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1.
Aerial photography from the 1930s serves as the earliest synoptic depiction of vegetation cover. We generated a spatially explicit database of shrub (Prosopis velutina) stand structure within two 1.8 ha field plots established in 1932 to address two questions: (1) What are the detection limits of panchromatic 1936 aerial photography?, and (2) How do these influence P. velutina biomass estimates? Shrub polygons were manually digitized on 1936 imagery and linked to 1932 field measurements of P. velutina canopy area. Aboveground 1932 P. velutina biomass was estimated using a site-specific allometric relationship for field-measured canopy area. Shrub canopy detection limits on the 1936 imagery were comparable to those reported for contemporary imagery. Based on a conservative shrub size detection threshold of 3.8 m2, 5.8% of P. velutina biomass was missed. Spatial resolution (0.6 vs. 1.0 m) did not influence detection limits, but the overall accuracy of shrub cover estimates was greater on 1.0 m images. Presence of the sub-shrub Isocoma tenuisecta may also have significantly influenced estimates of P. velutina canopy area. These analyses illustrate the importance of standardizing aerial photo interpretation protocols, accounting for uncertainty estimating shrub biomass, and caution species-specific interpretations for historic aerial photography.  相似文献   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
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   

8.
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   

9.
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   

10.
Using intact samples of Gobi surfaces, we conducted wind tunnel experiments and related analyses to describe the surface characteristics of Gobi deserts and their significance for dust emissions in the Ala Shan Plateau, an area of Central Asia with high dust emissions. Under relatively high wind velocities (22 m s−1), the total sediment transport approached 700 g m−2, with the fine fractions (<50 μm in diameter) accounting for up to 26 g m−2. In consecutive experiments, the emission rates decreased greatly due to depletion of erodible particles. In the Ala Shan Plateau, coverage by gravels varied, and aeolian sediment transport increased with increasing gravel cover (to about 30%); above that level, transport decreased. Because gravel cover was less than 30% in most areas, the gravel may not play important role in dust emissions in this region. Although the Gobi surfaces are covered by gravel, high clay contents that may restrain sediment transport. In the Ala Shan Plateau, dust emissions therefore appear to be controlled mainly by the availability of fine particles, which is in turn controlled by their deposition by ephemeral streams, by their creation via physical, salt, and chemical weathering, and by other processes such as aeolian abrasion.  相似文献   

11.
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   

12.
Increasing wildfire activity is one of the most pressing management concerns in arid lands of the American West. To examine post-fire recovery of perennial vegetation in the Mojave and Sonoran Deserts, I analyzed data systematically synthesized from the literature. Post-fire sprouting by desert perennials is generally limited but varies among species. For example, only 3–37% of Larrea tridentata sprouted compared to 64–86% of Yucca schidigera. Four of five studies measuring recovery of perennial cover reported close relationships (r2 = 0.67–0.99) between time since fire (TSF) and cover. In fact, three studies measuring the longest TSF (≥37 years) found that cover had returned to within 10% cover of unburned areas within approximately 40 years. Conversely, post-fire species composition exhibited little convergence with unburned composition in five of six studies even 47 years after fire. Sphaeralcea ambigua, Gutierrezia spp., Achnatherum speciosum, Encelia spp., Hymenoclea salsola, and Baileya multiradiata had the highest burned:unburned abundance ratios, although overall post-fire community composition differed between the Mojave and Sonoran Deserts. Analyzing the literature as a whole suggested some generalities (e.g., that perennial cover reestablishes faster than composition), but more work is required for improving specific knowledge about plant recovery among fires, sites, species, and climates.  相似文献   

13.
Analyses of landscape change using remotely sensed satellite imagery constitute a large component of forest transition research, allowing for assessments of large areas. In the western highlands of Honduras is an area of complex forest dynamics (～45,000 ha) that has seen significant forest regeneration in recent years. However, analysis of the larger region (～500,000 ha) shows net forest loss. The comparative aspects highlight the importance of site selection and scale in forest transition analysis, a process often ignored in the land-use and land-cover change (LULCC) and forest transition literature. Results also highlight the importance of analyzing human-induced fragmentation at a variety of selected sites and a range of spatial scales, and producing quality, accurate forest cover and change maps.  相似文献   

14.
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   

15.
《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   

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.
The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion.  相似文献   

18.
Although Jatropha curcas, an important tropical biofuel crop, is reputed for its drought resistance, its ability to perform under dry conditions has hardly been investigated. In a greenhouse experiment we investigated the plant–water relationships of Jatropha seedlings from different accessions under different levels of drought stress. There was little difference in plant–water relations between accessions. Drought significantly reduced leaf area, biomass and relative growth rate, but had no effect on specific leaf area, daily range in leaf water potential, leaf water content, transpiration efficiency or aboveground biomass water productivity, corrected for atmospheric conditions. Stem wood density was equally low (0.26 g cm?3) for all treatments. Stem water content was lowest for dry treatment seedlings. Based on these results, Jatropha could be characterized as a stem-succulent tree. In contrast to other stem-succulent deciduous trees, leaves were not shed immediately after the seedlings were confronted with drought. Instead, at the onset of drought, leaves with a higher adaxial stomatal density were formed, after which leaves were only gradually shed. The role of the succulent stem in the water economy of Jatropha was confined to balancing the small water losses of the leaves during drought.  相似文献   

19.
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   

20.
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   

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