Enhancing Overland Flow Infiltration through Sustainable Well-Managed Thinning: Contour-Aligned Felled Log Placement in a Chamaecyparis obtusa Plantation

Contour felling is a restoration method used to decrease overland flow (OF) and soil erosion in the world. However, the impact of thinning and the placement of felled logs on OF remains inconclusive. Low ground cover and soil permeability promote OF in Chamaecyparis obtusa (Siebold et Zucc.) Endl plantations, making thinning a method for reducing runoff. We examined the relationship between OF and ground cover in a C. obtusa plantation in Japan. Event-based runoff was monitored in three plots from 2016 to 2021, with 40% thinning conducted in 2019. In plot T1, logs were randomly scattered, and, in T2, logs followed contour lines, while control plots stayed the same. After thinning, both treatment plots showed lower OF than the control plot. The ANCOVA test shows a significant slope reduction in treatment plots compared to the control plot from pre-thinning to post-thinning (T1: 0.67 to 0.26, T2: 0.66 to 0.12, p < 0.001, Tukey HSD test). However, in plot T2, OF remained stable for two years post-thinning, affirming the enduring effectiveness of contour-aligned log placement. This study backs the notion that aligning fallen logs with contour lines boosts long-term OF infiltration, supporting sustainable forest and soil management

Relationship between Very Fine Root Distribution and Soil Water Content in Pre- and Post-Harvest Areas of Two Coniferous Tree Species

Tree root system development alters forest soil properties, and differences in root diameter frequency and root length per soil volume reflect differences in root system function. In this study, the relationship between vertical distribution of very fine root and soil water content was investigated in intact tree and cut tree areas. The vertical distribution of root density with different diameter classes (very fine &lt;0.5 mm and fine 0.5&ndash;2.0 mm) and soil water content were examined along a slope with two coniferous tree species, Cryptomeria japonica (L.f.) D. Don and Chamaecyparis obtusa (Siebold et Zucc.) Endl. The root biomass and length density of very fine roots at soil depth of 0&ndash;5 cm were higher in the Ch. obtusa intact tree plot than in the Cr. japonica intact plot. Tree cutting caused a reduction in the biomass and length of very fine roots at 0&ndash;5 cm soil depth, and an increment in soil water content at 5&ndash;30 cm soil depth of the Ch. obtusa cut tree plot one year after cutting. However, very fine root density of the Cr. japonica intact tree plot was quite low and the soil water content in post-harvest areas did not change. The increase in soil water content at 5&ndash;30 cm soil depth of the Ch. obtusa cut tree plot could be caused by the decrease in very fine roots at 0&ndash;5 cm soil depth. These results suggest that the distribution of soil water content was changed after tree cutting of Ch. obtusa by the channels generated by the decay of very fine roots. It was also shown that differences in root system characteristics among different tree species affect soil water properties after cutting

Application of the Reformulated Gash Analytical Model for Rainfall Interception Loss to Unmanaged High-Density Coniferous Plantations Laden with Dead Branches

Interception loss (IL) by the forest canopy removes a substantial quantity of rainwater within forested ecosystems. The large-scale unmanaged Japanese coniferous plantations with high stand density (SD) in Japan raise concerns about an additional increasing IL as a result of a new influential factor of dead branches under canopies. Thus, evaluating the usage of IL estimation models is vital to regulating the water and environment in such coniferous plantations. This study aimed to examine the applicability of the reformulated Gash analytical model (RGAM) to unmanaged coniferous plantations with high SD laden with dead branches. We established two plots (P1 and P2) laden with dead branches under the same SD of 2250 stems ha−1 but with different numbers of dead branches (56 vs. 47 branches per tree) in an unmanaged Japanese coniferous plantation. Results demonstrated that a large difference was found in canopy storage capacity (S) in P1 and P2 (3.94 vs. 3.25 mm), which was influenced by the different number of dead branches; therefore, the IL ratio to gross rainfall differed considerably (32.7% in P1 and 26.7% in P2) regardless of the SD being the same. The difference in S enables the RGAM to reflect the influence of dead branch structures on IL, leading to an acceptable RGAM performance for both P1 and P2 (“fair” IL relative errors: −20.2% vs. −16.1%) in the present study of unmanaged coniferous plantations with high SD laden with dead branches

Factors Determining Soil Water Repellency in Two Coniferous Plantations on a Hillslope

Soil water repellency (SWR) is a cause of low water infiltration, overland flow and soil erosion in mountainous coniferous plantations in Japan. The factors determining SWR intensity were investigated in two coniferous plantations of Chamaecyparis obtusa (Siebold et Zucc.) Endl. and Cryptomeria japonica (L.f.) D. Don, using intact tree plots and cut tree plots on the same hillslope. The SWR of Ch. obtusa plots was stronger than that of Cr. japonica plots. SWR intensity decreased after tree cutting. There were no significant differences in SWR upslope and downslope of individual trees/stumps for both tree species, though areas downslope of individual Ch. obtusa trees had higher SWR intensity than those upslope. SWR intensity and soil aggregate stability were positively correlated in the Ch. obtusa intact tree plot (r = 0.88, p &lt; 0.01), whereas in the cut tree plot, this correlation was weak with no significance (r = 0.29, p = 0.41). Soil aggregate size had a non-significant influence on SWR intensity. These findings suggest that SWR intensity was not related to the soil aggregate size, but SWR intensity seemed have a role in soil aggregation in the Ch. obtusa intact tree plot. Destruction of soil aggregates could occur after tree cutting because of physical disturbances or increased input of different types of organic matter from other vegetation into soil. The presence of Ch. obtusa introduces a source of SWR, although uncertainty remains about how water repellency is distributed around soil aggregates. The distribution pattern of soil water content and soil hydraulic conductivity around Cr. japonica was related to other factors such as the litter layer and non-water-repellant soil

The Differences in Water Repellency in Root Mat (Biomat) and Soil Horizons of Thinned and Non-thinned Chamaecyparis obtusa (Siebold et Zucc.) Endl. Plantations

Water repellency (WR) is one cause of root mat (biomat) flow and soil surface runoff in dense Chamaecyparis obutsa (Siebold et Zucc.) Endl. plantations. However, the changes in WR of biomat and soil horizons are unclear in the thinned C. obtusa plantations. This study compares the WR of biomat and soil horizons in the thinned and non-thinned C. obtusa plantations by considering the water content and surface temperature of biomat and soil from July 2021 to June 2022. We selected one plot in each thinned and non-thinned area in a catchment at Obora Experimental Forest in Japan. Our results showed that the 40% thinned plot lacked a biomat horizon, whereas the non-thinned plot had a ca. 3 cm depth of biomat. The biomat WR of the non-thinned plot (none to very strong) was higher than the soil WR of the thinned plot (none to strong). There was no relationship between WR and both water content and surface temperature of biomat and/or soil in either thinned or non-thinned plots. Our findings show that the biomat horizon had an essential role in the severity of WR in C. obtusa plantations. The lack of biomat after thinning could substantially impact soil surface hydrology