| Based on the study of pollen, 39 samples from Songhua Lake and 37 ones from Hailong reservoir, in the middle of Jilin Province, the paper shows as follows research results:In Songhua Lake pollen assemblageⅠandⅡ(from 1961 to 1984), The Herbs dominated for 52% during the whole stage,and trees for 37%;In assemblageⅢandⅣ, pollen of trees average percentage is 54%,and pollens of herbs average percentage is 38.7%. Trees pollens are maily composed of Quercus, Betula;Herbs pollen are maily composed of Artemisia, Chenopodiaceae. the characteristics of vegetation show a broad-leaved forest. And the amounts of Quercus、Betula、Juglans increased, that of Artemisia decreased gradually, showing vegetation expanded .In Hailong reservoir pollen assemblage A and B (from 1955 to 1980), the proportion of trees and herbs are both equal;In combination C and D(from1980 to 2003), The trees dominated during the whole stage,average percentage 56%, pollen of herbs average percentage is 33% and decreasing, indicating the vegetation type of coniferous trees and broad-leaved mixed forest. Area of vegetation expanded.Based on analysis of the meteorological data from the middle area of Jilin, it is suggested that the annual mean rainfall fluctuates obviously, with the overall decreased since 1950s. The annual mean temperature tended to become high, and the annual mean relative humidity tended to become lower. In general, the temperature in the region rises, the evaporation becomes stronger, resulting lower effective moisture, as well as warm and dry climate.Characteristics of pollen assemblage reveal that in the disadvantageous background of warm and arid climate, the improved vegetation in Songhua lake and Hailong reservoir can be attributed to returning farmland to forests, as well as other efforts. It reveals the effects of human activities on the ecological environment change, and the adjustment of human behavior can make quite reasonable results. |
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