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This study investigated the impact of thinning and felled logs (random- and contour-felled logs) on overland flow, soil erosion, and litter erosion in a Japanese cypress forest plantation (2400 tree ha−1) with low ground cover, from 2018 to 2023 in central Japan. Monthly measurements of overland flow and soil and litter erosion were carried out using small-sized traps across three plots (two treatments and one control). In early 2020, a 40% thinning (tree ha−1) was conducted in the two treatment plots. Overland flow increased in the plot with random-felled logs during the first year post-thinning (from 139.1 to 422.0 L m−1), while it remained stable in the plot with contour-felled logs (from 341.8 to 337.1 L m−1). A paired-plot analysis showed no change in overland flow in the contour-felled logs plot compared to the control plot from the pre- to post-thinning periods (pre-thinning Y = 0.41X − 0.69, post-thinning Y = 0.5X + 5.46, ANCOVA: p > 0.05). However, exposure to direct rainfall on uncovered ground areas post-thinning led to increased soil and litter erosion in both treatment plots. These findings suggest that thinning combined with contour-felled logs effectively stabilizes overland flow. Therefore, thinning with contour-felled logs can be considered a viable method for mitigating overland flow in monoculture plantations with low ground cover. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Understanding baseline runoff characteristics is critical in any water balance study whether single- or multiple-catchment. As a product of interaction between many area-specific factors (scale, climate, geology, vegetation, land-use history), each catchment can only be well-understood through a reasonable period of baseline observation. In temperate regions, processes are complicated by a range of meteorological factors and plant phenological periods (growth and dormant) as well as the way vegetation reacts to these factors resulting in varying water uptake from soils. In the Inuyama Research Forest of the Ecohydrology Research Institute in Aichi prefecture, central Japan, streams in two small headwater catchments (cypress plantation, 2.2 ha; secondary mixed-broadleaf, 2.1 ha) were instrumented with 90° v-notch weirs and automatic data loggers that recorded water level at 5-minute intervals. These data were then converted to discharge via calibration. A weather station was also set up in the area to record rainfall and other meteorological data. After quality control and data filtering, 1,398 daily runoff and rainfall over five years (April 2013-April 2018) were computed and analysed. Data were separated according to plant phenology (growth period from 1st May to 31st October; dormant period from 1st November to 30th April) and explored via runoff coefficients (C) and rainfall-runoff regressions in two ways:- (i) growth vs. dormant periods within individual catchments (cypress and broadleaf); (ii) cypress vs. broadleaf forests within each phenological period (growth and dormant). C-values in the growth and dormant periods are 0.534 and 0.521 for cypress; and 0.506 and 0.525 for broadleaf respectively. Within individual catchments, the cypress catchment has a 2.5% higher C-value in the growth period (0.534) compared to in the dormant period (0.521) whilst the broadleaf catchment shows a 3.6% reduction instead (0.506 in growth; 0.525 in dormant). This suggests that during the warm and humid growth period, the increased water uptake by cypress trees does not offset the influence exerted by higher rainfall. However, the slopes of rainfall-runoff relationship are higher during the growth period in both catchments. For comparison between the two catchments within the same phenological period, the 5% lower C-value in broadleaf (0.506) compared to cypress (0.534) reflects higher water use by broadleaf trees during the growth season whereas similar values (0.521 in cypress, 0.525 in broadleaf) during the dormant season indicate continual rainfall interception by cypress even after leaf abscission in the broadleaf catchment. This is further supported by the higher slope of rainfall-runoff relationship in the broadleaf catchment during the dormant period. This study has shown that rainfall-runoff relationships in this area differ according to vegetation type and phenological periods. Hence, forthcoming catchment hydrological studies should be designed to control for these effects. [ABSTRACT FROM AUTHOR]

It is well known that a logging/harvesting practice in a forested catchment induces evapotranspiration (ET) decline and a subsequent runoff increase. In addition, most previous paired-catchment studies have revealed that, after re-planting the same tree species in the logged catchment, the post-logging runoff increase gradually diminished and returned to pre-logging level within short time period (typically 5-15 years), due to increasing water use by the planted trees with their growths. However, there is relatively little information on how longer-term catchment water balance changes with maturity of trees in the catchment. Marc and Robinson (2007) is one the few studies, in which, based on long-term catchment water balance data in UK, they reported that ET reduced by 140 mm per year in spruce/pine catchments with increasing forest ages. In order to fill the knowledge gap further, we revisited long-term water balance data for a Japanese catchment in which the soil and vegetation had been disturbed due to forest overuse, until its establishment in 1929 by University of Tokyo. Initially, the catchment was covered by young pines with scattered denuded ridges. After the catchment establishment, the poor vegetation had gradually been matured and the scattered denuded ridges had been decreased particularly after 1960s. Because the matured pine trees in the catchment had been largely declined due to an epidemic (pine wilt disease) during 1980s, we examined annual loss of the catchment from 1930s to 1970s. The result showed that the annual loss decreased with the decades, i.e., pine maturing, by about 100-150 mm per year. A further event-basis water balance analysis (Linsley et al. 1982) showed that the decline in the annual loss was caused by ET decline in the mid-growing season while ET in the dormant season exhibited almost no change throughout decades. Though, in order to draw a solid conclusion, a further investigation is still needed considering fluctuations in meteorological factors (such as air temperature) controlling ET, we currently attribute the ET decline in the mid-growing season to age-related transpiration decline, as was found for maritime pine stands in France (Delzon and Loustau 2005), because canopy interception would be augmented in later decades due to larger above-ground biomass and leave amount. [ABSTRACT FROM AUTHOR]