黄土高原人工林地水分亏缺的补偿与恢复特征

Compensation and rehabilitation characteristics of soil water deficit at a planted forest site of the drought-prone Loess Plateau

采用单株平衡法 ,通过定位监测与对比分析 ,对黄土高原半湿润落叶阔叶林区 ,持续干旱期末人工林地土壤水分亏缺度、雨季末降雨补偿深度及土壤贮水亏缺补偿度的时空分布进行了研究。结果表明 ,在水分条件较为优越的半湿润落叶阔叶林带 ,丰水年雨季过后的土壤贮水亏缺补偿深度不超过 2 80 cm,平水年 2 4 0 cm,枯水年仅 4 0 cm,而耗水深度超过 32 0 cm。丰水年0～ 32 0 cm土层 ,土壤贮水亏缺补偿度仅 14 .5 3%。自然状况下 ,单纯依赖天然降水 ,人工林地土壤水分亏缺状况将难以恢复。人工林地土壤贮水亏缺补偿度具有极为明显的空间分布规律。其中 ,水平方向土壤贮水亏缺补偿度与距主干距离成反相关 ,垂直方向因林分类型不同而异。林木根系的存在 ,削弱了天然降雨对土壤水分亏缺的补偿作用。在黄土高原南部半湿润落叶阔叶林带 ,仅采用传统的一维垂向土壤水分观测结果 ,或以 3m以上土层的土壤水分动态监测资料为基础 ,进行有关土壤水量平衡及蒸散量的计算 。

The dried soil layer at a forest plantation site of the drought-prone Loess Plateau in China has threatened the survival and sustainability of the existent plantation, and also hindered the restoration and rebuilding of artificial forest vegetation. There have been numerous studies on the location and depth of dried soil layer's, and the influence of species composition. However, studies on root functions under soil water deficit, compensation and rehabilitation characteristics after rainy seasons in different precipitation years have been sparse. In this paper, the temporal and spatial distributions of compensation degree of soil water storage deficit in a plantation site of black locust (Robinia pseudoacacia Linn.), Chinese pine (Pinus tabulaeformis Carr.), and oriental arborvitae (Platycladus orientalis Franco) were studied using the method of single-tree water balance. Field experiment plot was divided into two zones, root zone and non-root zone, and soil water measurements were made every 5 days from the beginning of June to the end of November by a soil moisture meter of neutron scattering. The monitoring period covered continental-climate wet periods in dry years, normal years and rainy years, respectively. The root zones were divided into 3 or 4 sections from the tree's root crown to 1/2 spacing in the row at an interval of 30 cm to characterize the horizontal distribution of soil water content, and 16 layers from the surface to 320 cm depth at an interval of 20 cm to characterize the vertical distribution of soil water content. The non-root zone, 1.0 m×1.0 m on the surface, was at the same site with and near the root zone, but the roots from the nearby trees were cut off by digging a ditch around their periphery, followed by wrapping the soil column of 2.5 m in depth with 4 layers plastic sheet to prevent from root penetration, water uptake and soil water transport between the root and non-root zones. The annular ditch was then re-filled and a tube was put into the central of non-root zone to determine the vertical distribution of soil water content in the non-root zone at an interval of 20 cm. At the same time, two new quantitative parameters and mathematical expression demonstrated the compensation and rehabilitation characteristics of soil water storage deficit. The results showed that soil water deficit of the plantation site cannot be restored by natural precipitations, even in the sub-humid vegetation zone of the region. The average compensation degree of soil water storage deficit is only 14.53 % at the root zone of 0~320 cm after rainy season in rainy year; and the compensation depth is less than 280 cm in the rainy year, 240 cm in the normal year, and only 40 cm in the dry year after rainy season. The compensation degree of soil water storage deficit in the root zone is less than that of the non-root zone, indicating that roots decrease the compensation of soil water storage deficit by natural rainfall. The soil depth, at which water may easily lose, is no less than 320 cm. The spatial distributions of compensation degree of soil water storage deficit at the root zone have an inverse relationship with the distance from the root crown, different from that of the non-root zone. Due to the root distribution and stem flow of tree species, it is not accurate for the calculation of water balance by measuring vertical soil water content of either one dimension traditionally or no more than 3 m in depth at plantation site in the sub-humid vegetation zone of Loess Plateau.