低覆盖度固沙林的乔木分布格局与防风效果

Pattern oflow-covered sand-fixing woodland and its windbreak effect

低覆盖度植被是我国干旱、半干旱区经过漫长的自然演替过程逐步发育形成且广泛分布的植被类型。前人研究认为,植被覆盖度达到40%为固定沙地,40%—20%为半固定-半流动沙地。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。在干旱区、半干旱区,存在着大量天然的乔木疏林,其覆盖度均在低于40%,地表处于半流动状态,而配置成行带式后,即使覆盖度降低到20%时,地面也不会出现风蚀现象。因此,在内蒙古浑善达克沙地,针对覆盖度在20%左右的乔木疏林,同时测定了随机与行带式两种分布格局的防风阻沙效果。结果表明:(1)在不同的对照风速下,行带式配置的林内的相对风速均低于随机分布,其中在200 cm高度处行带式配置的平均相对风速比随机分布的低53.89%,在50 cm高度处低36.82%;(2)行带式林内的水平风速流场变化有一定规律,而随机分布林内风速流场变化主要受树冠在空间的分布影响,变化非常复杂;(3)随机分布的疏林内出现风速超过旷野对照的现象,在50 cm和200 cm的两个观测高度上分别有约27.45%和22.55%的风速测定值超过对照风速,说明出现明显的局部风速"抬升"现象,形成了强的涡流;(4)行带式配置林内的平均地表粗糙度达到1.01 cm,比随机分布的疏林内增大约5倍之多;(5)总体而论,行带式分布格局第1带降低风速的作用最显著,第2带及其以后各带间的风速均比第1带后的风速小,但逐带降低的叠加效益不明显;(6)由于乔木基本(枝下高)没有枝条对风的阻碍,乔木行带式固沙林在迎风面的第1林带的基部有一定的风力"抬升"作用,对林带基部地面产生较强的侵蚀作用,多数第1带树木的根系被侵蚀裸露,过境的风沙流只能在林带后树冠外侧堆积;(7)随机分布林内在许多位置出现了非常低的地表粗糙度,地表粗糙度低的位置基本与风速"抬升"区相吻合,这种"抬升"区形成的强的涡流是疏林内出现风蚀坑的重要因素,这也是浑善达克沙地出现榆树与风蚀坑相间分布的主要原因。

Low-covered vegetation is one of typical vegetation patterns in arid and semi-arid areas of China through a long-term natural succession.Conventional studies concluded that when vegetation cover arrives at 40%,sandland is fixed.When vegetation cover is from 20% to 40%,sandland is under semi-shifting and semi-fixing.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.There have many artificial or natural sand-fixing woodlands in arid and semi-arid areas of China which vegetation cover was lower than 40%.This paper tests the hypothesis that the horizontal distribution pattern of sand-fixing woodland at low vegetation coverage has the same law as shrubs.We compared the windbreak and sand-fixing effects of woodland with 20% coverage between belted and random patterns.Results show：（1） Compared to wind speed in open field,the relative wind speed of belted pattern is lower than random pattern,with 53.89% and 36.82% reduction rate at 200cm and 50cm height respectively;（2） compared to relatively regular changes in belted pattern,the horizontal flow field of wind speed in random pattern is very complicated,mainly affected by spatial pattern of crown;（3）The wind speed in random pattern is 27.45% and 22.55% higher than that in open field at 50cm and 200cm height respectively,the local wind uplift forms a strong eddy;（4） Average surface roughness of the belted pattern is 1.01cm,about 5 times of random pattern;（5） On the whole,among belted patterns the first belt was showed significant effects in reducing wind speed;and the wind speed after the second belt was is lower than that after the first belt;but superposition benefit was not obvious of reduced wind speed effect one by one belts;（6）the belted pattern has local uplift function at the base of the first belts windward which has relatively strong erosion effect in the basal part of forest belt,and the root systems for most of trees were eroded down and outcropped in the first belt,and the sand flow through a region was accumulated at the lateral of crown behind the belt;（7）Random pattern exists many low roughness zones in random pattern matching with local wind uplift zones,the strong eddy in these local uplift zones is an important driving factor of blowout pits.Arid and semiarid areas occupy about one third of China territory with less than 400 mm of annual precipitation.Traditional approaches to ecosystem restoration in China have focused on afforestation with high stand density such as the Three Norths Shelter Forest System Project and the Sand Control Program.However,the long-term results of those practices have been proved economically costly and ecologically unsustainable to some extent,with groundwater shortage and soil erosion increasing.We conclude from our experiment that soundly horizontal distribution pattern of sand-fixing woodlands at low vegetation coverage can fix shifting sands effectively,which may be helpful to environmental improvement and sustainable development of arid and semiarid regions in China.