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下植被的影响。疏伐作业以密植林路（128 m/ha）为基础。以森林径流浊度为指标，首先，在一条疏伐森林的溪流中，上、下两个观测点的浊度在降雨过程中的某一时刻均表现出相当高的值;而在下游较短距离内（约300 m），同一降雨期内的平均浊度值比上游测点降低约1/2，在无降雨时，采伐林排水的浊度值高于未采伐林排水。但是，随着间伐作业的开始，未采伐林的浊度没有观察到差异。在某些情况下，它只是相反的。在对采伐迹地和未采伐迹地排水管网进行精确调查的基础上， ...更多信息 因此，可以推断，浊度增加的原因是来自森林道路的土壤入流的重复发生。入渗土壤沉积在下坡处，入渗能力较低。随着这种土壤存款沿着坡面的延伸，下游含有地表径流水的悬浮泥沙增加。疏伐作业的主要目的之一是提高森林的水土保持功能，疏伐作业完成后，合理布置并长期保持疏伐设施是必要的。从不同的地点，从1年到30年前的森林道路建设分散的土壤样品收集。施工后的散土由各种大小的土颗粒组成。但随着年代的推移，降雨过程中细颗粒随径流不断流失，同一坡面上仅残留较多的粗颗粒。据推测，公路建成后4 ~ 10年，由于植被覆盖较厚，边坡看似稳定，但细土颗粒仍随水向下流动30年以上，特别是小于2.0mm的细土颗粒。在这种情况下，如果分散的土壤下游有一条河流作为水源，则应采取适当的对策，以防止细土流下。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