基于土壤水分承载力的防风固沙林密度配置格局与防护效应

Shelterbelts have been planted in a large scale in oasis peripheries in Northwest China to shelter oases in a long-term from shifting sands and to pretect the land surface from wind erosion. But it presented a decline effects for the deficit of the soil water in the grow process. So, the balance between the density and protection benefits needed to be solved for the construction of the oasis windbreak and sand-fixation forest. Certain researches showed that when the airflow travel through a continuous medium which composed of various porosity, the flow field changed significantly, and so changed the energy exchange intensity. Accordingly, the consumption of energy of the airflow will also be changed when the wind-sand flow cross through a forest belt which with a non-uniform density distribution. However, the application of plantation of windbreak and sand-fixation forest with a inhomogeneous density distribution still belong to blank, and till now,no system study related to shelterbelt has concerntrated on the effectiveness and protection mechanism of the windbreak and sand-fixation forest with a non-uniform density distribution. In the arid desert area of China. the oasis is determined by the prevailing wind and as the increase of the distance from the oasis, the soil water condition gets worse. Based on this, the basis theory and methods of desert ecology and aeolian physics will be used in this project, the response mechanism between the forest density and soil water and plant growth status will be got through a field investigation and then to sieve out a suitable density. Additional, the protection mechanism and protection benefits of the forest which be arranged with diffenert density along the wind direction will be explored by wind tunnel simulation experiments. And then, establish a mathematical model about the density distribution and the configuration system of density of windbreak and sand-fixation forest in the periphery of oasis will be explored by the study on the coupling effect of water, soil, climate, and protection benefit.The results can be applied to forest plantation as a technology strategy in transitional zones.

干旱沙区，绿洲外围大面积的防风固沙林在成林过程中往往因土壤水分亏缺而大量衰亡，因此，营建时需要在密度和防护效益间求得平衡。有研究表明，气流在孔隙度不同的连续介质中通过时，流场会发生显著变化，进而改变能量交换强度。据此，可以通过林分密度的非均匀配置,在降低林带平均密度的同时保证有效密度。在我国干旱沙区，绿洲所受主导风确定，且随着与绿洲距离的增大，土壤水分条件变差。基于此，本项目将在野外观测与风洞模拟的基础上，对林带密度沿主导风向的非均匀配置格局及其防护效益与机理进行系统研究。首先，通过野外调查与测定，探索植株生长状况和林分密度对土壤水分条件的响应，凝练适宜密度；在此基础上，通过风洞模拟实验，明确沿主风向密度非均匀配置格局及林带的防护效益与机理；最后，通过对绿洲外围关键生态因子与防护效益的耦合研究，构建密度配置数理模型,凝练适合荒漠绿洲外围生境条件的防风固沙林密度配置体系，为建植提供技术策略。

项目基于干旱区造林密度国家标准和低覆盖度治沙理论，围绕研究目标，对野外防风固沙林的现状及土壤水分状况进行了分析，在此基础上，通过风洞实验系统测定了不同类型单排林带、不同带宽、不同季相、不同空间配置模式的复合林带的流场结构和防风效能，并在野外利用仿真植物配置林带，在复杂环境中进一步观测林带周围的风速与风沙流。筛选了防风效能较优的林带并初步构建了数值模型。取得的主要成果有：（1）探明了防风固沙林内土壤水分状况及林带对风速和风沙流结构的影响。（2）探明了低覆盖度下，各林带的流场结构及防护效应。可知，粗糙元素空间分布的多元化对风速的影响极为重要，由两种不同个体特征的植物组成的复合林带的防护效益优于由单一植物组成的林带。复合林带中，行间配置优于株间配置；带状配置林带的防风效能与风向有关。（3）划分了林带区速度场。共分为5个相对速度区和3个绝对速度区。（4）筛选出了低覆盖度复合型防风固沙配置模式一套。依据各类型林带平均防护效益可知，林带的防风效果随带宽的增加而增大；行间配置（A-B型）优于株间配置（AB型）；两种不同的密度配置型复合林带中，AmAn型林带在相反风向作用下，防护效益接近，而AmBn型林带对风向敏感，对应于模拟的具体树种可知，当梭梭位于上风向而柠条位于下风向时，防护效益最优。复合林带中，防护距离最远的应为AmBn型林带，而防护距离最近的是其在相反风向作用下的防护距离，其余林带介于二者之间。据此，在营建防风固沙林时，可适当增加林带的宽度，种植行间配置的林带，或在具有明显主导风作用的地区，种植AmBn型林带。（5）依据风速在不同高度层的变化规律以及防风效益随风速增大而减小的规律，建立了典型防风固沙林风速及效益变化初级模型。项目研究所得的理论数据和结论，可以为西北干旱、半干旱风沙危害区的防风固沙林营建提供数据支撑和技术参考，以此促进沙区生态环境建设与发展。