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1.
Using Landsat TM data, this article examines the environmental impact of the East Port Said harbour project on the surrounding landscape. The optimum three-band combination and the most appropriate multispectral bands were selected to enhance the images and monitor land cover changes for the periods of 1984–1991 and 1991–2003. The results indicate that wetland areas declined from 103 km2 in 1984 to 30 km2 in 2003. In addition, the surface area of El-Malha Lake has shrunk from 27 km2 to 18 km2 over the same period. In contrast, the area covered by salt crust has increased from 11 km2 in 1984 to 19 km2 in 2003. Urban land use and designed cultivated lands were also significant in 2003, covering 49 km2 and 71 km2, respectively. The rate of shoreline change between 1984 and 2003, the period when the East Port Said harbour was constructed, was calculated. Vector data indicate that the rate of shoreline loss was ?13 m/year from 1984 to 1991 and ?15 m/year from 1991 to 2003. Despite the fact that construction of the East Port Said harbour caused significant changes in the study area, there are several factors controlling coastline and land cover changes including industrial development and fish cultivation farms.  相似文献   

2.
Clay pans (playas) are common landforms of semi-arid and arid desert regions ranging in size from 1 to 2 km2 to greater than 10,000 km2. The first observations of surface energy exchanges over a clay pan in central Australia using the eddy covariance method are presented. Measurements were obtained in the late dry season (September) before the onset of the Australian summer monsoon under clear skies and weak synoptic winds for five consecutive days. Sensible heat flux displayed an upward trend over the observation period reaching a peak of approximately 50% of net radiation over the period, while latent heat flux declined as the clay pan became dryer. Ground heat flux was similar in magnitude to sensible heat flux reaching maximum values near noon, approximately 2 h before daily maximum sensible heat flux. Net radiation and ground heat flux both reached minimum values 1–2 h after sunset and then gradually increased towards sunrise, while sensible and latent heat flux remained close to zero. Bowen Ratio values increased from 4.08 to 6.12 over the study period as the surface dried. Results highlight clay pans as significant sources of sensible heat in desert landscapes.  相似文献   

3.
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   

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.
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   

6.
Data from 40 monitoring wells across 9 sections of the lower Tarim River from 2000 to 2006 were analyzed to investigate the relationship between the transmission loss per unit river length and the change in groundwater depth. The relationship between the rise of the groundwater table (y) and the distance from the main river reach (x) was then assessed through regression analysis. We concluded that the maximum affected area was 1933 m away from the main river reach in the Alagan section, and the minimum affected area was 576 m away in the Kaogan section. In addition, after 8 water deliveries, the volume for recharging the groundwater was 78 248.7 × 104 m3. Using the Yingsu section as an example, we found that the volume for recharging the groundwater decreased with additional periods of delivery except after the second and sixth water delivery The results revealed that the beneficial effect of an ecological water conveyance project on the ecosystem in the lower Tarim River is a long-term process. These findings may be useful for guiding studies on instream flow requirements and provide a scientific basis for implementing similar ecological projects in other areas.  相似文献   

7.
Urban agriculture (UA) in Khartoum, Sudan, contributes to meeting the increasing food demand of the city's rapidly growing population, but its spatial extent, development over time and resource consumption are unknown. To fill this knowledge gap, we analyzed scenes captured by Landsat satellites in 1972, 1987 and 2000. For each dataset, we calculated the Normalized Difference Vegetation Index (NDVI) and classified all grid cells with NDVI values higher than one standard deviation above the mean as vegetated land. We also quantified agricultural vegetation on aerial photographs taken in 1958 and on a recent Google Earth image. The built-up area of the city was estimated for each point in time. Based on these spatial estimates, we computed water consumption by UA based on geographic, temporal and climatic datasets and official data on the cropping patterns of Khartoum State, using the Penman–Monteith equation to estimate crop water demands, and assuming a water use efficiency of 0.56.Urban agriculture increased from 8751 ha in 1972 to 13,249 ha today. In the municipal core zone, UA decreased from 4799 ha in 1958 to 2869 ha today. Along the Nile, UA persisted through time, whereas many urban irrigation schemes have been pushed into the periphery. The built-up area increased strongly and today exceeds the UA area 5.5-fold. Estimated crop water demand on UA land increased from 241 to 365 million m3 year?1 between 1972 and today.  相似文献   

8.
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   

9.
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   

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.
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   

12.
Juniperus communities are found on over 50 × 106 ha in arid and semiarid habitats in southwestern North America. The drought tolerant sedge Carex planostachys occurs below the canopy in some of these communities. Cover and biomass of C. planostachys are high below the canopy and low in associated gaps. The purposes of this study were to investigate the temporal and spatial physiologic response of C. planostachys to abiotic changes, and determine it's light response characteristics from four contiguous microsites. Net photosynthesis was highest in spring when temperature was cooler and soil water higher, but low carbon uptake continued during summer drought. In addition, C. planostachys demonstrates a capacity to recover from extreme drought, despite water potential measured below ?9.0 MPa. Based on physiological light response curves and gas-exchange measurements, C. planostachys appears tolerant of shaded and full sun habitats. Light levels below the canopy were reduced compared to the gaps, but light saturation of C. planostachys did not change and net CO2 uptake was only reduced slightly. Carbon uptake was coupled to light levels and not soil moisture. Observed differences in physiological attributes and variation in C. planostachys cover and biomass correspond to the presence or absence of the canopy. Low light compensation points, coupled with reduced respiratory demand, maximize photosynthetic gain in low light microsites. C. planostachys appears to acclimate across a range of light regimes, suggesting photosynthetic plasticity, allowing growth and survival in diverse light microhabitats. C. planostachys, tolerant of drought, appears anisohydric and demonstrates a capacity to acclimate to sun and shaded habitats, which could allow it to occur across a wider range of arid areas.  相似文献   

13.
《Polar Science》2008,2(3):163-174
Isotopic analysis of the Martian lherzolitic shergottite Yamato 000097 yields a Rb–Sr age of 147 ± 28 Ma with an initial 87Sr/86Sr ratio of 0.710490 ± 0.000072, a Sm–Nd age of 152 ± 13 Ma with an initial ɛ143Nd-value of +11.7 ± 0.2, and a 39Ar–40Ar age of ∼260 Ma. The near concordance of these ages, in combination with the Rb–Sr and Sm–Nd initial isotopic signatures, suggests that Yamato 000097 crystallized from low Rb/Sr, light rare earth element depleted source materials ∼150 Ma ago. Although the obtained 39Ar–40Ar age is significantly higher than the Rb–Sr and Sm–Nd ages, Yamato 000097 shows little or no evidence of trapped Martian atmospheric 40Ar. The trapped 40Ar concentration of Yamato 000097 is similar to that of Zagami, suggesting that both basaltic and lherzolitic shergottites may have similarly inherited excess 40Ar from their magmas.The Rb–Sr and Sm–Nd ages, and initial 87Sr/86Sr and ɛ143Nd-values of Yamato 000097 and Yamato 793605 lie on the same isotopic ingrowth curves, suggesting that they came from very similar mantle sources. Allan Hills 77005 was also probably derived from the same source, but Lewis Cliff 88516 appears to be from a distinct but similar source. Yamato 000097 represents the most recent known magmatism from its source, and is the youngest Martian meteorite for which concordant Rb–Sr and Sm–Nd ages have been determined.  相似文献   

14.
When using rift lakes as proxies for palaeoclimate it is essential to know if water level changes are of geological or climatic origin, and whether a reinterpretation of palaeo-lake levels derived from lithostratigraphic sequences is required. The saline, endorheic Lake Beseka is located in the tectonically active Main Ethiopian Rift. Despite the aridity of the rift valley the lake's surface area quadrupled from 11.1 km2 in 1973 to 39.5 km2 in 2002. We quantify the lake growth by means of a detailed bathymetric model and high-resolution satellite time series. We analyse the potential climatic, anthropogenic, and tectonic agents of Lake Beseka's growth. Multitemporal remote sensing data and meteorological records were compared with in-situ measurements of hydrochemical parameters and water depth to address the reason for the lake level rise. Our results suggest that Lake Beseka's expansion originates from an increased discharge of the hot springs. The combined analysis of satellite data and in-situ measurements proved to be a valid tool for the quantification of lake level changes and can help to detect the causes of these variations.  相似文献   

15.
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   

16.
Present-day Sahelian vegetation in a highly anthropized semi-arid region is assessed from local to regional scales, through the joint analysis of MODIS LAI (1 km2 and 8-day resolutions), daily rainfall, morphopedological and land cover datasets covering the period 2000–2008. The study area is located in northwest Senegal and consists of the “Niayes” and the northwestern “Peanut Basin” eco-regions, characterized by market gardening and rain-fed cultivated crops, respectively. The objectives are i) to analyse at pixel scale LAI time series and their relation to vegetation and soil types, ii) the estimation of phenological metrics (start of season SOS, end of season EOS, growing season length GSL) and their inter-annual variability, iii) to recognize the vegetation responses to rainfall trends (mean annual precipitation, MAP; frequency of rainy events, K; combination of MAP and K, called F).Pixel-scale analyses show that LAI time series 1) describe the actual phenology (agreeing with ground-truth AGHRYMET data), and thus can be used as a proxy for Sahelian vegetation dynamics, 2) are strongly dependent on soil types. Median maps of SOS and EOS suggest an increase of the GSL from Saint-Louis to Dakar, in agreement with both the North-South rainfall gradient and the intensification of agricultural practices around Dakar. Significant correlations (R: 0.64) between annual variation coefficient of LAI and MAP for both herbaceous crops and natural vegetation are highlighted; this correlation is reinforced (R: 0.7) using the rainfall distribution factors K and F. Rainfall thresholds allowing the SOS can be defined for each type of vegetation. These thresholds are estimated at 0–5 mm, 20 mm and 40 mm for natural herbs, herbaceous crops and shrublands, respectively.If previous works revealed the close link between the MAP and the SOS, our results highlight that LAI dynamics are also controlled by rainfall distribution during the Monsoon season. In this study, climatic indicators are proposed for estimating vegetation dynamics and monitoring SOS. Coupling Earth Observation data, such as MODIS LAI, with rainfall data, vegetation and soil information is found to be a reliable method for vegetation monitoring and for assessing the impact of human pressure on vegetation degradation.  相似文献   

17.
There is a growing requirement for techniques to assess land susceptibility to wind erosion, i.e. land erodibility, over large geographic areas (>104 km2). This requirement stems from a lack of wind erosion research between the field (101 km2) and regional scales, and a need to evaluate the performance of spatially explicit wind erosion models across these scales. This paper addresses this issue by presenting a methodology for monitoring land erodibility at the landscape scale (103 km2). First, we define criteria suitable for evaluating land erodibility based on empirical relationships between soil texture, vegetation cover, geomorphology, and wind erosion. The criteria were used to visually assess land erodibility over long distances (103 km) using vehicle-based transects run through the rangelands of western Queensland, Australia. Application of the data for testing the performance of a spatially explicit land erodibility model (AUSLEM) is then demonstrated by comparing the visual assessments of land erodibility with the model output. The model performed best in the west of the study area in the open rangelands. In regions with higher woody shrub and tree cover the model performance decreases. This highlights the need for research to better parameterise controls on erodibility in semi-arid landscapes consisting of forested and rangeland mosaics.  相似文献   

18.
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   

19.
《Polar Science》2008,2(3):195-214
We used total melting and stepwise heating methods to measure noble gases within bulk samples from Yamato 000027 and Yamato 000097 lherzolites as well as a melt-vein sample from Yamato 000027. He and Ne are dominated by cosmogenic noble gases. The obtained cosmic-ray exposure age of 4.6 ± 1.5 Ma, an average age based on cosmogenic 3He, 21Ne, and 38Ar for these samples, is consistent with the ages of other lherzolitic shergottites, indicating a common impact event for the ejection of lherzolites from Mars. Heavy noble gases released from the bulk samples at low temperatures were elementally fractionated terrestrial atmosphere. Martian noble-gas isotopic signatures, 40Ar/36Ar = 1900 and 129Xe/132Xe = 1.3, were observed at high temperatures (>1000 °C). The melt-vein sample released greater amounts of atmospheric Ar, Kr, and Xe at low temperatures than the bulk samples. Large amounts of Ar and Kr, as well as excess 40Ar and 129Xe, were evolved from the melt-vein sample at 1400 °C, and the gas shows very high 36Ar/132Xe (=3100) and 84Kr/132Xe (=76) values. Maximum 40Ar/36Ar and 129Xe/132Xe values of the melt sample were 1100 and 1.6, respectively, at 1600 °C. Cosmogenic Kr shows an absence of 80Kr and 82Kr produced by neutron capture on Br, which suggests a small pre-atmospheric body. Overall noble gas compositions for Y000027 and Y000097 support pairing for the Yamato 00 shergottites.  相似文献   

20.
The direct short-term impact of three rates of stocking (4, 8 and 16 small-stock units [SSU] ha?1) was quantified in terms of soil characteristics of arid Nama Karoo vegetation (subshrub/grass). Mature Merino wethers grazed in the experimental plots throughout May in 1995 and 1996 (the plots were not subjected to grazing at any other time). Stocking rate proved inversely related to initial infiltration rate. Light trampling (4 SSU ha?1) loosened the topsoil sufficiently to increase the initial infiltration rate: infiltration capacity of soil in fields stocked at 4 SSU ha?1 and 16 SSU ha?1 was 17% higher and 14% lower respectively than that of soil of ungrazed rangeland over the two grazing periods. Increased soil compaction and greater bulk density due to higher stocking rates significant decreased the infiltration rate. Compared to ungrazed rangeland stocking rates of 4, 8 and 16 SSU ha?1 over the two grazing periods increased bulk density respectively by 2.73%, 6.67% and 8.945% and compaction by 10.90%, 16.78% and 20.90%. No grazing also increased bulk density and soil compaction and decreased infiltration rate. Light stocking (4 SSU ha?1) influenced all soil parameters most favourably. From a hydrologic point of view, grazing levels and rotation schemes need to be tailored for sustainable utilization of arid subshrub/grass vegetation by livestock.  相似文献   

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