence; (2) density; and (3) biomass.More rapid vegetation dynamics, in terms of change of type of vegetation and distance moved in DCA species space between sampling occasions, were observed in smaller plots and in early seral stages, where species were few and alpha diversity low. Only the plots recently protected from grazing showed a directional vegetation change; those protected for more than 3 years and those under continued grazing showed, at this scale, non-directional dynamics.  相似文献   

16.
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   

17.
The effect of fire on growth of Piptochaetium napostaense, Stipa tenuis, and Stipa gynerioides, three important native perennial grasses in the semi-arid region of central Argentina, was evaluated under different fire temperature regimes: 300–400°C (low temperature regime), 500–600°C (high temperature regime) and no fire (control). Fire treatments were applied with a portable propane plant burner in April and December 1994, May 1995, and January 1996. Overall results indicate that during the first months after fire occurrence, average total green length of S. tenuis, P. napostaense and S. gynerioides tillers was severely reduced (p<0.05) by fire. This effect was more pronounced in plants burned with the high temperature treatment. The observed patterns of response to fire for height of tillers were very similar to those already reported for total green length of tillers. Towards the end of each growth cycle, the number of green leaves per tiller of burned plants of P. napostaense, S. tenuis, and S. gynerioides were similar or greater (p<0.05) than the number of green leaves on tillers of control plants. Relative growth rates for total green length and for height in tillers of burned plants of S. tenuis, P. napostaense and S. gynerioides were greater than in tillers of control plants. Our results indicate that fire affected differentially the growth of the studied species; S. gynerioides was more affected by fire than P. napostaense and S. tenuis. The species most tolerant to fire was P. napostaense.  相似文献   

18.
Decomposition of various litter forms including dead tussocks of two native perennial grasses, woollybutt (Eragrostis eriopoda) and mulga mitchell (Thyridolepis mitchelliana), as well as roots of woollybutt, dung of sheep and kangaroo, and bleached toilet rolls, was studied in contrasting grazing exclosures, half of which had termites excluded by biocide (termiticide) treatment. Dead mulga mitchell tussocks decayed more rapidly than woollybutt tussocks during the first 17 months post mortem. Thereafter, rate of decay differed little between species. After 3 years, only small amounts of tussock residues of either species remained and only then did the impact of biocide treatment become significant.Decomposition of kangaroo pellets was typically bimodal with significantly higher decomposition recorded in the controls (no biocide) up to 40 months after treatment, and many intact pellets remaining in the biocide-treated plots. While decomposition of sheep pellets showed similar bimodality, decomposition remained significantly lower in the biocide treatments for the entire duration of the experiment.Results suggest that abiotic processes, including those induced by UV radiation, may be dominant influences mediating decomposition of litter in these semi-arid ecosystems, especially following high-rainfall seasons when abundant grass biomass has been generated providing a surfeit of potential forage for harvester termites.  相似文献   

19.
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   

20.
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   

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