| The tree lawn is the window to display the image of city, tree lawn not only meet the ecological function with "sunshade" of city, also need to meet increasing the value of landscape, according to the survey, the Fraxinus in Beijing city exist more serious segregation phenomenon, "shapes" affecting the uniformity of green with high-rise structure. Through the phenology period survey and growth characteristic analysis of Fraxinus which is Beijing street trees, focusing on the phenotype diversity characteristics of Fraxinus trees, analysis of anatomical structure and cold resistance of different types of Fraxinus leaves, emphasizes on exploring the reasons of green period appear obvious differentiation in Fraxinus, draws the following conclusion:1. Through the phenological survey that the longest green period of Fraxinus leaves can reach more than 260 days, and the shortest days only about 200 days. The green period of Fraxinus leaves in deep color is generally longer than light ones, the small type of Fraxinus leaves still have longer green period.2. Statistics on 80 samples of Fraxinus trees, found that the length, base, width, shape, form, and dent in leaves, Crown Traits Variation, the coefficient of variation is above 40%, the coefficient of variation with the width is more than 70%, but also reflects the characters separation of Fraxinus. Divided the Fraxinus trees into 5 types which according to the size by using principal component analysis and cluster analysis, the first type is the Fraxinus with small green leaves, the second one is green leaves with big type, the third kinds of Fraxinus have light green and big leaves, the fourth type of Fraxinus with huge deep green leaveas, the fifth kinds of Fraxinus are deep green leaves with small type.3. The test was carried out to study the anatomical features of Fraxinus leaves in different defoliation, found the thickness of leaves between 145-205μm, the main vein thickness was about 391.9-582.Oμm, upper epidermal thickness was about 11.3-18.3μmm, the thickness of under epidermal cells was about 11.5-12.9μm. The thickness ratio between palisade and spongy organization cells of Fraxinus leaves were 0.88-1.31, with the defoliation was delayed, the ratio between palisade and spongy gradually reduced. With longer green period of Fraxinus leaves, the thickness of leaves and main vein,stoma cells were more huge, the size of stomatal cell had highest sensitivity.4. Measuring the electrolyte leakage of Fraxinus leaves under low temperature treatment, the Logistic fitting effect was good, using it to calculate half semilethal temperature, the winter resistance was higher, the value of LT50 was smaller, on the other hand, cold resistance was weak. The chlorophyll content of Fraxinus leaves gradually reduced as the temperature decrease, but the range was different, the chlorophyll in type of deciduous leaveas decreased most.In summary, the results of rough classification by direct observation and principal component analysis or cluster analysis were basically consistent. The Fraxinus in Beijing area can be divided into 5 types, the type of deep green leaves in small size which period can reach more than 260 days, found by anatomy that the leaves were thick and high fleshy, chlorophyll content change with the environment reflect sensitive, helps the plants to self-protection, therefore, the type of deep green leaves with small size in Fraxinus can better adapt to environmental change. |
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