In this study, the effects of forest clearing were quantitatively investigated by monitoring the water content and ground temperature of the surface soils within and outside a cutover area. First, the soil types, microtopographical units, and surface soil thickness were determined, and then the soil water content and ground temperature as well as their seasonal changes were measured and compared at 34 points. Except for the winter, the soil water content and ground temperature showed quite different values between outside and within the cutover area; especially at a head slope, the difference reached nearly 10%. And more, within the cutover area, there was major difference between upper and lower slopes on soil water content. Drier condition was observed at the lower slopes where would be originally wetter condition; the authors visually confirmed weakly dried yellow soils (Y_C type) after digging some soil pits. In tropics, buffer zones are usually established along streams in mountainous area for the sake of controlling soil runoff. In case of clearing in sub-tropical areas including Okinawa Island, it is necessary to avoid felling and establishing buffer zone at lower slope where is easily dried.

We added “dwarf-bamboo-dominant type” to the conventional types of undergrowth in planted Japanese cypress (Chamaecyparis obtusa) forests (“fern-dominant type”, “herbaceous type”, “shrub type”, and “poor vegetation type”), as classified from an examination of the coverage and dominant species in southern Gifu Prefecture. We also reordered the undergrowth types in terms of their ability to deter surface soil erosion. The dwarf-bamboo-dominant type appeared at higher altitudes than the fern-dominant type, and the dwarf-bamboo-dominant type could appear on forest floors where there was poor light. Its greater vegetation cover led to a reduction in the soil erosion susceptibility index, which is an indicator of the relative risk of surface soil erosion. The index of the dwarf-bamboo-dominant type was lower than those of the herbaceous type and the shrub type, but the vegetation cover of the dwarf-bamboo-dominant type was similar to those of these two types. The dwarf-bamboo-dominant type had a deterrent effect on surface soil erosion because of the greater vegetation cover of its herbaceous layer and the multilayered vegetation and litter cover. Consequently, adding the dwarf-bamboo-dominant type to the types of undergrowth in Japanese cypress stands is effective in estimating the surface-soil-erosion deterrence of undergrowth. Among these undergrowth types, the soil erosion susceptibility index increases in the order of fern-dominant type, dwarf-bamboo-dominant type, herbaceous type, shrub type, and poor vegetation type. This result should help to improve the versatility of methods used to estimate the surface-soil-erosion deterrence of different types of undergrowth.

We investigated the relationship between vegetation development and sediment transport rates during 13 months after clearcutting of a 100-year old sugi (Cryptomeria japonica) plantation in order to examine the factor of limiting the interaction of vegetation and sediments in the early development process of ground vegetation after logging. Following two hypotheses were examined; 1) vegetation cover controls sediment transport rates, 2) sediment transport inhibits vegetation development. For the first hypothesis, there was no significant relationship between the periodical average vegetation cover and the sediment transport rates per unit rainfall. A decision tree analysis did not also detect threshold values of the vegetation cover to discriminate the sediment transport rate. For the second hypothesis, in contrast, the inclement of vegetation cover standardized within each of their periodical average classes tended to be lower when the periodical sediment transports were larger. A decision tree analysis demonstrated that the standardized inclement of vegetation cover became larger when the periodical sediment transport rates were lower than a threshold of 22.25(g/m/day). These results indicated that the sediment transport rates are the limiting factor of the interaction with the vegetation development at the early stage after clearcutting when the vegetation cover is low and the sediment transport rate is high; the effect of sediment transport to inhibit vegetation development is predominant to the effect of vegetation to control the sediment transport. The analyses also suggested that the spatial heterogeneity of vegetation development was partly affected by the distribution of unstable sediments such generated by road construction and of the remaining course and/or fine woody debris on the logged forest floor.

Soil scarification is a natural regeneration practice in which both understory dwarf bamboo and surface soil are removed using civil engineering machinery. We provide a historical review of soil scarification practices in Hokkaido, the northernmost island of Japan. Soil scarification has been conducted widely in Hokkaido since the 1960s. It has been shown to be efficient for natural regeneration where Betula spp. are dominant. As a low-cost reforestation technique, soil scarification has recently received renewed attention regarding a large area of Abies sachalinensis plantation facing a final cutting period in Hokkaido. On the basis of past findings focusing on soil disturbance, scarification practices should strike a balance between maintaining soil physicochemical quality and entirely removing dwarf bamboo, Betula trees show better growth in scarified stands than in stands regenerated without scarification, and soil scarification may significantly reduce nutrient status. Previous reports on scarification-regenerated birch forests were derived mainly from young stands (<20 years old). Thus, it remains unclear how much timber of specific volume and size can eventually be harvested from scarification-regenerated birch forest. To achieve well-executed birch forest management, current scarification practices need to be improved depending on local conditions. Some issues still need to be addressed, for example, a countermeasure to tall forbs of concern in low-altitude areas and investigation of the causes of previously failed scarification practices.

Change of biomass expansion factor (BEF) by stand age was analyzed using the published data of aboveground biomass of Cryptomeria japonica stands. The BEF tended to become small according to stand age and mean height. The relationships between BEF and stand age or mean height of the University of Tokyo Chiba Forest weren't different from those of other prefectures. The mean BEF of the stands older than 80 years old was 1.14 ± 0.05 (mean ± sd), which was smaller than 1.23 of the BEF used in the IPCC report for the stands older than 21 years old. Leaf biomass of the old stand tended to be same or less than that of young stand. Branch biomass of the old stand tended to be same or more than that of young stand. On the relationship between the stem biomass and the leaf and branch biomass which determines BEF, mean leaf and branch biomass of the stands with stem biomass less than 100 t ha^－1, 100 to 200 t ha^－1 and more than 200 t ha^－1 were 31.7 ± 6.6 t ha^－1, 36.4 ± 6.1 t ha^－1, 49.6 ± 9.5 t ha^－1, respectively. On the leaf and branch biomass of the stands with stem biomass more than 200 t ha^－1, there wasn't a tendency to increase with stem biomass. Although large branch biomass of old stand mitigates the decrease in BEF, the effect on BEF was limited. BEF of old stands with large stem volume seemed to be smaller than that of young stands.