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Earth & Life (http://www.geofinds.com), 2006-10-1, Vol.1. , No.1: 12-15
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2 Comparison to modern florasModern terrestrial plants are divided as 6 floral kingdoms: the tundra, the boreal forest, the temperate deciduous forest, the tropical rain-forest, the grassland, and the desert (Emberlin, 1983). The tundra occupies the zone between latitude 57 degrees north and the polar regions and has no growing season. It contains low weeds including heaths, grasses, sedges, mosses and lichens, lacking any trees. The tundra communities lack substantial layering and the overall species diversity is low. Most of the species, however, have wide distributions. The boreal forest occupies the zone from the edge of the tundra to about 800 km southwards. Containing a few dominant trees, some shrub and a few herbaceous plants, the communities are homogeneous, lacking diversity and layering. Most of the trees have wide distributions. For example, species of pine, spruce and fir are found throughout. Some large animals and many insects dwell in this flora. The temperate deciduous forests contain diverse species of tree, with 12~60 dominant, and prolific undergrowth. The vegetation structure may have several layers. The animals inhabit the flora are diverse and abundant. The tropical rain-forests are dominated by broad-leafed evergreen trees and are extremely diverse, highly stratified, inhabited by diverse and abundant animals. The temperate grasslands are dominated by Gramineae. The deserts contain few plants or animals. The aftermath flora differed from the tundra and grasslands in that the latter has no trees. It was different from modern temperate deciduous forests and tropical rain-forests in the development of broad-leafed trees and other tall broad-leafed plants in the latter. Its difference from modern deserts lies in the absence of conifers in the latter. In terms of total appearance and community structure, the aftermath flora was very similar to modern boreal forest. The similarities between them lie in: (1) both are homogeneous in species composition, (2) both are composed of a few species of conifers, lycopods, and some others, (3) both are dominated by the plants of needle-shaped leaves, (4) though homogeneous, most species are cosmopolitan in distribution, encountered throughout; (5) both lack layering of communities; (6) both are inhabited by a few large animals but relatively abundant insects. Because of their similarity, the aftermath flora should be adapted to the climate conditions similar to those of modern boreal forest. The climate of modern boreal forest is characterized by the annually average temperatures of below 0 ºC, the common temperatures of 0~18ºC during the warmest months (2~4 months per year), and the short growth season of 3-4 month annually. The replacement of the flourishing tropical to subtropical floras of the Latest Permian by the aftermath flora of cold climate was one of the key aspects of the end-Permian mass extinction. It should be caused by a drastic climatic cooling event, in the order of a decrease in temperature by 10~20ºC. Such a cooling not only totally destroyed the terrestrial ecosystems but also the diverse marine ecosystems. Since the aftermath flora was cosmopolitan in distribution, the aftermath Earth was speculated to have no climatic zonation. That is, the whole Earth was a “cooling ball”. |
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