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1.
A portable wind tunnel was used to test the contribution of biological and physical elements to overall soil aggregation on a soil dominated by biological soil crusts in south-eastern Australia. After moderate disturbance and simulated wind erosion, 90% of surface aggregates on the loamy soil and 76% on the sandy soil were dominated by biological elements (cryptogams). Lower levels of biological bonding were observed on the severely disturbed treatment. Linear regression indicated a significant positive relationship (r2=0·72) between biological soil crust cover and dry aggregation levels greater than 0·85mm. To maintain sediment transport below an erosion control target of 5gm−1s−1 for a 65kmh−1 wind at 10m height, a crust cover of approximately 20% is required. When a multiple regression model which sequentially fitted biological crust cover and dry aggregation greater than 0·85mm was applied to the data, dry aggregation accounted for more of the variation in sediment transport rate than biological crust cover. These data were used to develop a conceptual model which integrates crust cover and dry aggregation, and provides a useful framework within which to predict the likely impacts of changes in soil crust cover and aggregation.  相似文献   

2.
We studied the relationship between primary productivity and species richness of small mammals at both large and small spatial scales in the arid and semi-arid grasslands of north China. The productivity (x)–species richness (y) pattern at a large spatial scale can be described by a unimodal quadratic regression curve (y=7·41+0·1*x−0·0003*x2,p =0·008, r2=0·788). At a small spatial scale, however, neither linear nor quadratic regression fit the data for 1980 and 1994 (p>0·25). Primary productivity may not be an appropriate predictor of the species richness of small mammals at a small spatial scale. We conclude, therefore, that the primary productivity–species richness pattern of small mammals may be scale-dependent in the arid and semi-arid grasslands of north China. Landscape complexity should be considered in future studies of productivity–richness relationships.  相似文献   

3.
Four plant communities were determined through floristical and non-metric analysis: a Dolichlasium lagascae community in crevices on sunny rock outcrops; a Cercidium praecox ssp. glaucum community on sunny slopes; an Artemisia mendozana–Adesmia trijuga community on shady slopes; and a Eupatorium buniifolium community on dry riverbanks. The discriminant function analysis showed that the first group corresponds to the sunny rock outcrops discriminated by Na content (r=0·53, p<0·05), the second group is for sunny slopes discriminated by electric conductivity and the third for shady slopes–riparian discriminated by the (Ca+Mg) content (r=0·54, p<0·05). A pattern of primary succession on outcrop surfaces is proposed.  相似文献   

4.
The standard metabolic rate ofUromastyx philbyiwas measured at 20–40 °C, using constant pressure manometric respirometers. Standard metabolic rate was mass-dependent. Values for the mass exponent ‘b’ ranged from 0·75 at 20 °C to 0·82 at 40 °C. Standard metabolic rate increased as temperature increased with high Q10values at low temperatures (20–25 °C). Low standard metabolic rates in this species are probably an adaptation to herbivory in hot desert environments.  相似文献   

5.
This paper presents a detailed cost accounting of the processes involved in the production of flours from the mesocarps of the fruits of Prosopis alba and P. pallida to be used in human foods. The flours from both species are similar and contain 7–8% protein, 0·8–2% fat, 26–32% total dietary fiber and 48–59% sucrose. The data were based on actual production rates in use at the Universidad de Piura pilot plant using simple machinery. Machinery hourly charges were based on equivalent rental costs in order to avoid interest charge calculations. As opposed to other analyses which only included processing costs delivered to the plant, our analysis ascribed value to the owner of the trees in order to provide incentives to plant and care for the trees. The sensitivity analyses showed that the most economically important factor was the conversion efficiency of pods into flour. When a 40% conversion efficiency, such as that which has been obtained in both Peru and Argentina was obtained, and by-product credits for high fiber containing fraction were assumed, the wholesale selling price with a 5% marketing cost and a 40% profit direct costs was estimated to be $0·66 kg−1 for Peru (using a $0·40 h−1 labor rate ) and $1·76 kg−1 for Argentina (using a $1·5 h−1 labor rate).  相似文献   

6.
Sedimentary deposits from the Smith Canyon dune field, south-central Columbia Basin, Washington, U.S.A. document climatically-influenced Late Pleistocene and Holocene aeolian and fluvial deposition in a region impacted by glacial outburst floods and tephra falls. The depositional history is summarized by five environmentally distinctive and climatically sensitive sedimentary units (temporal limits estimated): Unit 1 (c. 15·5–8 ka), pedogenically altered glacial outburst flood and minor aeolian silt and clay; Unit 2 (c. 8–6·9 ka), fluvial and minor aeolian sand; Unit 3 (c. 6·9–6·8 ka), flood-induced fluvial sand with gravel-sized tephra clasts; Unit 4 (c. 6·8–3·9 ka), aeolian dune sand; Unit 5 (c. 3·9 ka to present), pedogenically altered, stabilized dune sand. Estimated age ranges are based on stratigraphic position, tephrochronology, and correlation with temporally constrained strata from elsewhere in the region.  相似文献   

7.
The profile distribution of total, diethylene triamine penta-acetic acid (DTPA)- and 0·1 HCl-extractable Fe and Mn were determined in 12 pedons formed on three contiguous sand dunes in the semi-arid savanna of Nigeria. The total Fe and Mn contents varied from 100 to 3750 and 40 to 11,375 μg g−1, respectively. Values of 0·1 HCl-extractable Fe and Mn varied from 8·0 to 123 and 1·5 to 43·5 μg g−1, respectively. The corresponding values of DTPA-extractable Fe and Mn were 4·5 to 16·0 and 1·0 to 38·8 μg g−1. Total Fe and Mn correlated significantly with clay in nearly all the dunes (p≤ 0·01) but were not significantly correlated with organic matter. The 0·1 HCl-extractable Mn had a positive significant correlation with soil pH (r= 0·58*) in Illela dunes but a negative significant correlation with pH in the Sangiwa dunes (r= −0·75***). The values of extractable Fe and Mn in the sand dune soils are above the critical limits reported in other literature, thus deficiencies of these micronutrients do not pose a problem to crop production in the study area.  相似文献   

8.
The responses of desert rams to diet composition (concentrate vs. lucerne hay) in unshaded and shaded environments were investigated. The rectal temperature was higher (p < 0·01) with the concentrate diet but only in the afternoon, whereas the respiration rate was higher (p < 0·01) with the concentrate diet in unshaded and shaded environments during both morning and afternoon. Dry matter intake (DMI) was higher (p < 0·05) with the concentrate diet, while the ratio of water intake to DMI was higher (p < 0·01) with lucerne hay. Water gain through preformed and metabolic avenues and water loss through faeces and evaporative avenues were higher (p < 0·01) with the concentrate diet. With both diets, water turnover rate was higher (p < 0·05) in the unshaded environment. Feeding lucerne hay increased plasma total protein (p < 0·01) and albumin (p < 0·05) in unshaded and shaded environments. Plasma urea was higher (p < 0·01) with the concentrate diet. Plasma glucose level and plasma osmalality were higher (p < 0.01) and level of free fatty acids was lower (p < 0·01) with the concentrate diet in both thermal environments. Plasma cholesterol level was higher (p < 0·05) with the concentrate diet in the shaded environment, but lower (p < 0·05) in the unshaded environment. With lucerne hay, plasma cholesterol level remained the same in both environments.  相似文献   

9.
Sand wedges, formed mainly in a Late Pleistocene alluvial gravel in the Hexi Corridor are described in detail in this paper. 14C ages of the infilling eolian sand indicate that the sand wedges were products of the Last Ice Age. During the period of formation, the mean annual air temperature in the Hexi Corridor was about −5.3 °C, i.e. 12.6–14.6 °C lower than that of the present day. This estimated value is coincident with the fall in mean air temperature of 13 °C, which is predicated from a mirabilite (Na2SO4·10H2O) sedimentary layer, and also agrees with research on the estimated amplitude of air temperature lowering in middle and high latitudes of the Northern Hemisphere during the last glacial period. The annual precipitation in the Western Hexi Corridor at that time was probably 100–200 mm, about 100 mm more than at present.  相似文献   

10.
The natural abundance15N/14N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N2fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha−1P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha−1year−1to 0·701 Mg ha−1year−1for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha−1year−1to 4·4 kg ha−1year−1.Due to the range in δ15N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ15N per tree in calculations of the percent N derived from N2fixation (%Ndfa). There was considerable variability in δ15N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ15N of nearby soils (7·0±1·0). As the total above-ground biomass δ15N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ15N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ15N to calculate % of N derived from N2fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ15N and total N per tree and biomass per tree (R2= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N2fixation than larger trees which accumulated 1200 kg ha−1more N under their canopies. The percentage N derived from N2fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N2fixation, the N2fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha−1year−1, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ15N to background soil δ15N provides a more reasonable approach to estimate % N2fixation than comparisons of leaves of fixers and reference plants.  相似文献   

11.
The effects of temperature, water stress, hydration–dehydration cycles and seed priming on the germination of Callitris verrucosa and Callitris preissii, two Australian semi-arid coniferous tree species, were investigated. Optimum germination occurred at 18°C, with a minimum germination time of 8–9 days for both species. At this temperature, germination was inhibited at osmotic potentials lower than −1·0 MPa, but the capacity to germinate at low osmotic stress increased as the temperature decreased. Seed priming and hydration–dehydration cycles did not reduce seed viability, and Callitris seeds appear to retain the physiological changes induced by short-term hydration, as the time to the onset of germination was decreased to about 3 days. The capacity of Callitris seeds for incremental germination is likely to increase overall germination success in a low rainfall environment.  相似文献   

12.
Three-week old soybean (Glycine max) plants were subjected to a factorial combination of four regimes of soil matric water potential (ψm=−0·03, −0·5, −1·0 and −1·5 MPa), two levels of supplementary Zn (O and 20 mgl−1) and two levels of foliar IAA application (O and 10 mgl−1). Under control conditions (no Zn, no IAA), increasing soil drying progressively retarded shoot and root growth (length and dry mass production), reduced leaf relative water content (RWC) and decreased the contents of chlorophyll (Chl) and shoot soluble sugars (SS), but increased soluble sugar content of roots and lowered osmotic water potential of shoots and roots (osmotic adjustment). Total free amino acid (TAA) content increased in shoots but decreased in roots whereas contents of soluble proteins (SP) decreased in shoots and roots. The effect of water stress was statistically significant (p<0·05) and had a major effect (as indicated by η2values) on leaf RWC, shoot and root dry masses and osmotic potential. Supplementary Zn improved root growth at all levels of stress and shoot growth under severe stress. Improvement of growth was positively correlated with the internal tissue Zn concentrations (r=0·91 and 0·86 for shoot and 0·94 and 0·82 for root length and dry mass respectively). Exogenous IAA raised (p<0·05) RWC, Chl, DM (slightly), root SS, and SP, whereas shoot TAA was lowered. Effects on root TAA and shoot SS were more complex: they were lowered at zero stress and raised under severe stress. IAA and Zn in combination had additive effects on Chl, growth and osmotic potential, but their combined effects on SP and TAA were more complex. It is concluded that the treatment of soybean plants grown under conditions of low soil water potentials and Zn deficiency with Zn and IAA solutions counteracted the deleterious effects of stress, especially at high stress levels, and helped stressed plants to grow successfully under these adverse unfavourable conditions.  相似文献   

13.
An experiment was carried out at Nahshala Farm, north west of Al-Ain City, U.A.E. during the 1998–2000 growing seasons, using six halophytes (Batis maritima, Distichlis spicata, Juncus roemerianus, Paspalum vaginatum, Salicornia bigelovii and Spartina alterniflora) and two levels of leaching fraction (0·25 and 0·50) under three irrigation salinity levels (10, 20, and 40 gL−1) in a randomized complete block design arranged in split plots. The purpose of the research was to apply the theory of crop salt tolerance on aboveground yields and agronomic characteristics of halophytes to determine their thresholds for salinity and to determine leaching requirements. The results indicated that the halophyte species tested can grow with minimum reduction in the growth potential at <20 gL−1of mean salinity of soil solution. Leaching fraction (LF) of 0·25 at the highest salinity of irrigation water (40 gL−1) was inadequate to attain the steady-state salt balance during the growth period, although the drainage salinity reached more than 90 g L−1. Furthermore, if the same level of LF is used for longer period, soil salinity under this high salt treatment will continue to rise, and plant growth may deteriorate. Leaching fraction of 0·50 is preferable if salinity of irrigation water was more than 20 gL−1and dry matter production is considered, although the amount of water use will be excessive  相似文献   

14.
We assessed the geographic distribution of Tillandsia lomas in northern Chile, from Arica (18°20′S) to the Loa river (21°25′S) and discussed the factors that might potentially underlie the observed patterns. We carried out extensive field survey complemented with aerial surveys and analysis of specimens deposited in herbaria. We detected over 30 Tillandsia stands most of which corresponded to the species Tillandsia landbeckii and can be grouped in 10 large systems. Other two species were also detected Tillandsia marconae and Tillandsia virescens, both of which show a restricted distribution in the area. Our results provide evidence on the wide distribution of Tillandsia lomas in northern Chile and its association with fog corridors.  相似文献   

15.
A greenhouse experiment was conducted to investigate the effects of moderate (14 cm) vs. severe (7 cm) defoliation on tiller recruitment and mortality within the interior and on the perimeter of weeping lovegrass (Eragrostis curvula) tussocks. Tiller number per marked shoot within the interior was significantly greater in plants defoliated at 14-cm stubble height than at 7-cm stubble height (2·37 vs. 0·43, p<0·05) 28 weeks after defoliation; likewise, tiller number per marked shoot was also greater on the perimeter of tussocks in the higher stubble than the lower stubble defoliation treatment (5·10 vs. 2·03, p<0·05). Tiller natality increased significantly 4 weeks after defoliation except for tillers cut to 7-cm stubble height within the interior of tussocks where tiller natality reached a peak at 4·5 weeks after defoliation. During the later stages of canopy development, tiller numbers per marked shoot gradually declined as tiller mortality increased. Tillers began senescing 13 weeks after defoliation, and the senescence rate was 52% greater in the 7-cm stubble height treatment than in the 14-cm stubble height treatment (2·33 vs. 1·53, p<0·0001). Within the interior of tussocks, number of the marked tillers more than doubled under the 14-cm stubble height treatment, whereas the plants lost 57% of the monitored tillers under the 7-cm stubble height treatment. We concluded that the central dieback process in weeping lovegrass is accelerated by severe defoliation.  相似文献   

16.
The objectives of this study were to determine the composition, density and spatial distribution of the soil seed bank of woody species, as well as their regeneration pattern in two different land use systems, controlled (ranch) and open grazing, in an Acacia woodland of the Rift Valley in Ethiopia. We also compared the species composition of the soil seed bank and the above-ground vegetation to find out if differences exist in the soil seed bank and advance regeneration between the two land use systems. The germination requirements of seeds of the woody species were also investigated under laboratory conditions. Acacia senegal, Acacia seyal, Acacia tortilis, Dichrostacys cinerea and Balanites aegyptiaca were encountered in the above-ground vegetation in both systems. Seeds of only A. tortilis (90±32 seeds m−2) from the ranch, and A. senegal (5±3 seeds m−2) and A. tortilis (72±34 seeds m−2) from the open system were found in the soil seed bank along transects with patchy horizontal pattern. The two systems were not significantly different in density of soil seed banks of A. senegal andA. tortilis . Jaccard's similarity index showed that only a few woody species were common in the soil seed flora and above ground vegetation. However, all of the species accumulated seeds (58±43–331±130 seeds m−2) in/on the soil under the canopy. Very large numbers of seeds of A. tortilis (19382±9722 seeds m−2) and D. cinerea (1278±494 seeds m−2) were also found in barns. Most of the seeds recovered from the soil samples (60–80%) were found in the litter layer. Acid and mechanical scarification improved legume germination (36–99% and 60–99%, respectively) over boiling water (0–48%). Treatment means differed significantly for all the legumes (p<0·001) but not for Balanites. Height and diameter class distribution of regeneration of A. tortilis and D. cinerea in both systems and A. senegal in the open system had a negative exponential correlation (rs=−0·5, −0·25 and −0·86, respectively). A. seyal and B. aegyptiaca showed poor regeneration. Horizontal distribution of advance regeneration of all the species was patchy. Advance regeneration of A. seyal, A. tortilis and B. aegyptiaca were not significantly different, while that of A. senegal and D. cinerea were significantly different between the two systems. Poor representation of species in the soil seed bank along transects and in the different height and diameter classes may be attributed to the low density of mature trees as well as the mode and strategy of seed dispersal. Ungulate and wind dispersed species (e.g. A. senegal, A. tortilis and D. cinerea) were highly favoured. Patchiness in the distribution of seeds and advance regeneration was also a result of endozoochory. Dispersal of non-ungulate dispersed seeds (A. senegal, A. seyal and B. aegyptiaca) was restricted to the canopy zone. Piles of seeds ofA. tortilis and D. cinerea that were found in barns were a result of consumption of their pods by cattle. High concentration of seeds in the litter layer may be due to low soil disturbance and larger size of seeds. The height and diameter class distribution of A. senegal (in the open system), A. tortilis and D. cinerea also indicated that the species have good regeneration. Results from the germination tests indicated that seeds of the legumes require pre-sowing treatments to give a rapid, uniform and improved germination. Intervention through artificial regeneration should be employed to improve the density and regeneration capacity of those species with hampered regeneration at both systems.  相似文献   

17.
The columnar cactus Polaskia chende, endemic to Central Mexico, occurs in the wild and in silviculturally managed populations where artificial selection for better edible fruits operates by sparing and favouring desirable phenotypes during vegetation clearance. Wild and managed populations were compared in morphology and reproductive biology to analyse if human selection has influenced phenotypic divergence, if it has modified breeding systems and if reproductive isolation exists between them. Fruits from managed populations were larger (6·36±0·17 cm3), heavier (19·62±1·32), sweeter (9·07±0·25 °Brix), and with thinner peel (2·09±0·13 mm) than those from wild populations (5·77±0·18 cm3, 15·73±0·85 g, 8·23±0·31 °Brix, and 2·58±0·14 mm, respectively). Both population types bloom synchronically, show diurnal anthesis, with bees as the most likely pollinators, and have breeding system predominantly self-incompatible. Spatial and temporal barriers to pollen exchange between wild and managed populations are unlikely, and morphological differences would be related with an ongoing process of artificial selection.  相似文献   

18.
Impacts of six initial treatments and rainfall on maximum above-ground herbaceous standing crop were assessed in 1978 and annually from 1984 to 1992 in rangelands of east-central semi-arid Argentina. Treatments applied in 1977–1978 were: (1) untreated control; (2) burning; (3) herbicide application (shrub control); (4) Old Field 1 or (5) Old Field 2 (areas previously exposed to different degrees of mechanical soil disturbance for 25 years); and (6) overgrazing. Domestic herbivores were excluded thereafter from all treatments until 1993.During 1984–1992, total above-ground herbaceous standing crop often remained greater in both Old Fields than in the other treatments; highest mean values were 2336 and 1640 kg ha−1for Old Field 1 and Old Field 2 treatments, respectively. Lowest total above-ground standing crops in all treatments (M = 296–475 kg ha−1) occurred in 1989, the year with the lowest annual rainfall (257·5 mm). Desirable perennial grasses contributed most (44–100%) to total herbaceous standing crop in all treatments during the study period. Most of this standing crop was made up of the cool-season grassesPiptochaetium napostaense, Poa ligularis, Stipa clarazii, S. papposaandS. tenuis. Annual rainfall was closely related (p< 0·05) to total herbaceous and desirable perennial grass standing crops in most treatments, and accounted for most of the variation in herbage production between years.  相似文献   

19.
We report on the phenology, seed production, and three assays (field, greenhouse and laboratory) to detect the optimal temperature for germination (OPT) and the temperature with fastest germination rate (TFASG) of seven Mimosa species, all endemic to Mexico: Mimosa adenantheroides (M. Martens and Galeotti) Benth., M. calcicola B. L. Rob., M. lacerata Rose, M. luisana Brandegee, M polyantha Benth., M. purpusii Brandegee and M. texana (A. Gray) Small var. filipes (Britton and Rose) Barneby. Flowering and fruiting take place during the wet season (May–September). Though flowering periods overlap among species, no hybrid species were found. The reproductive strategy includes a large number of seeds produced (4000–15000 per plant); however, a high percentage (30–75%) is not viable due to bruchid predation (Acanthoscelides mexicanus (Sharp) and Stator pruininus Horn). In the laboratory, OPT was established at (23°C)24°C(27–28°C) and TFASG at (20°C)25°C(30°C), with 100% of germination in six of the seven Mimosa species. In the field, the maximum germination rate (MAXG) was 92% to 100%, whereas in the greenhouse this value was 56% to 80%. Our results show that heat and moisture do not necessarily stress Mimosa seedling germination, but shade seems to be an important factor influencing germination.  相似文献   

20.
Prosopis ruscifolia, with its 15 cm long spines, has spread onto vast areas of Argentine grazing lands and formed dense impenetrable stands. We examined grafting the highly respected Prosopis alba (algarrobo blanco) onto Prosopis ruscifolia (vinal) rootstock, with the objective of converting undesirable stands of vinal into desirable stands of P. alba. This grafting experiment examined five dates and three light treatments (full-sunlight, half-shade and ‘tent’ treatment 30×40 cm black plastic shade). The full-sun, half-shade and tent treatments gave 2·4±2·02, 6·0±1·22 and 6·6±0·99 (mean±95% confidence interval) successful grafts out of 10, respectively. Future work should evaluate grafting of P. ruscifolia coppice resprouts with P. alba scions, after the P. ruscifolia trees have been harvested for fuelwood or lumber. EliteP. alba clones are now available for use in grafting.  相似文献   

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