| In recent years, food security has become a key issue which is closely related to human survival quality in the background of global change. As the barrier of farmland ecosystem, the complete structure of farmland protection forest in the hot wind and sand disasters reduced, improving the climate, soil, hydrological conditions. However, missing plants and gaps in the forest belt which increased the wind speed in the forest network, changed the spatial distribution pattern and reduced the output and quality of crops severely exist generally due to the anthropogenic destruction, plant diseases, and insect pests in the process of the farmland shelterbelt management. Depth study of the mechanism of the effect of the imperfect shelterbelt structure on the wind fields of farmland shelterbelt network has become a key factor in dispute resolution. In this study, we reveal the spatial and temporal variation in wind speed by the method of real-time synchronous observation in a farmland shelterbelt network based on ecological field theory. Its aim is to illustrate the spatial heterogeneities of various ecological factors in the forest network and explore its sphere of influence in the different imperfect shelterbelt structure, and provide theoretical basis and technological guidance for sustainable management of farmland shelter forest construction. The main conclusions are as follows:(1) Influence of missing plants on shelterbelt porosity Shelterbelt porosity of different missing plants types increase with the increase of the number of missing plants in a certain range. When the numbers of missing plants were the same, missing six plants continuously type of missing plants resulted in the largest increasing degree of shelterbelt porosity, while discrete type of of missing plants resulted in the smallest. Using binomial and linear fit methods made the best impact on the model between missing plants numbers and shelterbelt porosities of different types of missing plants, correlation coefficients were all above 0.9.(2) Wind speed changes in the forest All the Different kinds of wilderness wind speeds showed a increase-decrease trend in the process of going through the forest(-9 ~ 9.2m). The wind speed increases gradually with the increase of the wind speed in the field, and the wind speed disincreases gradually from the-4.5m to the 4.7m. Wind speeds increased with increasing wind angle and wind speed increased significantly(P <0.05) at 0-9.2m under the same direction. In the different wilderness wind speed or wind direction, wind speeds inside the forest belt along the strike showed wavy trend.(3)The influence of the gaps in the primary forest belt on the wind field The gaps in the primary forest belt change the spatial distribution pattern in the wind field, and the degree of the increasing wind speed is closely related to the length of the shelterbelt structure in the same desert wind conditions:(1) The longer the length of the gaps, the greater the wind speed of the sample, and the wind speed level of the wind field is gap 13 m > gap 7m > gap 4m.(2) The bigger the length of the gaps, the longer the distance of windbreak effect reached 50%, and length range of the downwind area(over desert wind speed) is gap 4m < gap 13 m < gap 7m.(3) The main shelter belt gaps affect the effective protection area significantly, and the longer gap, the less effective protection area, the greater the downwind area. The wind-protection effects of 10 to 20% area in the plots reflects that gap 7m> gap 13m> gap 4m, and the downwind area showed gap 13m> gap 7m>gap 4m.(4) semi variogram analysis showed that the Gaussian model has a strong correlation in the theoretical model of wind speed variation, the structural heterogeneity are in more than 90%, and variation of the random heterogeneity is small. The space distribution of continuity of the wind speed in the sample is stronge, and the sampling interval is reasonable. The degree of the influence of the gaps on the wind field in the sample depends on the desert wind speed under the same length of gaps conditions:Wind speed increased significantly with the increasing of the desert wind speed, and the higher the desert wind speed, the longer distance from the windbreak when the windbreak efficiency reachs 50%, and the larger the downwind area(more than or equal to the desert wind speed), the smaller the effective area of wind protection when the wind efficiency is 50%(reduce 132.8%). The research shows that when the gap is greater than or equal to 7m, we should immediately replant, and the effect of 4m is not be ignored in some areas that gale occurs frequently.(4)The influence of the missing plants continuously in the primary forest belt on the wind field The degree of the increasing wind speed in the primary forest belt is closely related to the length of the missing plants continuously in the same desert wind conditions:(1) The longer the continuous missing length, the greater the wind speed of the net work, and the average wind speed of missing 6 plants continuously was the highest and higher than that of missing 4 plants continuously and missing 5 plants continuously of 7.5% and 9.6% significantly(P<0.05).(2) The missing plants continuously in the primary forest belt extend the distance of windbreak which reached 50% in the net work, and however, different length of missing plants affect not significiantly on the main wind drop zone(P>0.05).(3) The longer the continuous missing length, the smaller the effective protection of the sample.(4) Semi variance function analysis indicated that the wind speed variation theory model are Gaussian model, the spatial variation of the wind speed in the has strong self correlation and in the factors affecting the change of wind speed, the structure factors accounted for the major proportion, small influence of random factors, spatial continuity of strong. Under the same length of missing plants continuously conditions, the wind speeds in the plot overall level increased with the growing desert wind speed, the average wind speeds in the plot increased 113.8% with significant difference(P <0.05) as the desert wind speed increased 106.3%; and the distance when the wind protective effect reached 50% in the plots and the area of effectiveness wind area had no significant difference(P <0.05).Studies show that: it should be replanted immediately, when the length of missing plants continuously is equal or longer than 9m. Although impaction of missing plants 5m continuously had small influence on wind protection effect, its effects on other forest protection sould be reseached in the coming study. |
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