255 resultados para Malvaceae.


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The areas of marine pollen deposition are related to the pollen source areas by aeolian and fluvial transport regimes, whereas wind transport is much more important than river transport. Pollen distribution patterns of Pinus, Artemisia, Chenopodiaceae-Amaranthaceae, and Asteraceae Tubuliflorae trace atmospheric transport by the northeast trades. Pollen transport by the African Easterly Jet is reflected in the pollen distribution patterns of Chenopodiaceae-Amaranthaceae, Asteraceae Tubuliflorae, and Mitracarpus. Grass pollen distribution registers the latitudinal extension of Sahel, savannas and dry open forests. Marine pollen distribution patterns of Combretaceae-Melastomataceae, Alchornea, and Elaeis reflect the extension of wooded grasslands and transitional forests. Pollen from the Guinean-Congolian/Zambezian forest and from the Sudanian/Guinean vegetation zones mark the northernmost extension of the tropical rain forest. Rhizophora pollen in marine sediments traces the distribution of mangrove swamps. Only near the continent, pollen of Rhizophora, Mitracarpus, Chenopodiaceae-Amaranthaceae, and pollen from the Sudanian and Guinean vegetation zones are transported by the Upwelling Under Current and the Equatorial Under Current, where those currents act as bottom currents. The distribution of pollen in marine sediments, reflecting the position of major climatic zones (desert, dry tropics, humid tropics), can be used in tracing climatic changes in the past.

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A 200 m long marine pollen record from ODP Site 658 (21°N, 19°W) reveals cyclic fluctuations in vegetation and continental climate in northwestern Africa from 3.7 to 1.7 Ma. These cycles parallel oxygen isotope stages. Prior to 3.5 Ma, the distribution of tropical forests and mangrove swamps reached Cape Blanc, 5°N of the present distribution. Between 3.5 and 2.6 Ma, forests occurred at this latitude during irregular intervals and nearly disappeared afterwards. Likewise, a Saharan paleoriver flowed continuously until isotope Stage 134 (3.35 Ma). When river discharge ceased, wind transport of pollen grains prevailed over fluvial transport. Pollen indicators of trade winds gradually increased between 3.3 and 2.5 Ma. A strong aridification of the climate of northwestern Africa occurred during isotope Stage 130 (3.26 Ma). Afterwards, humid conditions reestablised followed by another aridification around 2.7 Ma. Repetitive latitudinal shifts of vegetation zones ranging from wooded savanna to desert flora dominated for the first time between between 2.6 and 2.4 Ma as a response to the glacial stages 104, 100 and 98. Although climatic conditions, recorded in the Pliocene, were not as dry as those of the middle and Late Pleistocene, latitudinal vegetation shifts near the end of the Pliocene resembled those of the interglacial-glacial cycles of the Brunhes chron.

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Mode of access: Internet.

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Malva parviflora L. (Malvaceae) is rapidly becoming a serious weed of Australian farming systems. An understanding of the variability of its seed behaviour is required to enable the development of integrated weed management strategies. Mature M. parviflora seeds were collected from four diverse locations in the Mediterranean-type climatic agricultural region of Western Australia. All of the seeds exhibited physical dormancy at collection; manual scarification or a period of fluctuating summer temperatures (50/20 degrees C or natural) were required to release dormancy. When scarified and germinated soon (1 month) after collection, the majority of seeds were able to germinate over a wide range of temperatures (5-37 degrees C) and had no light requirement. Germination was slower for seeds stored for 2 months than seeds stored for 2 years, suggesting the presence of shallow physiological dormancy. Seed populations from regions with similar annual rainfall exhibited similar dormancy release patterns; seeds from areas of low rainfall (337-344mm) were more responsive to fluctuating temperatures, releasing physical dormancy earlier than those from areas of high rainfall (436-444mm). After 36 months, maximum seedling emergence from soil in the field was 60%, with buried seeds producing 13-34% greater emergence than seeds on the surface. Scanning electron microscopy of the seed coat revealed structural differences in the chalazal region of permeable and impermeable seeds, suggesting the importance of this region in physical dormancy breakdown of M. parviflora seeds. The influence of rainfall during plant growth in determining dormancy release, and hence, germination and emergence timing, must be considered when developing management strategies for M. parviflora.

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The Gulf of Carpentaria is an epicontinental sea (maximum depth 70 m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12 m deep) and to the west by the Arafura Sill (53 m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf was separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. A preliminary interpretation is presented of the palaeoenvironments recorded in six sediment cores collected by the IMAGES program in the Gulf of Carpentaria. The longest core (approx. 15 m) spans the past 130 ka and includes a record of sea-level/lake-level changes, with particular complexity between 80 and 40 ka when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry clearly identifies a final marine transgression at about 9.7 ka (radiocarbon years). Before this transgression, Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600 km-300 km and a depth of about 15 m. The earlier rise in sea-level which accompanied the Marine Isotopic Stage 6/5 transgression at about 130 ka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

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