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1.
Pine plantations coexist with Stipa tenacissima grasslands in many semiarid western Mediterranean areas. We compared three microsites created by a 30-year-old Pinus halepensis plantation: below pine plantation line (BP), below canopy of pines (BC) and interline bare band (BA). They were evaluated in terms of soil properties, pine litter and suitability as recruitment niches for S. tenacissima. Next, in a manipulative experiment in growth chambers we tested the hypothesis that pine litter interferes with the seedling emergence of S. tenacissima. Three treatments in pots were compared: (a) soil from BA; (b) intact soil + litter from BP; and (c) soil + litter from BP, which was mixed in the laboratory (BPMX). In the field the main microsite differences were pine cover and litter cover and thickness. Seedling emergence was significantly greater in BA than in BP. Emergence and litter depth fits a linear regression model. In the growth chamber litter did not interfere with the emergence of S. tenacissima. However, seedlings grown without litter were 28% longer and their mass was 27% greater than in the litter treatments. The detected pine litter interference may be relevant for plant dynamics and might be considered in forestry management programs.  相似文献   

2.
Artificial water sources have been used for decades to enhance and restore wildlife habitat but the benefits of their use have been subject to debate. During the past century, the number of natural springs in Joshua Tree National Park, California, USA, has declined. In response to concerns about the viability of the bighorn sheep (Ovis canadensis nelsoni) population, a number of water developments were constructed throughout the park. We modeled potential historical and present-day summer habitat of female bighorn sheep to evaluate the effectiveness of the artificial and remaining natural water sources in maintaining habitat and to determine how loss of artificial sources might affect future habitat availability. Prior to 1950, 583.5 km2 of summer habitat was potentially available. Presently, only 170.6 km2 of habitat is available around natural water sources and 153.5 km2 is available around guzzlers. When all perennial water sources are included in the habitat model (minus overlap), 302.3 km2 of summer habitat is potentially available. This represents only 51.7% of summer habitat available prior to 1950. Without artificial water developments, 47.7% of present-day summer habitat would be lost, which raises important management questions regarding the debate about what is natural or artificial within otherwise protected areas.  相似文献   

3.
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   

4.
Allometric equations and community biomass stocks are presented for Guiera senegalensis J.F. Gmel (Gs) and Piliostigma reticulatum (DC.) Hochst (Pr) – two native shrub species in the Sahel. These shrubs are of interest because they dominate semi-arid sub-Sahalien Africa but have been largely overlooked as a key biomass component and regulator of ecosystem composition and function in this landscape. In Year 1, best predictors of aboveground biomass were height and number of stems (Gs) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R2 = 0.90 for Gs and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58 ton ha?1 (mean = 2.38 ton ha?1) in the Gs sites and from 0.33 to 7.38 ton ha?1 (mean = 3.71 ton ha?1) in the Pr communities. Both species exhibited unusually large root:shoot ratios (4.5:1 for Gs and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for Gs and Pr.  相似文献   

5.
The perennial saltgrass nipa (Distichlis palmeri, Poaceae) is endemic to northern Gulf of California tidal marshes flooded with hypersaline (38–42 g L−1) seawater. Nipa was a wild harvest staple of the Cocopah people of the Río Colorado delta. We investigated the physiology, anatomy, chromosome number, and agronomic potential of nipa as a global food crop. Nipa seeds had 60–93% germination on salinities ranging from 0 to 30 g L−1. Relative Growth Rates (RGR) on both flooded and aerobic conditions remained above 4% d−1 up to 30 g L−1, about half the RGR on freshwater. Nipa grain (caryopses) had 7–8% protein, 8% sugar and 79% total digestible carbohydrates (mostly starch) and only 2% ash and 8% fiber, equal to conventional grains in apparent nutritional value. Shoots were low in ash and sodium, and compared favorably to alfalfa forage in protein, digestible carbohydrates and energy contents. Mature female stands in the Colorado River delta produced an estimated 1.25 t ha−1 of grain, but over two years in the greenhouse only partial flowering was observed. Nevertheless, D. palmeri appears to be worth developing as a perennial grain and forage crop, especially for salinized, flooded soils.  相似文献   

6.
Laboratory experiments were conduced to assess the synergic effect of chilling and light on photosystem II photochemistry of the halophyte, Sarcocornia fruticosa, grown at different salinity concentrations (0, 170, 340, 510 and 1030 mM). Chlorophyll fluorescence was measured after chilling (at 5 °C in darkness) and light-chilling (at 5 °C and 700 μmol m?2 s?1) treatments, and after 24 h of recovery (at 20 °C and 75 μmol m?2 s?1). At 5 °C and 700 μmol m?2 s?1, plants grown with 0 and 170 mM NaCl showed the lowest Fv/Fm values, whereas quantum efficiency of PSII (ΦPSII) was higher for plants grown at 170 and 340 mM NaCl, these results being consistent after two exposures to chilling treatments. Susceptibility to photoinhibition decreases when low temperature and high light are combined with high salinity. Therefore, populations of S. fruticosa that occur in arid environments with salinities c. 340 mM could show a higher tolerance to light-chilling.  相似文献   

7.
Opuntia tomentosa seeds have physiological dormancy and a permeable but hard funicular envelope that restricts water uptake and embryo growth. Dormancy break, germination, and changes in the funiculus were compared in laboratory-stored seeds and in those buried for 7 months at different microsites and later exhumed and stored in the laboratory. The funicular envelopes of both lots were examined with a scanning electron microscope (SEM). Seeds in both lots were: scarified with H2SO4 (0, 45, 60 and 90 min), treated with gibberellins (0, 1000 and 2000 ppm) and germinated at the same ages (0, 2, 4, 7, and 10 months after exhumation) at 24 °C. Light effect was determined at 24 °C and 20–35 °C. In laboratory-stored seeds germination was low. Exhumed seeds germinated in ～50%, the remaining ones required scarification and gibberellins, but lost dormancy two months later. Gibberellins reduced germination heterogeneity, and scarification reduced germination; the adverse effect of scarification was offset by gibberellins. Exhumed seeds germinated mainly in darkness at 20–35 °C and partially in far red light at both temperatures. SEM showed that fungi eroded the funiculus reducing its resistance. O. tomentosa could form a seed bank, ensuring its survival under heterogeneous environments.  相似文献   

8.
Biological soil crusts cover large areas of the Gurbantunggut Desert in northwestern China where they make a significant contribution to soil stability and fertility. The aim of this study was to quantify the potential nitrogen-fixing activity (NA) of different types of biological soil crusts in the Gurbantunggut Desert. The results suggest that NA (nmol C2H4 m?2 h?1) for each type of crusts was highly variable. Seasonal variation was also important, with all three types of crusts responding in a similar way to changes in environmental conditions. From March to May, NA was relatively low for all crust types. During this season, NA was 2.26 × 103 for cyanobacterial crust followed by lichen crust (6.54 × 102) and moss crust (6.38 × 102). From June to October, all crust types reached their highest level of NA, especially lichen crust and moss crust (p < 0.01). The NA of cyanobacterial crust (9.81 × 103) was higher than that of lichen crust (9.06 × 103) and moss crust (2.03 × 103). From November to February, when temperatures were consistently low (<0 °C), NA was at its lowest level, especially in cyanobacterial crust (4.18 × 102) and moss crust (5.43 × 102) (p < 0.01). Our results indicate that species composition is critical when estimating N inputs in desert ecosystems. In addition, all three types of crusts generally responded in a similar way to environmental conditions. The presence of N fixation activity in all crusts may contribute to the maintenance of fertility in sparsely vegetated areas and provide surrounding vascular plant with fixed nitrogen.  相似文献   

9.
The purpose of our study was to better understand seed germination ecology of the spring annual short-lived Olimarabidopsis pumila, which grows in the Gurbantonggut Desert, China. Seeds underwent after-ripening at 4, 20 ± 2 (room temperature) and 30 °C. After dormancy was broken, germination capacity was a function of temperature and presence of light. For the temperature range studied (4–30 °C), germination capacity was significantly higher between 15 and 25 °C than at temperatures above or below them. Alternating temperatures of 20/10, 25/10 and 25/15 °C were favorable for germination. Although GA3 did promote dark germination of seeds, GA3 did not replace light for germination. Seeds germinated to >75% if light exposure time was over 8 h and temperature and moisture conditions were favorable. Seeds were able to germinate at relatively low water potentials (83% at ?0.41 MPa), but decreasing water potentials produced detrimental effects on germination percentage and rate. Thus, dormancy characteristics and germination behavior of O. pumila seeds ensure that germination occurs in the desert only when soil moisture conditions are favorable for seedling establishment and survival.  相似文献   

10.
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   

11.
《Polar Science》2008,2(3):215-222
Arctic copepods were collected using time-series sediment traps in the Amundsen Gulf, Canadian Arctic, as part of the CASES (Canadian Arctic Shelf Exchange Study) program. Four sediment traps were deployed at three stations (200 m depth for CA15, 200 and 400 m depths for CA18, and 200 m depth for CA20) from October 2003 to July 2004. We collected many copepod nauplii ranging in body length from 155 to 811 μm, among which nauplii (mostly N1–2) of Calanus with a size of ∼190 μm apparently increased in abundance from February to mid-March. Mature-stage adult females (AF) of Calanus hyperboreus were collected in the traps from February to March, and adult males of C. hyperboreus appeared from November to December at all stations. The likely spawning period of these AF coincided with the occurrence period of ∼190 μm-sized nauplii. This finding suggests that these nauplii were derived from C. hyperboreus and that their breeding began at the beginning of November or December at the latest, continuing through April in the Amundsen Gulf.  相似文献   

12.
A successful migratory strategy is the result of optimizing three important factors – time, energy and safety. Raptors mainly use soaring-and-gliding flight that results in time and energy minimization. I observed migratory flocks of Levant Sparrowhawk as they took off from nocturnal roost, to understand the environmental and anthropogenic factors affecting their early-morning flight behavior. The earliest flocks converged in a straight line from the roosts to the highway and flew above it for 40–70 min after sunrise. The flocks stayed ca. 40–100 m in above the highway which suggested that the Levant Sparrowhawk utilized the heat released by the highway surface to improve flight conditions. Twenty-five Levant Sparrowhawk that took off from the roost and headed across open areas had to flap an average of 16.8 ± 1.9 wing beats per minute, which was significantly greater (paired t = ?16.8, P = 0.0001) than the 8.8 ± 1.8 wing beats per minute for the same individuals when flying over the highway. The data indicate that Levant Sparrowhawk, that have behaviorally modified and adapted to flying along highways, are able to migrate an hour earlier in the day and to conserve almost 50% of the energy they would otherwise have had to invest in covering the same distance in flapping flight. The result of this strategy maximizes timing of migration while minimizing energy expenditure.  相似文献   

13.
《Polar Science》2009,3(4):235-245
  相似文献   

14.
This paper describes evaluation of forest stand density combining satellite imagery with forest inventory data set. The degree of canopy cover is described in terms of fractional vegetation cover (FVC) obtained by a linear mixture model applied on multi-spectral IKONOS image and canopy cover (CC). CC was calculated from field measurements of crown width of 646 standing trees sited within 72 circular (200 m2) plots. A comparison between CC and FVC shows that the former can be accurately represented by the latter linking in-situ measured forest characteristics with surface reflectance measured by a satellite.Stand density expressed as an absolute term (number of trees per unit area) showed high and significant positive correlation to FVC (R2 = 0.96) and to relative density measure (Crown Competition Factor; R2 = 0.89).In order to show the applicability of the presented approach for managerial practices, a map of the spatial distribution of stand density within the forest was produced using the above-mentioned correlations. Its quality was verified against an independent data set of ground measurements. The correlation between field- and map-based number of trees per unit area was found to be satisfactory (R2 = 0.4; p < 0.05), even though a slight lack of sensitivity was evident for low-density stands.  相似文献   

15.
In spite of the extremely low phosphorus (P) availability in soils of the Alxa Desert region (Inner-Mongolia, China), many shrubs grow well there. The objective of this study was to investigate the status of inorganic phosphorus (Pi) fractions in the rhizosphere of xerophytic shrubs. For all five selected shrubs, soil pH in the rhizosphere decreased by 0.4–0.8 units and the total P concentrations were greater than those in the corresponding bulk soils. In the rhizosphere of Reaumuria soongorica, Olsen P concentration was lower than that in the bulk soil. In general, the order of Pi fractions was Ca10–P > Ca8–P > Al–P > Fe–P > Ca2–P > O–P. The concentrations of all Pi fractions varied significantly with rhizosphere and corresponding bulk soils. The results of stepwise regression showed that Olsen P concentration in rhizosphere and bulk soils was contributed by different Pi fractions. The correlation analysis showed that the relationships between Pi fractions differed between rhizosphere and bulk soils. These results indicated that the soil Pi fractions are significantly affected by root activities, differing between the rhizosphere and bulk soils, and different xerophytic shrubs exhibited significantly different effect on soil Pi fractions.  相似文献   

16.
Agave angustifolia is taken from wild populations in the desert of Sonora, Mexico, to make the alcoholic drink known as “bacanora”. An increase in the demand for this beverage has led to overexploitation of these agaves. We used AFLP to measure the genetic variability within and between natural populations of the species. The fixation index (FST) and population genetic distances (GD) were determined for three populations of A. angustifolia. At the clonal level, the expected heterozygosis (HE), the proportion of polymorphic loci (P), and the genetic similarity indices (GSIs) were lower in one population. Total GSI for adults and offshoots was 0.924. There were no differences in the GSI within or between the populations and dendrogram showed three groups that partially reflect the geographical distribution of the populations. Some degree of genetic variation between mother plants and their vegetatively produced rhizomes was observed. At the species level, total expected heterozygosity was high (HT = 0.314) with a P value of 78%, and intermediate gene flow (Nm = 1.18), giving rise to a moderate fixation index (FST = 0.175). Nm, FST and GD values observed, indicate that urgent measures must be implemented to preserve the genetic diversity.  相似文献   

17.
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   

18.
Revegetation of disturbed land, particularly in arid environments, is often hindered by low seedling establishment. Information on seed biology and germination cues of keystone species is lacking, particularly in arid Australia; a major zone for mining developments. This study investigated seed characteristics and germination of 18 common species required for rehabilitation of disturbed areas at Shark Bay Salt in the Shark Bay World Heritage Area, Western Australia. Untreated seeds of seven species (Aphanopetalum clematideum, Atriplex bunburyana, Austrostipa elegantissima, Melaleuca cardiophylla, Pembertonia latisquamea, Rhagodia baccata, Salsola tragus) exhibited high germination percentages. Seeds of two species (Acacia tetragonophylla, Stylobasium spathulatum) had low imbibition, which increased with hot-water treatment, hence require scarification for germination. Gibberellic acid, smoke water and karrikinolide (a butenolide isolated from smoke) substantially increased germination percentages of three species (Anthocercis littorea, Diplolaena grandiflora, Solanum orbiculatum). Seeds of the remaining six species (Dioscorea hastifolia, Eremophila oldfieldii, Nitraria billardierei, Ptilotus exaltatus, Thryptomene baeckeacea, Zygophyllum fruticulosum) had low germination percentages regardless of treatment. Most species germinated equally well at 26/13 °C and 33/18 °C, however seven species had improved performance at 26/13 °C. This study is significant to land managers and conservation agencies with an interest in optimising germination of arid zone seeds for restoration.  相似文献   

19.
We studied in Opuntia tomentosa the effect of plant shade (orientation and distance), and the place where the seeds remained before germination. Seeds were collected in 1998 and a) sown immediately on soil (non-buried seeds), b) stored in the laboratory and sown on soil during the 1999 rainy season (control) or c) buried after collection (under the shade of a tree, a rock and in an open space), exhumed 7 months later and sown in 1999 (exhumed seeds). Seeds were sown under or outside the shade of several trees; or on the northern, eastern and western sides of a 14-m-tall tree. In 1999, germination was high in all samples, but the control seeds had delayed germination. After two rainy seasons, non-buried seeds had the lowest germination rate. Germination was not affected by the distance from the tree. Outside of the plant's shade, germination was low. Germination was affected more by the environmental heterogeneity than by the individual plant shade. Germination probabilities were the highest at 16–19 °C; field germination was zero at 20.6 °C. Seed polymorphism and microenvironmental heterogeneity spread germination across two rainy seasons creating a permanent seed bank.  相似文献   

20.
Medium-sized kangaroo rats (Dipodomys spp. Gray) function as keystone species in the dynamics of desert bunchgrasses. We tested the hypothesis that kangaroo rat graminivory leads to reduced grass growth and inflorescence production, and that kangaroo rat grass consumption reflects their preferences for open microhabitats. We excluded kangaroo rats from grasses, and measured tiller loss, leaf and tiller growth and inflorescence production. We recorded the extent of tiller loss in grasses varying in tussock size or the extent of surrounding cover. Consumption of tillers by kangaroo rats was extensive, peaking in late summer. Grasses protected from kangaroo rat graminivory showed elevated leaf and tiller growth and inflorescence production. Grass tussocks protected by surrounding vegetation or large grass tussocks suffered lower graminivory than tussocks in the open or small tussocks. Our data demonstrate that the mechanism of the keystone role of kangaroo rats is through direct graminivory. We hypothesize that kangaroo rat graminivory may contribute to the persistence of desertified shrublands by limiting grass recruitment and abundance. Medium-sized kangaroo rats prefer open microhabitats and increase in abundance as such microhabitats increase, suggesting a positive feedback, with kangaroo rats increasing in abundance in overgrazed, desertified landscapes, and then contributing to their persistence through graminivory.  相似文献   

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