The risk reduction effect of sediment production rate by understory coverage rate in granite area mountain forest
In this study, as shown in the conceptual image described in Fig. 2, we supposed that the cause to promote sediment production rate from forest areas to rivers is understory coverage rate decreasing. As the locations of the survey, we selected three granite low mountain forest areas in Japan; Hiei mountain, Kagami mountain, and Suzuka mountain. These places were the similar environmental conditions, forest woods, understory plants, the incline which almost less than 35°, and the latitude which is N35°04′-05′ at catchments of the river which inflow to Lake Biwa. We surveyed Shiga prefecture where is almost Lake Biwa basin in the western area of Japan. The forest area of Shiga prefecture is about 200,000 ha (about 60% occupied the prefecture area). About 80,000 ha of them are the artificial forest of cedar and cypress. Most of the planted forests are of an age that can be cut down and used. The locations of the field survey, we selected three low mountain forest areas on granite area and almost the same latitude in Lake Biwa basin; Hiei mountain, Kagami mountain, and Suzuka mountain (Fig. 3).Figure 2The concept ecosystem image diagram of the relationship between understory coverage rate and sediment production in the low mountain forest area. The understory disturbs sediments moving. The concept image diagram was drawn by T. Mizuno using Microsoft PowerPoint.Full size imageFigure 3The location map of the field survey forest of the low mountain on granite are between N35 06′ from N35 04′, where are the basin of Lake Biwa near Kyoto in the western area of Japan. The location map was based on the Digital Topographic Map 25,000 published by the Geospatial Information Authority of Japan (https://maps.gsi.go.jp), and was edited and processed by T. Mizuno using Microsoft PowerPoint.Full size imageFor the sediment production rate, annual actual measurement data of sediment receiving boxes, sediment receiving weirs, and dams were used. For the understory coverage rate, the Miura method was used18. The almost physical flow of forest surfaces is expressed in the form of a function of exponentiation19. Therefore, in the meta-analysis of all field data, the Poisson regression analysis (log function) of the generalized linear mixed-effects regression model was used. The dependent variable was the sediment production rate, the explanatory variable was the understory coverage rate and the random effect was the rain intensity. Because the rainfall intensity has a large effect on the surface flow of the forest when the amount of rainfall exceeds 90 (mm) on the moist soil4. Therefore, assuming that the maximum annual rainfall of 72 h is strongly related to sediment movement, it was used as an index of rainfall intensity. All-year data of Hiei, Kagami, and Suzuka of the maximum annual rainfall of 72 h exceeded 90 (mm). The rainfall intensity was categorized 6 levels; 100–200 (mm), 200–300 (mm), 300–400 (mm), 400–500 (mm), 500–600 (mm) and 600–700 (mm). Each rainfall intensity was inserted as the random effect in the Poisson regression analysis model. Statistical analysis software used R20 and performed calculations using the lme4 package21.Detail method of field survey of Hiei mountain forest (St.1)We carried out a field survey on the sediment production rate on a forest slope in the Hiei mountain forest owned by Enryakuji temple which was recorded world heritage. The stream order of the survey site is the zero-order basin of the Omiya River. The survey point was set in the forest of Sakamoto-Cho, Otsu City (Latitude and longitude notation; 35° 5′ 29ʺ N, 135° 50′ 10ʺ E) located in the uppermost stream of the Omiya River in the southwestern part of Shiga Prefecture. The bedrocks were mainly granite rock and the soil was brown forest soil. The altitude was about 760 (m), the slope direction was east, and the slope was 32°–35°. The main forest wood was the Japanese cypress (Chamaecyparis obtusa), which was about 100 years old, and the forest floor was relatively bright with moderate forest density. Besides, dwarf bamboo flourished on the forest floor of the study site and nearby forests until around 2005. At present, there are many areas where understory vegetation has disappeared due to deer feeding damage. At the survey point, the surface of the forest floor had become bare. On the forest slope where the understory vegetation had disappeared, we made a 5.0 (m) × 5.0 (m) survey area surrounded by a protective fence with a height of about 2.0 (m) to prevent deer feeding damage. At the lower end of the survey area, five sediment receiving boxes with a width of 25 (cm) and a height of 15 (cm) were installed at intervals of about 1.0 (m) along the contour lines. The survey started in June 2015, and the samples captured in the sediment receiving box were collected approximately once every two to four weeks, and after heavy rain appropriately. The collected sediment sample was air-dried, dried at 70 °C for 24 h or more, fractionated into sediment and litter, and the weight of each was measured. The sediment production rate was converted with a specific gravity of 1.8 (tons) per 1.0 (m3). As for the understory coverage rate, vegetation growth, litter, sediment, and gravel were evaluated by the point-counting by the Miura method18 in a range of 50 (cm) × 50 (cm) above each sediment receiving a box every autumn. Also, we checked the vegetation overgrowth around the field survey area. As the rainfall data used in the analysis, the observation data (observatory name: Hiei) closest to the survey site was used from the water quality hydrology database of the Ministry of Land, Infrastructure, Transport, and Tourism. The boxplot of the annual sediment production rate (m3/km2/year) was made by using soft-wares R3.6.120.Detail method of field survey of Kagami mountain (St.2)We collected data about the sediment production rate of the forest with 60% or more of the understory coverage rate in the Kagami mountain forest where no deer has been confirmed. The stream order of the survey site was a 0–3 order basin within the catchment area of the Hino River. The investigation point of sediment outflow from the forest was conducted at the forest mountain stream in Oshinohara, Yasu City (Latitude and longitude notation; 35° 4′ 2ʺ N, 136° 4′ 3ʺ E). The bedrock is granite, and the soil is brown forest soil. The catchment area of the study site is 20.0 ha, the altitude is about 150–280 (m), the slope of the mountain stream is north, and the slope of the mountain stream is about 11°. The main forest wood was the Japanese cypress (Chamaecyparis obtusa) and deciduous broad-leaved trees such as oak. No deer feeding damage to adult trees and understory was observed high density in the survey area. Mainly understory is the fern plant (Gleichenia japonica). Now the understory coverage rate is 60% or more anywhere. Sediment and litter that flowed out of the forest were collected from the upper part of the concrete weir (2.4 (m) wide × 1.2 (m) tall) installed at the downstream end of the survey site. We collected approximately once every two to four weeks after heavy rain in five years from 2015 to 2019. The collected sediment sample was air-dried for about 1 week, then dried at 70 °C for 24 h or more, and the weight of gravel and litter was measured. The boxplot of the annual sediment production rate (m3/km2/year) was made by using soft-wares R3.6.120.Detail method of data collection of Suzuka mountain (St.3)We collected data about the sediment production rate of the forest with both cases under 30% and 30%-60% of the understory coverage rate in the Suzuka mountain by using the Eigenji dam annual sediment deposit data. The elevation of the Eigenji Dam dam is 274 (m), and the maximum elevation of the catchment area of the Eigenji Dam is 1247 (m). The Eigenji dam is the stream order which is a 0–6 order basin within the catchment area of the Echi River. The Eigenji Dam was built in 1973 on the Echi River in Higashi-Omi City, Shiga Prefecture (Latitude and longitude notation; 35° 4′ 35ʺ N, 136° 20′ 7ʺ E), the catchment area is 131.5 km2. The catchment area is almost the forest area of the Suzuka mountain. The main bedrock of the Eigenji dam is granite, and the main bedrocks of the catchment are granite and sedimentary rock. The soil is brown forest soil. The slope of the watershed area is 10–20°. The report of Shiga prefecture referred to the damage caused by overgrazing by deer began to increase around 201022,23. In 2011, a large decrease in understory was confirmed in the entire watershed of the Eigenji Dam24. The boxplot of the annual sediment deposition (m3/km2/year) was made by dividing the period’s case 1 is when a 30–60% understory coverage rate from 1982 to 2009 and case 2 is when under 30% understory coverage rate from 2010 to 2015 by using soft-wares R3.6.120.Detail explains of the random effect of the equation of meta-analysisThe Poisson regression analysis (log function) of the generalized linear mixed-effects regression model was used. The dependent variable was the sediment production rate, the explanatory variable was the understory coverage rate and the random effect was the rainfall intensity. The rain intensity was categorized 6 levels; 100–200 (mm), 200–300 (mm), 300–400 (mm), 400–500 (mm), 500–600 (mm) and 600–700 (mm). Each rain intensity was inserted as a random effect in the Poisson mixed-effect regression analysis model. Statistical analysis software used R and performed calculations using the lme4 package21. More
