R3 - 6

令和４年度砂防学会研究発表会概要集

Evaluating the Effect of Forest Density on Root Reinforcement by Using a Flume Experiment
1*23144Rozaqqa NOVIANDI , Takashi GOMI , Roy C. SIDLE , Rasis P. RITONGA , Norihiro OTAKA , and Yuko HASUNUMA
1United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Japan
2
Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Japan3
Mountain Societies Research Institute, University of Central Asia, Tajikistan
4
Nippon Steel Metal Products, Co., Ltd., Japan

1. INTRODUCTION
Vegetation cover is the key factor for slope stabilization in forested hillslopes (Sidle, 1991). Vegetation root systems
reinforce soil structures and thereby improve slope stability (Schwarz et al., 2010). This reinforcement depends on many
contributing factors but is mainly affected by the characteristics of the root system architecture including root orientation andmass. Root orientation defines the reinforcement mechanisms, in which vertical roots enhance basal shear resistance while lateralroots moderate slope deformation (Cohen and Schwarz, 2017). Root mass including root size and distribution then controls themagnitude of root reinforcement (Wu et al., 1979).
In the hillslope, root reinforcement depends not only on the characteristics of root systems architecture but also on the root
connections between neighboring trees. Such connections are complex but may be largely affected by the length of theconnections (hereinafter referred to as root overlapping length). The overlapping length may not only control the continuity ofroot reinforcement within the slope but also the capability of the slope to resist deformation. Nevertheless, the effect of root
overlapping length on root reinforcement has never been studied in any studies because of the difficulties in measuring the actual
overlapping length even with dissection.
Forest density and distance between vegetation greatly influence the overlapping length (e.g., Babi et al., 2019), affecting
root reinforcement on the slope. Indeed, previous study suggested that the change in forest density from 400 to 1000 stems/ha
elevated root reinforcement by 1.6 times because of more overlapping roots (Schwarz et al., 2010). Since root reinforcement
affects slope stability, the change in forest density consequently alters landslide frequency (e.g., Imaizumi et al., 2007). Thus,
evaluating the effect of forest density on root reinforcement is the key point for landslide risk assessment in forested hillslopes.
However, such evaluation has been remaining a challenge since forest conditions are spatially and temporally complex.
Flume experiment with vegetation can be effective to evaluate the effect of forest density on root reinforcement. Indeed,
previous study mentioned the effectiveness of the flume experiment in evaluating the effect of vegetation root systems on slope
stability (Noviandi et al., 2021). Therefore, the objectives of this study are to (1) evaluate the effect of forest density on slope
stability by using a flume experiment and (2) elucidate the effect of root overlapping length on root reinforcement. Findings ofthis study provide insight for improving landslide risk assessment in forested hillslopes.
2. METHODOLOGY[1] Experimental slope
A flume constructed at a 1:70 scale was used to evaluate the effect of forest density on root reinforcement (Fig. 1). The
flume consisted of two segments representing landslide initiation (120 cm long, 35° inclination) and deposition (150 cm long,35° inclination). All segments were 80 cm wide, 15 cm high, and constructed with 1-cm thick acrylic material. Nozzles wereinstalled 2 m above the flume to apply surface water as rainfall. Since the edge of the deposition segment was open, water fromthe slope flowed freely on this segment. Hence, we placed a tipping bucket at the end of the deposition segment to measure theoutflow discharge. Sand (density = 1.4 g/cm3, D50 = 0.23 mm) was placed in the initiation segment to a depth of 10 cm.
2] Application of vegetation
We grew pea (Pisum sativum L.) bean sprouts in the sandto simulate the root reinforcement effect on the slope. Beans
were placed at 3, 5, and 7 cm intervals to represent a forest
with 2,200, 800, and 400 stems/ha of density. After 2 weeksof planting, the sprouts reached 25-30 cm in height that
representing 17.5-21 m height of Japanese Cedar plantation.
[3] Measurement and analysis
We applied rainfall at an intensity of 90 mm/h. When thelandslide was initiated, we measured the time from surface
water application to landslide initiation. Mean volumetricwater content was estimated based on 10 TDR (Time DomainReflectometer) sensors placed at 3 and 7 cm depths from soilsurface. Nine intact root system samples (individualvegetation) were taken from each experiment to measure thefresh individual root mass and the maximum root radius. Total
Fig. 1 Schematic illustration of flume apparatus
root mass was estimated on the basis of the mean individualroot mass multiplied by the total number of vegetation per unit area. Root overlapping length was then estimated based on themean root radius and vegetation intervals, assuming 60° of lateral root inclination on pea beans (e.g., Rubio et al., 2003).

－ 233 －3.

RESULTS
Root distribution differed depending on vegetation interval (Fig.
2a). The individual root mass was the highest for 7 cm (n = 18, mean= 0.58 g, SD = 0.07 g), followed by 5 cm (n = 27, mean = 0.48 g, SD= 0.07 g) and 3 cm intervals (n = 27, mean = 0.33 g, SD = 0.06 g). In
contrast, total root mass was the highest for 3 cm (mean = 371 g/m2,SD = 73 g/m2), followed by 5 cm (mean = 192 g/m2, SD = 29 g/m2)
and 7 cm intervals (mean = 119 g/m2, SD = 15 g/m2). Root overlapping
length was the highest for 3 cm (mean = 65 mm, SD = 18 mm), whilethat in 5 cm (mean = 51 mm, SD = 11 mm) and 7 cm intervals (mean= 50 mm, SD = 18 mm) was almost similar.
Differences in root distribution affects landslide characteristics(Fig. 2b). Indeed, condition with 3 cm interval had the gratest
capability to delay landslide initiation (n = 3, mean = 924 s, SD = 41s), while landslides in 5 cm (n = 3, mean = 810 s, SD = 113 s) and 7
cm intervals (n = 3, mean = 820 s, SD = 58 s) were initiated at almost
the same time. Similarly, mean volumetric water content duringlandslide initiation was the highest for 3 cm interval (mean = 0.22, SD= 0.01), followed by 7 cm (mean = 0.19, SD = 0.01) and 5 cm (mean= 0.18, SD = 0.01).

Fig. 2 (a) Root overlapping length and (b) timing forlandslide initiation for different vegetation intervals

4.

DISCUSSION
Forest density greatly influences root mass since tree spacing affects the competition between trees for getting light, water,and minerals (Babi et al., 2019). Indeed, we found that the highest individual root mass was for 7 cm since less vegetation spacing
promoted more root growth, whereas the lowest one was for 3 cm interval. In contrast, total root mass was the highest for 3 cm
interval because it produced the greatest amount of vegetation. Similarly, root overlapping length was the longest for 3 cm
intervals because less vegetation spacing resulted in more overlapping length. Nevertheless, the overlapping length for 5 cm and7 cm intervals was almost similar (Fig. 2a). Since less spatial competition promoted more root growth, whereas less vegetation
spacing resulted in more overlapping, conditions with 5 cm and 7 cm intervals generated similar values of overlapping length.
Previous studies mentioned the effect of root mass on root reinforcement (Wu et al., 1979). In general, the larger root mass
generated greater reinforcement effects (e.g., Wu et al., 1997; Sakals and Sidle, 2004; Genet et al., 2008). Indeed, we found that3 cm interval, having the largest total root mass among other conditions, had the greatest capability to delay landslide initiation.
Nevertheless, we found a similar response on landslide initiation timing for 5 cm and 7 cm intervals even though they produced
a dissimilar total root mass (Fig. 2b). Instead of total root mass, root reinforcement under these conditions is likely influenced
by the overlapping length since they produced a similar value of this parameter.
Roots at overlapping zone may fail under pull-out because of nonintact root connections, unlike an individual intact root.
Since root length controls root pullout resistance (Schwarz et al., 2010), root overlapping length thereby influences root failure
within the overlapping zone. Thus, the longer overlapping length may generate greater reinforcement and capability toaccommodate deformation because of the larger pull-out resistance. Such condition consequently alters the rainfall threshold(e.g., duration) for landslide initiation. Indeed, we found that 3 cm interval with the longest overlapping length had the longest
duration for landslide initiation, whereas that for 5 and 7 cm intervals was similar since these intervals generated similar valuesof overlapping length. Thus, our findings concur with previous studies that root mass influences root reinforcement. However,
we highlight that root overlapping length is also a key parameter for controlling root reinforcement in forested hillslopes.
5. CONCLUSION
We suggested that root overlapping length is also influential for root reinforcement in addition to root mass. Since forest
density affects the overlapping length, forest density greatly influences the vulnerability toward slope instabilities, particularly
shallow landslides. Based on these findings, possible land use management for mitigation measures against landslide hazards
can be implemented. We plan to estimate root reinforcement under various forest densities in future studies.
REFERENCES
Babi, K., Guittonny, M., Larocque, G. R., Bussière, B., 2018. Effects of spacing and herbaceous hydroseeding on water stress exposure and root development of poplars planted i nsoil-covered waste rock slopes. Écoscience, 1-16.
Cohen, D., Schwarz, M., 2017. Tree-root control of shallow landslides. Earth Surf. Dynam. 5, 451-477.
Genet, M., Kokutse, N., Stokes, A., Fourcaud, T., Cai, X., Ji, J, Mickovski, S., 2008. Root reinforcement in plantations of Cryptomeria japonica D. Don: effect of tree age and standstructure on slope stability. For. Ecol. Manag. 256(8), 1517-1526.
Imaizumi, F., Sidle, R. C., Kamei, R., 2007. Effects of forest harvesting on the occurrence of landslides and debris flow in steep terrain of central Japan. Earth Surf. Process. Landf.
33(6), 827-840.
Noviandi, R., Gomi, T., Sidle, R. C., Iwasa, N., Ritonga, R. P., 2021. Developing an Experimental Method to Evaluate the Effect of Root Network on Shallow Landslide s by Using aFlume Experiment. Japan Society of Erosion Control Engineering annual meeting 2021, 261-262.
Rubio, G., Liao, H., Yan., X., Lynch, J. P., 2003. Topsoil Foraging and Its Role in Plant Competitiveness for Phosphorus in Common Bean. Crop Sci. 43, 598-607.
Schwarz, M., Lehmann, P., Or, D., 2010. Quantifying lateral root reinforcement in steep slopes – from a bundle of roots to tree stands. Earth Surf. Process. Landf. 35, 354-367.
Wu, T. H., McKinnell, W. P., Swanston, D. N., 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geoth. J. 16, 19-33.
Sakals, M. E., Sidle, R. C., 2004. A spatial and temporal model of root cohesion in forest soils. Can. J. For. Res. 34, 950-958.
Sidle, R. C., 1991. A Conceptual Model of Changes in Root Cohesion in Response to Vegetation Management. J. Environ. Qual. 20, 43-52.

Keywords: Flume experiment, root reinforcement, forest density, root overlapping length－ 234 －