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Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
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M. A. Tzec-Simá R. Orellana M. L. Robert 《In vitro cellular & developmental biology. Plant》2006,42(1):54-58
Summary
Bactris major and Desmoncus orthacanthos are native palms from the Yucatan Peninsula which could be used as substitutes for rattan. When their seeds were germinated
in vivo and in vitro they proved to be highly recalcitrant. Therefore, the culture of isolated embryos was studied as an alternative means of
producing planting material for nurseries. It was found that the in vitro germination of the isolated embryos was gradually reduced by storage, falling to zero by 5 wk. However, isolated embryos
from freshly collected seeds germinated at ∼100% frequency. The presence of the endosperm, whether still attached to the embryos
or separated from them but in direct contact with the nutrient medium, greatly reduced germination in both species. High concentrations
of abscisic acid (ABA, 100 μM) only slightly diminished it, suggesting a different cause for the observed endosperm-induced inhibition. This embryo rescue
method permits the production of sufficient plants for in vitro micropropagation and the establishment of experimental plots to evaluate the full potential of these materials. 相似文献
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Robert Knox 《BMJ (Clinical research ed.)》1923,1(3242):305-306
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The seven currently recognized species of Geranium endemic to the Hawaiian Islands are unusual in their shrubby or arborescent habit and unlobed, parallel-veined leaves rather than the palmately cleft or lobed leaves and herbaceous habit typical of the genus. Their placement within the genus and their biogeographic source have been obscured by this morphological distictiveness and the limited resolution of relationships on the basis of morphology in the very speciose subgenus Geranium. Phylogenetic analysis of rbcL gene sequences provides strong support for the monophyly of the Hawaiian group, and indicates that the Hawaiian clade is deeply nested within section Geranium rather than comprising a separate section. The continental relatives studied to date with the greatest similarity in sequence to the Hawaiian group are native to the Americas rather than Asia or the Pacific. The Hawaiian species are extremely similar to one another in rbcL sequence, while the tree topology obtained is consistent with a basal position for Geranium arboreum within the group. 相似文献