Seed germination of five Helianthemum species: Effect of temperature and presowing treatments     

F. Prez-García  M.E. Gonzlez-Benito 《Journal of Arid Environments》2006,65(4):688-693

The germination behaviour of five Helianthemum species (H. almeriense, H. appeninum, H. cinereum, H. hirtum, H. squamatum) has been studied under controlled conditions. Constant 15, 20 or 25 °C and alternating 25/15 °C temperature regimes and 16/8 h light/dark photoperiod conditions were used. Presowing treatments applied were manual scarification, boiling water, hot water, dry heat and sulphuric acid. Germination values recorded were final germination percentage and germination rate expressed as days to reach 50% of the final germination percentage (T₅₀). Incubation temperature had no significant effect on final germination percentage for untreated seeds of the five Helianthemum species. However, variation due to temperature was significant for scarified seeds, with the lowest germination percentage attained at 25 °C. In all Helianthemum species studied, the highest germination percentages were obtained by manual scarification of seeds. Germination rate of scarified seeds decreased as germination temperature increased. The different presowing treatments investigated allowed some germination in some species, but none were any better than manual scarification. The high germination among most species studied, following mechanical rupture of the seed coat, shows that the mechanism of dormancy lies in the seed coat. The physical dormancy caused by impermeable seed-coat appears to be the main reason of poor germination of untreated seeds of Helianthemum species studied.  相似文献   

Passage time, viability, and germination of seeds ingested by foxes     

O. Varela  E.H. Bucher   《Journal of Arid Environments》2006,67(4):566-578

Seed passage through the gut of vertebrates is critical for endozoochorus seed dispersal because it may influence seed viability and germination capacity. Foxes are important fruit-eaters and presumed seed dispersers in the Chaco ecoregion. We experimentally assessed the seed passage time (SPT) and its effect on seed viability and germination in two fox species (Pseudalopex gymnocercus and Cerdocyon thous) that occur in the Chaco. Passage time was measured using both artificial seeds of three sizes and wild seeds of four native fleshy-fruit species commonly consumed by foxes (Ziziphus mistol, Acacia aroma, Celtis tala, and Syagrus romanzoffiana). The average SPT was 6.8 h for the Pampa fox and 7.7 h for the crab-eating fox. Passage time did not differ either among seeds of different sizes or between fox species. Passage through digestive tract of both fox species did not affect seed survival, whereas germination rate was slightly improved. Our data suggest that both fox species are legitimate seed dispersers of the four species studied, having a positive effect on both germination rate and dispersal distance from the parent plant.  相似文献   

Food Deserts in the Prairies? Supermarket Accessibility and Neighborhood Need in Edmonton,Canada*     

Karen E. Smoyer‐Tomic  John C. Spence  Carl Amrhein 《The Professional geographer》2006,58(3):307-326

The U.S. and U.K. literatures have discussed “food deserts,” reflecting populated, typically urban, low‐income areas with limited access to full‐service supermarkets. Less is known about supermarket accessibility within Canadian cities. This article uses the minimum distance and coverage methods to determine supermarket accessibility within the city of Edmonton, Canada, with a focus on high‐need and inner‐city neighborhoods. The results show that for 1999 both of these areas generally had higher accessibility than the remainder of the city, but six high‐need neighborhoods had poor supermarket accessibility. We conclude by examining potential reasons for differences in supermarket accessibility between Canadian, U.S., and U.K. cities.  相似文献   

Reconstructing hydrological variability from testate amoebae analysis in Carpathian peatlands     

C. Schnitchen  D. J. Charman  E. Magyari  M. Braun  I. Grigorszky  B. Tóthmérész  M. Molnár  Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17

Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content, confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania, Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP, 3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow which affected eastern Europe as well as western and central Europe.  相似文献   

Interglacial History of a Palaeo-lake and Regional Environment: A Multi-proxy Study of a Permafrost Deposit from Bol’shoy Lyakhovsky Island, Arctic Siberia     

Boris P. Ilyashuk  Andrei A. Andreev  Anatoly A. Bobrov  Vladimir E. Tumskoy  Elena A. Ilyashuk 《Journal of Paleolimnology》2006,35(4):855-872

Chironomid, pollen, and rhizopod records from a permafrost sequence at Bol’shoy Lyakhovsky Island (New Siberian Archipelago) document the development of a thermokarst palaeo-lake and environmental conditions in the region during the last Interglacial (MIS 5e). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool initially existed at the site. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in the basin, which could in turn induce thermokarst and create a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. The chironomid record points to three successive stages during the history of the lake: (1) a peatland pool; (2) a pond (i.e., shallower than the maximum ice-cover thickness); and (3) a shallow lake (i.e., deeper than the maximum ice-cover thickness). The trend of palaeo-lake development indicates that intensive thermokarst processes occurred in the region during the last Interglacial. Shrub tundra communities with Alnus fruticosa and Betula nana dominated the vegetation during the interglacial optimum. The climate was moister and warmer than present. The results of this study suggest that quantitative chironomid-based temperature reconstructions from Arctic thermokarst ponds/lakes may be problematic due to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblages.  相似文献   

Post-depositional 137Cs Mobility in the Sediments of Three Shallow Coastal Lagoons, SW England     

I. D. L. Foster  T. M. Mighall  H. Proffitt  D. E. Walling  P. N. Owens 《Journal of Paleolimnology》2006,35(4):881-895

We present ¹³⁷Cs profiles for three low lying coastal lagoons in Southwest England that show a decline in activity with sediment depth. ¹³⁷Cs inventories are lower than expected by comparison with local reference inventories despite the fact that sampling was undertaken in the deep-water zone of each lake where sediment and ¹³⁷Cs focusing would be expected. At all three locations, lake sediment ⁷Be and unsupported ²¹⁰Pb (²¹⁰Pb_un) inventories are not significantly lower than the local reference inventory. ¹³⁷Cs inventories in the study cores range from 38 to 95% of local reference inventories. The standing water level and mud: water interface at two sites are below maximum tide level and, at all three sites, salinity increases significantly in the water columns between low and high tide and in the pore waters of the underlying sediments. We suggest that the difference in hydrostatic pressure between sea level and standing water levels in the lagoons forces salt water up through the sediment column and that monovalent cations (especially Na⁺ and K⁺) replace ¹³⁷Cs on exchange sites leading to the upward migration and loss of ¹³⁷Cs. Rising sea levels may therefore contribute to remobilisation and release of ¹³⁷Cs to the aquatic environment from the sediments of coastal lagoons.  相似文献   

Sources of Sediment in Lake Pepin on the Upper Mississippi River in Response to Holocene Climatic Changes     

David W. Kelley  Stefanie A. Brachfeld  Edward A. Nater  Herbert E. Wright Jr. 《Journal of Paleolimnology》2006,35(1):193-206

Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source areas. We observed a decreased contribution from the Minnesota River during the interval ~6700–5500 ¹⁴C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation, snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America.  相似文献   

Physiography and tectonic setting of the subglacial lake district between Vostok and Belgica subglacial highlands (Antarctica)     

I. E. Tabacco  P. Cianfarra  A. Forieri  F. Salvini  A. Zirizotti 《Geophysical Journal International》2006,165(3):1029-1040

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Dual simulations of fluid flow and seismic wave propagation in a fractured network: effects of pore pressure on seismic signature   总被引：1，自引：0，他引：1  

S. Vlastos  E. Liu  I. G. Main  M. Schoenberg  C. Narteau  X. Y. Li  B. Maillot 《Geophysical Journal International》2006,166(2):825-838

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Palaeomagnetism of the teschenitic rocks (Lower Cretaceous) in the Outer Western Carpathians of Poland: constraints for tectonic rotations in the Silesian unit     

J. Grabowski  L. Krzemi&#;ski  P. Nescieruk  E. Starnawska 《Geophysical Journal International》2006,166(3):1077-1094

Results of palaeomagnetic investigations of the Lower Cretaceous teschenitic rocks in the Silesian unit of the Outer Western Carpathians in Poland bring evidence for pre-folding magnetization of these rocks. The mixed-polarity component reveals inclinations, between 56° and 69°, which might be either of Cretaceous or Tertiary age. Apparently positive results of fold and contact tests in some localities and presence of pyrhotite in the contact aureole suggest that magnetization is primary, although a Neogene or earlier remagnetization cannot be totally excluded since inclination-only test between localities gives 'syn-folding' results. Higher palaeoinclinations (66°–69°) correlate with a younger variety of teschenitic rocks dated for 122–120 Ma, while lower inclinations (56°–60°) with an older variety (138–133 Ma). This would support relatively high palaeolatitudes for the southern margin of the Eurasian plate in the late part of the Early Cretaceous and relatively quick northward drift of the plate in this epoch, together with the Silesian basin at its southern margin. Declinations are similar to the Cretaceous–Tertiary palaeodeclinations of stable Europe in the eastern part of the studied area but rotated ca. 14°–70° counter-clockwise in the western part. This indicates, together with older results from Czech and Slovakian sectors of the Silesian unit, a change in the rotation pattern from counter-clockwise to clockwise at the meridian of 19°E. The rotations took place before the final collision of the Outer Carpathians nappe stack with the European foreland.  相似文献