This study aimed to investigate an active central Shikoku Island forestry site to assess the growth performance of cedar saplings in the presence of weed competition. By examining the potential of using high-performing cedar varieties, we tried to identify opportunities for reducing weeding efforts. Furthermore, we evaluated the influence of soil layer thickness and terrain ruggedness index (TRI) on the development of randomly selected cedar saplings and discussed the feasibility of selectively targeting weeding efforts in part of a forest compartment. The results showed that the selected variety ‘Takaoka#1’ exhibited superior sapling height, stem diameter, and crown width compared with the common variety ‘Tanoaka’ three years after plantation. Although the advantage of Takaoka#1 in terms of sapling height was somewhat mitigated by the lanceolate tree form of Tanoaka, logistic regression analysis revealed that using Takaoka#1 increased the likelihood of cedar saplings outcompeting weeds for sunlight. Moreover, cedar saplings were found to perform better at sites with thicker soil layers and lower TRI, indicating the need for a gentler terrain. Conversely, weed height was not associated with soil layer thickness or TRI. Logistic regression analyses incorporating soil layer thickness and TRI as explanatory variables confirmed their significant impact on the probability of cedar saplings thriving amidst weed competition. Overall, our study suggests that reducing weeding efforts in a young cedar plantation is possible when using high-performing varieties and strategically identifying sites characterized by thicker soil layers and lower TRI, thus enabling enhanced cedar sapling growth.

In October 2018, spur road construction and small-area thinning were conducted in No. 2 (along the ridge) and No. 4 (near the stream) of the Kamabuchi experimental watershed. Logging rates by volume were estimated at 3.5％ and 4.4％ respectively. To assess the effects of small-scale practice on water runoff in snowy regions, we compared monthly runoff before and after thinning. In the analysis of the monthly runoff ratio to the No. 1 control watershed, the runoff ratio in No. 4 increased after thinning in June only. However, No. 4 had water seepage problems, and the monthly runoff ratio was positively correlated with monthly precipitation. The increased runoff is likely due to several years of high precipitation after thinning. The monthly runoff ratios in No. 2 and No. 3 gradually increased in August, possibly unrelated to the thinning. Then the relationships between the monthly runoff in the No. 1 and the other three watersheds were analyzed for the three seasons (midwinter, snowmelt, and warm seasons). No differences were observed before and after thinning in any season or watershed. Given the limited tree removal during small-area thinning, its impact on water runoff is considered insignificant.