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

研究了利用高效林业机械进行机械化间伐作业对林分和林地的影响。间伐作业以密林公路(128m/ha)为基础。以森林径流的浑浊度为指标，研究了森林径流的浑浊度对降雨的影响，首先，从原木林排水的溪流上、下两个观测点的浑浊度在降雨过程中的某一时刻表现为相当的高值。但同一降雨期间，下游短距离(约300m)上观测点的平均浊度降低约1/2，无降雨时，伐木林出水的浊度高于未伐木林出水的浊度。但是，随着未伐林间伐的开始，混浊度没有明显的变化。在某些情况下，情况恰恰相反。基于对有记录和无记录FORE…排水网络的精确调查进一步推断，浑浊度增加的原因是林道入渗土壤的反复出现。入渗土壤堆积在下坡，入渗能力低。根据这种土壤沉积沿坡面的扩展，含有地表径流水的悬浮泥沙在下游增加。疏伐作业的主要目的之一是在长期作业完成后对排水设施进行适当的布置和持续维护，以达到增强林木水土保持功能的目的。从1-30年前修建林道的不同地点采集了零星的土壤样本。施工后散落的土体由不同大小的土粒组成。但随着时间的推移，细小的土壤颗粒在降雨过程中继续与径流的水一起流失，只有更多的粗土壤颗粒留在同一坡面上。据推测，修路后4~10年，由于植被覆盖较厚，边坡看似稳定，但30多年来，细粒土壤随水继续向下流，特别是直径小于2.0 mm的细粒土壤。在这种情况下，如果在分散的土壤下游有一条溪流作为水源，应采取适当的对策，防止细粒土壤向下流淌。较少

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