森林の成立および皆伐が土壌の2・3の性質に及ぼす影響について : 第2報 皆伐による土壌中諸物質量の変化

Abstract

It is a very important problem in forestry to understand the degradation process of forest soil due to the clear cutting, but the scientific researches were very few because it is difficult to study on account of the complexity of site conditions. In this report, the changes in the amount of organic matter and nutrients in forest soils caused by clear cuttings are considered. The studied forests were as follows: (a) Sugi and Hinoki (Cryptomeria japonica and Chamaecyparis obtusa) forests in Mt. Kōya. (b) Sugi forests in Kitō. (c) Hinoki forests in Owase. In Mt. Kōya, the author picked out the five stands which were arranged with age series, from cut area to natural forest. In Kitō, four stands which were different in the frequency of clear cutting each other, were studied to understand the influence of the frequency of cutting on soil properties. In Owase, the author aimed to make clear the relationship between the site condition and the degradation by clear cutting, and took four stands which were different in site but same in stand age. The amount of organic matter of A0-layer decreased after clear cutting because the supply of organic matter to A0-layer by litter fall ceased, on the contrary the rate of decomposition of A0-layer which were likely to hasten this decreasing process did not continue for long time. It stopped soon after the next generation grew up and the crown closed, and thereafter A0-layer became to increase. The amount of organic matter in soils did not show the definite decrease by one clear cutting. It may be due to some factors such as follows: (1) The supply of organic matter to soils does not stop by clear cutting, because there is A0-layer on the surface of soil and roots of cut tree as resources of soil organic matter. (2) Generally, the rate of decomposition of soil organic matter is very small. (3) There are local diversities in the amount of soil organic matter in these forests soil. In Kitō Sugi forest, it was found that the amount of soil organic matter and soil nitrogen became to decrease with the frequency of clear cuttings, and in Owase Hinoki forests, the poorest site seemed to be most severely influenced. The loss of carbon and nitrogen by clear cutting is recovered with the development of the forest vegetation, as the forest can accumulate carbon and nitrogen as shown in the previous paper (Tsutsumi 1963). When the rate of recover is very rapid, the decrease in the amount of soil organic matter and soil nitrogen will not occur even if cuttings are repeatde many times. But usually it needs a very long time to recover as compare with the cutting period. In Kitō Sugi forest, the cutting age was very short. It used to be cut in 30 or 40 years old, and cutting area was cultivated and there some crops were planted in a few years. Therefore in this region, the next cutting might be carried out before the recovering process progressed quite enough. On the other hand, as the poorer the site, the slower the rate of recovering, it severely influenced on the poor site as shown in Owase Hinoki forests. Mineral nutrients in the soils will be lost by clear cuttings by the way of the harvested trees containing some mineral nutrients. However, the author could not find the definite decrease in the amount of mineral nutrients in the soils, except in the amount of calcium in this research. Usually, the large parts of mineral fund of the forest ecosystem are in soils, and very small in stem or top (Table 1, 3). Therefore, the diminution in the amount of minerals by clear cutting may be imperceptible, if they are lost only by the way of the harvested trees. But, when they are scarce in the soils, the rate of loss with the harvested trees begins to grow up large. On the researched forests, calcium content in the soils was very scarce compared with other forests (Table 2), and the author found the tendency that the amount of calcium decreased with the frequency of clear cutting, even if it was very vague. As the circulation system of mineral nutrients is closed, the forest does not have the ability to accumulate the mineral nutrients. If the supply of mineral nutrients by the way of rain water and the weathering of the rock does not compensate the loss by clear cutting, it will lead to the gradual diminution of mineral nutrients in the soil. Then, the current cycle by litter fall will be disturbed, and the growth of forest will be slower. It is presumed that the bad effects of clear cutting on forest soil do not definitely appear soon after one clear cutting, especially on good site, but they become gradually appeared after the repetition of clear cuttings. Unfortunately, it is presumed to need a long time to recover the productivity of forest soil. To maintain the forest soil productivity, it will be better to avoid the cutting methods that have the bad effects on soil.