Optimal Models Of Farmland Shelterbelts On Chongming Island, Shanghai

Farmland shelterbelts are the main component of Chongming Island's shelter forest network. The objective of this study was to select the optimum farmland shelterbelt type by determining the optimal shelter forest porosity. After choosing the five main farmland shelterbelts models and surveying the community structure, wind protecting efficiency, soil physical and chemical properties and yield increase benefit were determined for each shelter forest. The following conclusion was drawn through the research:1. The yield and 1000-grain weight were improved by the forest protection. Significant difference happened in Grain yield and 1000-grain weight at different distances at the same sample plot, and the changing trend was the same. Leaf area index was also improved. Great differences occured in the yield increase among different models, the benefit of yield increase of taxodium ascendens was the greatest, followed by mixed forest of water fir and cryptomeria, water fir, and poplar, and that of cryptomeria was the lowest.2. The range of five farmland shelterbelts models'porosity was between 0.31 and 0.39, and it was called porosity structure belt, which have better windbreak effect. While the average porosity for mixed forest of water fir and cryptomeria was close to the best porosity value. Among the different models, the wind protecting efficiency was in the order mixed forest of water fir and cryptomeria> taxodium ascendens> water fir> poplar> cryptomeria.3. Based on the measurement of wind protecting efficiency and yield increase benefit, the most effective protective distance:taxodium ascendens was 1H (fivefold height of the tree)-5H, mixed forest of water fir and cryptomeria was 3H-10H, water fir was 3H-10H, poplar was 3H-10H, and cryptomeria was 1H-5H. Among the different models, the most effective protective area was in the order poplar> taxodium ascendens> mixed forest of water fir and cryptomeria> water fir> cryptomeria.4. Soil organic matter, total nitrogen, total phosphorus, moist content of different layers of five farmland shelterbelts models' plot were bigger than bottomland's while soil bulk density and pH value of different layers of five farmland shelterbelts models' plot were smaller than bottomland's. These indicated that each protective forest can improve coastal soil. Because of the different of tree species and time of afforesting, the benefits of improvement were different. The optimum farmland shelterbelt types were taxodium ascendens, mixed forest of water fir and cryptomeria, followed by water fir, and poplar, and that of cryptomeria was the lowest. 5. Based on the measurement of wind protecting efficiency, soil physical and chemical properties and yield increase benefit, the mixed forest was better than the monotypic forest with the mixed forest of water fir and cryptomeria being the optimal shelterbelt model. Now the protective forests such as cryptomeria were too short, and most were pure models. They were easily harmed by illness and did no good to trees. Some of them might be cut, and then we could plant taxodium ascendens, water fir or poplar with cryptomeria. Thus the forests could still prevent the wind they were replaced, and also could remedy soil degradation, using the productive power of forest soil batter.