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1.
We studied the effect of ant herbivory on the establishment and survival of the annual plant Schismus barbatus. We hypothesized that ants may control this species when biomass of native plants is generally low during their vegetative period. We predicted that ant herbivory will decrease plant survival and reproduction. We tested our prediction with an insect exclosure experiment in a sandy desert of Northern-Central Monte.We found more than 12 000 established seedlings per square meter on early May, after two consecutive rain pulses of ca. 20 mm each. Overall, we found that almost 75% of recruited plants survived by the end of the cool season (September), and that 22–24% of the initially established plants survived as mature reproductive plants by the end of the growing season (December). Contrary to our expectation, insect herbivory did not affect plant establishment, plant survival or the proportion of flowering and fruiting individuals of S. barbatus.The large number of seedlings reported, the ability to exploit a temporal window free of plant competitors and enemies, and the availability of microsites where this species can succeed, all suggest that S. barbatus may have the potential to become an important plant invader in the Monte Desert.  相似文献   

2.
Understanding the role competition intensity and importance play in directing vegetation dynamics is central to developing restoration strategies, especially in resource poor environments. We hypothesized 1) competition would be intense among invasive and native species, but 2) competition would be unimportant in explaining variation in target plant biomass and survivorship relative to other factors driving these variables. We performed a two year addition series field experiment to quantify competition intensity and importance. Densities of two invasive (cheatgrass and medusahead) and two native (Sandberg’s bluegrass and bluebunch wheatgrass) species were arranged in monocultures and mixtures of two, three and four species, producing varying total densities and species proportions. Multiple linear regression models predicting individual plant biomass and survivorship were developed. Based on biomass, competition intensity coefficients ranged from −0.38 to 0.63 with R2 < 0.06. All survivorship data produced poor fitting regression models (R2 < 0.05). Our results suggest neither competition intensity nor importance influenced plant dominance in resource poor environments during the two years of establishment. Land managers may be more successful at restoration of resource poor ecosystems by overcoming abiotic barriers to plant establishment rather than focusing on plant-plant interactions.  相似文献   

3.
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   

4.
A field experiment was conducted to test the hypothesis that competitive interactions affect demographic responses of desert dune annuals to gradients in sand stability. The experiment was based on a three-way factorial design with two types of neighbor treatments (complete neighbor removalvs. control), two types of habitats (stablevs. unstable sands) and two species which characterize sandy areas of the western Negev Desert (Neurada procumbensandBromus fasciculatus). Each of the eight treatment/habitat/species combinations was characterized by three demographic variables: seedling survival to reproduction, fruit number per reproductive plant and fruit number per seedling. All of these variables exhibited significant interactions between the effects of neighbor removal and habitat type, indicating that neighbor conditions were important in determining the observed patterns of demographic variation. Moreover, while plants ofNeurada procumbensgrowing without neighbors suffered more mortality on unstable sands, conspecific plants growing at natural density conditions suffered more mortality on stable sands. In the case ofBromus fasciculatus, individuals growing in the absence of competitive effects produced more fruits on stable sands, but conspecific plants growing at natural density conditions exhibited higher fruit production on unstable sands. These findings indicate that competitive effects not only modify, but may even reverse demographic responses of desert dune annuals to gradients in sand stability. Based on these results and evidence from two other studies it is suggested that seed dispersal from abiotically favorable into less favorable habitats may increase fitness of desert annuals by reducing the average effect of competition.  相似文献   

5.
This paper explores relations between assemblages of carpological remains and vegetation in and around a small, shallow reservoir in southern Poland. The study was conducted from 2006 to 2008. Quantity and distribution of species in the reservoir were recorded annually during the growing season. In October 2008, 40 samples of surface sediment (top 2?cm) were collected along transects at 10?m intervals. Samples of 100?cm3 were prepared for analysis of plant macroremains. Assemblages of carpological remains generally reflect local vegetation well. In some cases, however, even analysis of numerous samples failed to fully capture the species composition or reflect plant ratios in the parent phytocenosis. Reasons for this include factors that affect seed production, transport and fossilization, which differ among species. Among the best-represented macroremains were plants of the rush phytocenosis. In analysed samples, macroremains of 68.8?% of extant rushes were identified. Sixty percent of submerged and floating-leaf taxa were found in carpological samples, whereas 26.7?% of the trees and bushes were represented in sediment deposits. Species composition of phytocenoses in the reservoir and in surrounding areas was best reflected by macroremains from the nearby reed bed. Numbers of diaspores of Mentha aquatica, Hippuris vulgaris and Carex reflected well their relative abundance in phytocenoses. Chara sp., Juncus inflexus and Eupatorium cannabinum were overrepresented, whereas Typha latifolia and Sparganium minimum were poorly represented in relation to contemporary plant cover. There were no diaspores of Phragmites australis, which dominates the contemporary reed bed. Besides the shape of a reservoir, the key factor influencing diaspore numbers is distribution of plant cover. In many cases, single diaspores (Potentilla erecta, Myosotis scorpioides, Lythrum salicaria, Scutellaria galericulata), or higher concentrations (Hippuris vulgaris, Mentha aquatica, Eleocharis palustris, Schoenoplectus tabernaemontani, Chara sp.) reflected well the location of parent vegetation. The findings indicate that carpological remains in sediments can be an important source of information about plants in and around lakes. They generally reflect well local vegetation and in some cases may be used to identify taxa that dominated in the past.  相似文献   

6.
Regression equations were developed to estimate above ground biomass and carbon and nitrogen mass of foliage and stem size fractions from plant size dimensions (basal diameter, canopy area, height, canopy volume) for a tall shrub species (Prosopis velutina) that has increased in abundance in arid and semi-arid grasslands in the southwestern United States and northwestern Mexico. Regression equations were also developed to describe relationships among the dimensions of plant size. All equations were significant (p < 0.001); and all but two had r2 values >0.72. In addition to species-specific information, we found support for the global patterns of foliar biomass increasing to the ¾ power of stem biomass and height increasing to the ½ power of stem diameter. We provide a comprehensive report of all equations, which can support a variety of in situ (ground-based), modeling, and remote-sensing objectives related to quantifying changes in ecosystem function and carbon sequestration accompanying changes in woody plant abundance. We advocate that comprehensive reporting should become more common for arid and semi-arid woody species in order to support a broad spectrum of users while laying the foundation for the development of global generalizations similar to those available for forest trees.  相似文献   

7.
Light may be an important limiting resource that influences community structure of chenopod shrublands. As part of a larger study that aimed to determine the factors that influence chenopod community structure, the focus of this study was the influence of plant canopy on the growth and establishment of smaller plants. We therefore measured the height and cover of three chenopods (Enchylaena tomentosa, Maireana brevifolia and Maireana georgei) when growing within and outside of the canopy of Atriplex bunburyana under field conditions. All three chenopods had lower cover and E. tomentosa was taller when growing within the canopy of A. bunburyana in comparison to those growing outside of the canopy. The chenopods were then grown under three artificial shade regimes. Plant height, cover, biomass, relative leaf area and photosynthetic surface area measurements showed that each species responded differently to shade. E. tomentosa biomass was facilitated by shade. This was inferred by an increase in total plant biomass. M. brevifolia, in contrast, tolerated shade by increasing above-ground biomass allocation. M. georgei was adversely affected by the shade regimes: root biomass decreased in response to shade. Competition for light is, therefore, likely to influence chenopod community structure of semi-arid and arid environments.  相似文献   

8.
In semi-arid regions of North America and Australia, Pennisetum ciliare (L.) Link (syn. Cenchrus ciliaris; buffelgrass) is highly invasive and has the potential to introduce fire to fire-intolerant ecosystems. Major efforts to remove P. ciliare continue and it is essential that P. ciliare be prevented from recolonizing. This study investigated potential methods to revegetate with native herbaceous plants: sowing seeds; sowing seeds and mulching; sowing seeds and transplanting seedlings; and relying on natural revegetation from the seedbank. The treatments were applied in 2009 and 2010 at sites in the Sonoran Desert which had undergone P. ciliare removal. Monsoon precipitation was below average each year and seedling emergence and establishment rates were low. There are indications that soil disturbance associated with planting seedlings promoted P. ciliare emergence and increased mortality of brittlebush (Encelia farinosa A. Gray ex Torr.), a common native perennial forb. Addition of mulch may have promoted P. ciliare over native grasses, and seeding had no effect. We did not find competition between herbaceous seedlings. Rather, native and exotic grass seedling densities were positively correlated across sites. Under prevailing conditions, low precipitation appeared to limit herbaceous plant establishment and none of the treatments reduced P. ciliare abundance.  相似文献   

9.
Chronic wind is an important ecological factor, but its direct roles in shaping plant communities remain poorly understood. We hypothesized that chronic wind can modulate community productivity, inter-specific competition, and species abundance in inland dunes. We conducted an experiment with three shrubs (Artemisia ordosica, Caragana intermedia, and Hedysarum laeve) common to semi-arid sandlands, set up seven kinds of plant communities (i.e. Artemisia monoculture, Caragana monoculture, Hedysarum monoculture, Artemisia–Caragana mixture, Artemisia–Hedysarum mixture, Caragana–Hedysarum mixture, and Artemisia–Caragana–Hedysarum mixture), and communities subjected to two levels of wind exposure: shielded (by means of fencing) or exposed (no fencing). We measured total biomass per plot, competitive effects, and relative species abundance. Wind exposure did not significantly affect the total biomass of monocultures but increased their root weight ratio. However, wind exposure enhanced the total biomass of three-species mixtures but not two-species mixtures, and had no effects on root weight ratio of all mixtures. Wind exposed condition increased the competitive ability and relative abundance of Artemisia, decreased the competitive ability of Hedysarum but had no effects on its abundance, and did not affect the competitive ability of Caragana but decreased its abundance. These results suggest that chronic wind, as an environmental filter, can directly modulate plant communities through altering competitive outcomes and thus affect community functioning.  相似文献   

10.
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   

11.
Throughout the western United States, the invasive annual grass, medusahead (Taeniatherum caput-medusae L. Nevski), is rapidly invading grasslands once dominated by native perennial grasses, such as bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A). It is also invading grasslands dominated by less undesirable invasive annual grasses, especially cheatgrass (Bromus tectorum L.). Understanding medusahead growth dynamics relative to native perennial grasses and cheatgrass is central to predicting and managing medusahead invasion. We hypothesized that medusahead would have a higher relative growth rate (RGR), a longer period of growth, and as a consequence, more total biomass at the end of the growing season than the native perennial grass and cheatgrass. In 2008 (dry conditions), 250 seeds and in 2009 (wet conditions), 250 and 100 seeds of each species were sown in 1 m2 plots with 5 replicates. Shoots were harvested on 3-25 day intervals throughout the growing season. The native perennial grass had more biomass and higher RGR than medusahead in the dry year, but the relationship was reversed in the wet year. Precipitation in 2008 was well-below average and this level of drought is very infrequent based on historical weather data. Medusahead had a longer period of growth and more total biomass than cheatgrass for both years. We expect that medusahead will continue to invade both native perennial and less undesirable invasive annual grasslands because of its higher RGR and extended period of growth.  相似文献   

12.
In desert ecosystems, selective foraging by seed consumers affects distribution and composition of soil seed banks, influencing plant population dynamics. However, the roles of consumers in burned habitats where direct seeding is used during ecological restoration to replenish depleted seed banks have not been well established. We evaluated seed removal for nine seed species over 12 months in burned and unburned shrublands in the Mojave Desert, USA. Percentage of total seed removed was highest during spring (16% of offered seed) and summer (21%). Rodents removed much of the large-seeded Coleogyne ramosissima in both burned and unburned habitats, while seed removal of this species by ants was low in burned and moderate in unburned habitats. Ants removed the greatest amount of small-seeded species (Penstemon bicolor, Encelia farinosa, and Sphaeralcea ambigua) in unburned habitat, indicating that ants can exploit different seed masses. Seed removal imposes limitations on seed availability, particularly for large-seeded species, as both rodents and ants selected seeds of C. ramosissima. Successful restoration seeding projects in arid lands may require protecting seed from granivore pressure, and seed species selection and season of seeding warrant consideration to reduce seed loss.  相似文献   

13.
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   

14.
We investigated the influence of linear habitats, three types of hedgerows and two types of field margins, on the small mammal community within a semi-arid agricultural landscape in south-central British Columbia, Canada. We examined whether abundance, species richness, and species diversity of small mammals in tree fruit orchards differed from linear, edge habitats along orchard margins. Our five edges spanned grassy to wooded habitats. Linear habitats were intensively sampled for vascular plants in 2003 and small mammal communities from 2003 to 2007. Vegetation and total abundance of small mammal species among three types of hedgerows were similar and population changes followed those within nearby apple orchards. Species richness and diversity of small mammals, however, were significantly higher in hedgerows than orchards. Fewer mammals occupied hedgerows with high volumes of herb and shrub biomass, but richness and diversity of mammals did increase with shrub volume. All seven species of small mammals were represented in the hedgerow communities, including two species at risk: the Great Basin pocket mouse Perognathus parvus and the western harvest mouse (Reithrodontomys megalotis). Two types of field margins (orchard-sagebrush and orchard-old field) along fencerows provided habitat for P. parvus. Field margins managed for herbaceous plant species along fencerows and other such linear configurations provided habitat for R. megalotis. Linear non-crop habitats should maintain the overall small mammal community, as well as the two species at risk, within this semi-arid agricultural landscape.  相似文献   

15.
Community composition of arbuscular mycorrhizal (AM) fungi was analyzed in the roots of five representative shrub species (Genista cinerea, Lavandula latifolia, Rosmarinus officinalis, Thymus mastichina and Thymus zygis) growing in a typical semi-arid Mediterranean ecosystem. Roots from a well-preserved area of the ecosystem were extracted from soil and analyzed by nested PCR, single strand conformation polymorphism and sequencing of the NS31-AM1 and NS8-ARCH1311 regions of the small subunit of the ribosomal DNA (SSU rDNA). Ten AM fungal phylotypes were identified; eight belonged to the Glomeraceae and two to the Diversisporaceae. Only two of the phylotypes clustered with sequences of morphologically defined species and a high dominance by one AM group (Glomus intraradices) was detected. Our diversity analyses revealed that the AM fungal communities of G. cinerea, L. latifolia and T. mastichina did not significantly differ while the AM fungal communities of R. officinalis and T. zygis were distant from this cluster and from each other. The highest diversity was found in the roots of T. zygis. Our data indicate that co-occurring plant species may house distinct communities of AM fungi.  相似文献   

16.
The objective of this work was to evaluate the effects of post-fire defoliation on axillary bud viability and mortality of Piptochaetium napostaense (Speg.) Hack. and Poa ligularis Nees. at different phenological stages. Both species are cool-season perennial native grasses highly preferred by cattle in the semi-arid region of central Argentina. The following treatments were established for each species: no defoliation (control), early-season defoliation (vegetative stage) and late-season defoliation (internode elongation stage). Ten plants were assigned randomly to each treatment. Fire treatment was applied at the end of the annual growing cycle of both species.Our results would suggest that early-season defoliation during the first year after fire might reduce axillary bud viability in P. napostaense and P. ligularis. In addition, P. napostaense was more susceptible to early-season defoliation than P. ligularis. Post-fire defoliation, principally early-season defoliation, might increase plant mortality in P. napostaense, but it does not seem to affect P. ligularis. Therefore, a period with no grazing after fire might be the key factor to protect axillary buds, and subsequently permit the re-establishment of the photosynthetic canopy through the production of new tillers, favouring mainly the persistence of P. napostaense in the community.  相似文献   

17.
In Mediterranean ecosystems, fire influences plant population dynamics and changes plant community structure by affecting germination and establishment of seeder shrubs. Fabiana imbricata is a long-lived seeder shrub with a broad distribution in South America. In Northwestern Patagonia grasslands there are many records of F. imbricata recruitment after fires. We hypothesized that recruitment is promoted by: 1) soil erosion that exposes deeply buried seeds; 2) direct fire effects such as heat that could break seed dormancy; 3) indirect fire effects (i.e. increase of light availability and elimination of allelopathic substances) and 4) water availability in spring. In field conditions, we tested the effects of shading and soil disturbance on F. imbricata recruitment and seed availability and distribution in the soil profile. Under controlled conditions, we investigated the influence of leaf leachate, light, heat and water on seed germination and seedling emergence. Seed germination was inhibited by leachate, and seedling emergence was inhibited by seed heated to 80 °C and 120° for 5 min. F. imbricata seedlings only emerged in germination treatments that simulated wet and very wet spring conditions. Fire and postfire wind that favor seed exposure, combined with high precipitation early in the growing season, would be necessary for successful shrub recruitment. When F. imbricata shrubland colonizes the grassland, it strongly modifies local vegetation structure, reduces species richness and increases fuel loads. F. imbricata is a key species in the ecosystem specially related to fire regime. Information on the postfire regeneration of this species would provide valuable knowledge of the changes in biomass accumulation in a fire-prone Mediterranean ecosystem.  相似文献   

18.
The North African houbara bustard (Chlamydotis undulata undulata) is a medium-sized steppeland bird described as omnivorous and opportunistic. Its diet reflects the local abundances of plants and invertebrates. However, comprehensive studies based on both plant and animal materials are lacking to confirm or invalidate this characterization. The diet of the species during the non-breeding season was studied based on the contents of the stomachs of 42 adults collected during 2004, 2006 and 2007 in two regions of eastern Morocco. Taxa were identified to species or genus using a herbarium, a plant epidermis reference collection, an invertebrate reference collection, and an invertebrate fragment reference collection. We analyzed variation in the diet according to sex, region and year. The diet primarily included plant material (66 ± 25% of the dry weight of the stomach contents). The relative proportions of plants and invertebrates did not differ between the sexes but showed variations in space and time. The ingested plant and invertebrate assemblages differed between year and area but not between the sexes. These results suggest that both sexes forage in similar habitats and that their diet reflects spatial and temporal variations in plant and invertebrate availability, as expected for an opportunistic feeder.  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.
In arid environments, soil fertility exhibits a high degree of spatial and temporal heterogeneity, which results from high climatic variability seasonally and heterogeneous plant distribution. However, because most desert areas have been altered by human activities, heterogeneous fertility would originate from grazing or logging activities. We evaluated spatial and temporal heterogeneity of soil fertility in cattle-excluded sites under and outside woody plant cover (Prosopis flexuosa and Larrea divaricata), and in sites disturbed by tree removal during wet and dry season in Ñacuñán Biosphere Reserve (Central Monte desert of Argentina). Soil organic matter, fulvic acids, bioavailable organic matter, and nitrate were lower outside plant canopy (8.9 mg g?1, 0.03 mg g?1, 8.2 mg g?1, and 4.17 mg kg?1, respectively). Total N, humic acids, and abundance of microbial functional groups did not show differences among sites. Most parameters differed between seasons, tending to be higher in the wet season. Overall soils of Ñacuñán Reserve are characterized by: a) more homogenous spatial pattern than expected from woody plant presence; b) very heterogeneous temporal pattern; and c) after two years, tree removal does not seem to induce infertile soil formation.  相似文献   

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