A Study of the Effects of Mechanized Thinning Operation on the Soil and Water Conservation Function of Forest in Mountainous Region

机械化稀释效果操作由高效林业机研究了林分和森林地板上。疏伐作业以密林道路为基础（128米/公顷）。以森林径流水浊度为指标，对其效果进行了研究。首先，从采伐森林流出的河流上、下两个观测点的浑浊度在降雨时的某一时刻显示出相当的高值。而同一降雨期的平均浑浊度在下游近距离（约300m）下降了约1/2以上观测点。在无降雨条件下，采伐林排水的浊度值高于未采伐林排水。但在未采伐的森林，随着疏伐作业的开始，浑浊度没有变化。在某些情况下，情况正好相反。通过对采伐前和未采伐前排水网的精确调查，可以推断出浊度增加的原因是森林道路引起的土壤流入的反复发生。流入土在下坡淤积，入渗能力较低。随着这种土壤沉积物沿斜坡的延伸，含有地表径流水的悬浮泥沙在下游增加。疏林作业的主要目的之一是在完成作业后，对排水设施进行长期的合理布置和持续维护，以达到增强森林水土保持功能的目的。对森林公路施工过程中开挖的散粒土进行了粒度分析。从1至30年前修建森林道路的各个地点收集了分散的土壤样本。施工后的散土是由大小不一的土粒组成的。但随着时间的推移，细土颗粒在降雨期间随着径流继续流走，只有更多的粗土颗粒留在同一坡面。由此推断，道路建设后4 ~ 10年，边坡因植被覆盖较厚而趋于稳定，但30多年来，细土颗粒仍在随水持续向下流动，尤其是粒径小于2.0 mm的细土颗粒。在这种情况下，如果在散土的下游有溪流作为水资源，则应采取相应的对策，防止细土的流入。少

Effect of mechanized thinning operation carried out by highly efficient forestry machine on forest stand and forest floor was studied. The thinning operation was based on dense forest road (128m/ha). Turbidity of run-off water from the concerned forest is taken as an index to study the effect.First of all, turbidity of water at two upper and lower observation points in a stream which drain water from logged forest, indicated equivalent high value in a moment during rainfall. But average turbidity during the same rainfall period decreased by about 1/2 of the upper observation point in a short distance (about 300m) in down stream.Water drained from logged forest showed higher turbidity value compared with the water from unlogged forest, when there was no rainfall. But, with the beginning of thinning operation in unlogged forest, difference of turbidity was not observed. In some cases, it was just reverse. Based on the precise investigation of drainage net work in logged and unlogged fore … More st it can be inferred that the cause of increase in turbidity is repeated occurrence of soil inflow originated from the forest roads. Inflow soil deposits in down slope, which has low infiltration capacity. Depending on the extension of such soil deposit along the slope suspended sediment containing surface run-off water increases in the down stream. Appropriate arrangement and continuing maintenance of drainage facilities after the completion of operation for a long time is necessary to attain the enhancement of soil and water conservation function of the forest, which is one of the main purposes of thinning operation.Particle size analysis of scattered soil, which was excavated during construction of forest road, was carried out. Scattered soil samples were collected from various sites where forest roads were constructed from 1 to 30 years ago. The scattered soil just after construction is composed of various sizes of soil particles. But with the lapse of years fine soil particles continue to flow away with run-off water during rainfall and only more coarse soil particles remain in the same slope. It is inferred that 4 to 10 years after roads construction the slope seems stable by thick vegetation cover, but fine soil particles are continuing to flow down with water for more than 30 years, specially fine soil particles less than 2.0 mm in size. In such cases if there is a stream used as water resource in the down stream of scattered soil, appropriate counter measures should be implemented to prevent the flowing down of fine soil. Less