Selective sample plots of 0.01∼1.0 ha and square, circle, and diamond in shape were placed at random in 4 ha model stands consisting of survey data from actual Japanese cedar plantations. From this computer simulation, we developed an estimation model to determine the coefficient of the stand volume variation in the sample plot based on the plot size and shape and the stand density. Assuming aiming precision of 10%, the recommended plot size is 0.1 ha for a young stand of about 2,000 stem/ha, 0.2∼0.3 ha for an immature or mature stand, and 0.4 ha or larger for a final aged stand of about 500 stem/ha, respectively. If more than 270 trees are included in the sample plot, the precision declines below 10%, regardless of plot shape and stand density. Particularly in the case of a small plot, the coefficient of variation by square plots exceeded that with circle or diamond varieties, when the sample plots were placed at random, regardless of stand disposition. More accurate results can be expected from the circle or diamond plot than the square plot, regardless of the skill of the surveyor selecting the sample plot.

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To clarify the temporal effects of different plant species on the nutrient dynamics and soil biochemical characteristics in the same site location, we investigated the amounts and nutrient contents of the litter fall, soil chemical characteristics (every 40 cm from the tree trunk, 6 points and 0∼25 cm depth, 5 cm intervals), microbial analyses and cellulose decomposition rates at two beech (Fagus crenata) forest and cypress (Chamaecyparis obtusa) forest consisted from two different ages (40 and 80 years old). Amount of the litter fall were almost same in the beech and cypress forest, and N content of the litter fall in the cypress forest was approximately 0.5 time of the beech forest. On the other hand, Ca, Mg contents of the litter fall in cypress forest were about 1.4∼1.7, 1.2∼1.8 times larger than those of beech forest, respectively. As a result of compared the soil property of four sites, cypress plantation enhanced acidification of tree trunk soil, reduced the cellulose decomposition rates. However the nutrient concentrations were not significantly decreased in cypress forests. Moreover, in the cypress forest (80 years old), soil organic matter and base cations were accumulated around tree trunk soil. Present results suggest that if cypress forest located in sloping terrain, management that reduced erosion of the forest floor is carried out appropriately and the soil nutrients could be maintained.

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To clarify the effects of different forest management practices on the quantitative variation in surface soil (fine earth, sediment, and litter) movement, we used sediment traps to measure the transport of materials on a slope for 4 years before and after clear cutting, heavy thinning, and normal thinning. As indicators of the degree of topsoil erosion, the transport rates (g m⁻¹ mm⁻¹) of fine earth, sediment, and litter were compared. The transport rates of all three materials increased significantly after clear cutting. By contrast, the transport rates of fine earth and sediment did not change after heavy or normal thinning. The greater the cutting intensity in the order normal thinning, heavy thinning, and clear cutting, the greater the increase in the relative light intensity of the understory and floor vegetation. We showed that heavy or normal thinning had little effect on fine earth and sediment movements, as compared to clear cutting. At the clear cut and heavily thinned sites, the surface soil disturbance caused by handling the felled trees was thought to offset by the inhibitory effects of vegetation recovery on sediment transport. We believe that it is important to conserve forest soils in order to minimize surface disturbance during harvest and encourage rapid revegetation when managing forests on steep slopes in Chamaecyparis obtusa plantations.

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We conducted two questionnaires on wood characteristics to understand the demands of end-users in terms of wood properties. We asked which properties were disliked, and which properties consumers hoped will be improved. In both questionnaires, the most common response was a preference for wood that is resistant to termites. Based on the responses from potential wood end-users, we conducted a study to select termite-resistant sugi (Cryptomeria japonica) cultivars. The termites Coptotermes formosanus and Reticulitermes speratus were cultured in Petri dishes with meals consisting of heartwood of various sugi cultivars, which were identified by DNA analysis. The cultivars Akaba and Iwao showed higher rates of termite mortality and fewer days to reach 50% termite mortality than those of the other tested cultivars. These results suggested that Akaba and Iwao had strong termiticidal activities. In Akaba, the termite mortality rates were equally high in two distinct ramets from different plantations. In contrast, Honsugi consistently showed weak termiticidal activities in all tests. When R. speratus was cultured with heartwood meals from which volatile compounds had been extracted, the termiticidal activities of all tested sugi cultivars including Akaba greatly decreased. Under culture conditions in which R. speratus was not fed sugi meals but exposed to volatile compounds extracted from the heartwood, Iwao and Akaba showed high termite mortality rates compared with the negative control. These results suggest that termiticidal activity of sugi is closely related to cultivar-specific extractives including volatile compounds. Together with reproducibility of cultivar-specific termiticidal activity, clonal forestry with termite-resistant sugi cultivars is one option for foresters to meet the demands of end-users.

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In order to clarify the influence of non-weeding treatment on the early growth of Chamaecyparis obtusa, we examined the factors affecting the size and growth in terms of vertical and lateral suppression through comparison of tree size and growth with weeded site. Mean tree height and diameter at breast height (DBH) at the end of the 4th growing season did not differ significantly between treatments. However, mean crown projection area was far smaller in the non-weeding site. Decision tree analysis demonstrated the relationships between tree height or DBH and vertical suppression, showing that vertical suppression is the predominant factor restricting the early growth of hinoki. The comparison of the growth between treatments showed the influence of both vertical and lateral suppression on every measure. Decision tree analysis, however, indicated the stronger influence of lateral suppression than vertical suppression. Lateral suppression might reduce the light availability and inhibited the increment of leaf mass of hinoki, resulting in the decrease of mass production. We concluded that avoiding vertical suppression on tree top is essentially required to maintain the ordinary growth of hinoki. However, as the cumulative effects of lateral suppression might realize even after weeding, the timing of weeding should be decided based on not only the status of vertical suppression but also the crown volume and/or leaf mass affected by lateral suppression.

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The reduction of moisture in split logs of Japanese cypress (Chamaecyparis obtusa) and Japanese blue oak (Quercus glauca) was compared in two drying seasons (autumn to winter and spring to summer) under certain conditions differentiated in terms of split size (one-quarter or one-eighth) and drying circumstances (on a paved or unpaved surface, and under a roof or exposed to the outside). Regardless of the drying season, (i) Japanese cypress dried out more rapidly than oak having higher wood density, (ii) drying under paved or roofed circumstances was more effective than under unpaved or unroofed circumstances, and (iii) fine splitting enabled more rapid drying, especially in oak of higher wood density. The effect of the split size tended to be concealed during initial drying due to differences of moisture content by heartwood ratio among the sampled logs. Overall, natural seasoning from spring to summer achieved a faster and more extensive initial reduction of moisture. Nevertheless, the benefits of longer natural seasoning are apparently marginal in both drying seasons, as the drying speed of the split logs rapidly decreased during the first four weeks of drying in both seasons. And longer drying decreased the advantage of splitting.

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