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71.
W. O. van der Knaap Jacqueline F. N. van Leeuwen Tomasz Goslar Robert Krisai Willy Tinner 《Vegetation History and Archaeobotany》2012,21(1):37-60
Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the
last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed
peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting
variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in
samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized
in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population
sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as
0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is
difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques
are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales
in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results
in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human
action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All
mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied
small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800. 相似文献
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Summary Eight Hawaiian Dubautia species grow in habitats as varied as exposed lava, dry scrub, mesic forest, wet forest, and bog. These species also differ in diploid chromosome number, with four species having 13 pairs of chromosomes and four species having 14 pairs. This ecological and chromosomal variation is paralleled by significant interspecific variation in tissue elastic properties. The four 13-paired species from dry habitats exhibit significantly lower tissue elastic moduli near full hydration (E
i) than the four 14-paired species from mesic to wet habitats. Values of E
i range from 2 to 4 MPa among the former species and from 9 to 18 MPa among the latter species. The turgor dependence of the elastic modulus also differs markedly between the two groups of species. As a result of these differences in tissue elastic properties, the capacity for maintaining high turgor pressures as tissue water content decreases is much greater in the 13-paired species from dry habitats than in the 14-paired species from mesic to wet habitats. These results indicate that the evolutionary diversification of the Dubautia species has been accompanied by a significant degree of change at the physiological level. 相似文献
80.
Robert Kirkland 《BMJ (Clinical research ed.)》1907,2(2427):8-12