【摘要】 传统观点研究认为:植被覆盖度低于40%,沙地处于半固定半流动状态.但在实践观察中发现:在低密度(或覆盖度)时,灌丛的水平分布格局对固定流沙和阻止风沙流的作用差异显著。以低覆盖度(20%~25%)的柠条固沙林为研究对象,采用多点式自记风速仪(GB-228),在2~4月份的盛风季节,测定了行带式配置和随机分布的柠条固沙林以及完整的行带式配置和其中一带出现缺口的行带式固沙林内不同部位、不同高度的风速.统计分析风速测定结果发现:(1)当覆盖度在20%~25%时,行带式配置的柠条固沙林内的防风效果比同覆盖度随机分布的在20cm高处高48.4%;50cm高处高30.7%;200cm高处高27.4%,且风速越大,行带式配置降低风速的效果越显著,当200cm高风速达到6~7m/s时,行带式的平均防风效果比同覆盖度随机分布的高48.2%;反映出在低覆盖度时,灌丛的水平分布格局成为制约固沙林沙防风效果的重要因素,行带式配置具有显著的防风效果;(2)在灌丛与灌丛之间形成的类似“狭管”流场的局部,有提升风速的作用,导致其林内观测的风速有约41.3%的观测结果超过旷野对照风速,行带式配置的柠条固沙林内没有一个观测结果超过对照风速;这个结果反映出随机分布的柠条固沙林内流场结构复杂、变化多样,也成为低覆盖度时,沙地处于半固定、半流动状态,疏林内同时存在风蚀和积沙和缺口处风速升高的重要因素;(3)行带式配置林内地表粗糙度比随机配置的高5.4~114.4倍,说明行带式配置具有显著的防止风蚀、固定流沙的作用;(4)行带式林内出现断带缺口处,其缺口处的风速降低有明显的累加现象;风速抬升现象在一定程度受到制约。这些结果为发展低覆盖度行带式配置的固沙林提供重要的科学依据。更多还原

【Abstract】 Conventional research thinks that when vegetation cover is less than 40%,sandland would be in a state of semi-mobile or semi-fixed.However,in practice it has been found that at low vegetation coverage,the horizontal distribution pattern of shrubs can have marked effects on fixation of shifting sands.In this study we investigated the windbreak effects of a Caragana korshinskili kom plantation with 20%25% cover established for the purpose of sand-fixation. Wind velocity was measured at different locations and heights on sites of the intact or gapped belt scheme plantation and randomly distributed plantation by using multiple-sensor automatic anemoscope(GB-228) during windy season(February to April).Statistical analysis indicated that when cover was at 20%25%,the belt scheme C.korshinskili plantation was 48.4%,30.7%,and 27.4% more effective for wind breaking than the randomly distributed plantation at 20 cm,50 cm,and 200 cm above the ground surface.The effectiveness of wind breaking increased with wind velocity in the belt scheme.When the wind velocity at 200 cm reached 67 m/s,the mean windbreak effect of the belt scheme was on average 48.2% more than the randomly distributed plantation scheme at the same vegetation coverage.This shows that when vegetation cover is sparse,the horizontal distribution pattern of shrubs becomes an important factor for restricting windbreak effect,and that the belt scheme is the most effective plantation in the windbreak effect.The "narrow pipe" flow from gaps between shrubs can have an effect on increasing wind velocity,which results in 41.3% of observations displaying wind velocity greater than that measured in the open.Within the belt scheme C.korshinskili plantation,none of the observations was greater than observations made in the open ground.The flow field structure of the randomly distributed C.korshinskili plantation was complex and changeable,and was an important factor for the occurrence of wind erosion and sand accumulation in the thin stands and increased wind velocity in the gaps of stands low in coverage in the semi-mobile or semi-fixed sand-dunes.Soil surface roughness was found to be about 5.4114.4 times greater in the belt scheme than in the randomly distributed plantation,indicating that the belt scheme had marked effect on preventing wind erosion and fixing drift sand.When there were gaps in the belt scheme plantation,the reduced wind velocity apparently displayed accumulation phenomena and increased wind velocity was limited to a certain extent in gaps.This study provided scientific basis for developing low cover and belt scheme shrub plantations for sand-fixation.更多还原