在干旱地区种植乔木,由于缺乏必要的水分供应,其生长会处于停滞状态,几十年也不能成材,即形成所谓“小老树”。水分亏缺的原因,一是大气降水少,二是地形和土壤类型的制约。同样是300毫米的降水量,在地下水位较浅的风沙坨沼区及沿河平川,乔木可依靠地下水来补充降水的不足,所以能够长成材。而在地下水位较深的漫岗缓坡栗钙土上及地形破碎、水土流失严重的黄土 Growing arbor in arid regions, due to a lack of necessary water supply, is stagnant in growth and can not be achieved for decades, forming what is called a “small tree.” Water deficit reasons, one is less precipitation, the second is topography and soil type constraints. The same is the rainfall of 300 mm. In the shallow sandstorm area and the rivers along the river, the trees can rely on groundwater to supplement the lack of precipitation, so they can grow more timber. In the groundwater level of Manchuang gentle chestnut chestnut soil and crustal terrain, soil erosion of loess