Large roots dominate the contribution of trees to slope stability

Abstract

Tree roots provide surface erosion protection and improve slope stability through highly complex interactions with the soil due to the nature of root systems. Root reinforcement estimation is usually performed by in situ pullout tests, in which roots are pulled out of the soil to reliably estimate the root strength of compact soils. However, this test is not suitable for the scenario where a soil progressively fails in a series of slump blocks – for example, in unsupported soils near streambanks and road cuts where the soil has no compressive resistance at the base of the hillslope. The scenario where a soil is unsupported on its downslope extent and progressively deforms at a slow strain rate has received little attention, and we are unaware of any study on root reinforcement that estimates the additional strength provided by roots in this situation. We therefore designed two complementary laboratory experiments to compare the force required to pull the root out. The results indicate that the force required to pull out roots is reduced by up to 50% when the soil fails as slump blocks compared to pullout tests. We also found that, for slump block failure, roots had a higher tendency to slip than to break, showing the importance of active earth pressure on root reinforcement behaviour, which contributes to reduced friction between soil and roots. These results were then scaled up to a full tree and tree stand using the root bundle and field‐measured spatial distributions of root density. Although effects on the force mobilized in small roots can be relevant, small roots have virtually no effect on root reinforcement at the tree or stand scale on hillslopes. When root distribution has a wide range of diameters, the root reinforcement results are controlled by large roots, which hold much more force than small roots. © 2019 John Wiley & Sons, Ltd.

Effects of Slope Gradient on Runoff and Sediment Yield on Machine-Induced Compacted Soil in Temperate Forests

“…The forest-covered basin has relatively little surface erosion, but timber harvesting changes canopy cover and exposes mineral soil to water and wind, resulting in a lower amount of evapotranspiration [1][2][3]. In addition, the change of vegetation has a profound effect on the water cycle and the reduction of vegetation, which are caused by logging operations increases surface runoff volume and overall water yield [4]. The change in land use can significantly affect the hydrology of a forested watershed [5].…”

Minimum representative root distribution sampling for calculating slope stability in Pinus radiata D.Don plantations in New Zealand

“…Root tensile strength predicted by the regression model gives completely different results if the model only considers laboratory test data (Watson et al 1999), or data obtained from field pull-out tests such as carried out in this study. The use of laboratory-only tests will greatly overestimate root tensile strength, as highlighted in Docker and Hubble (2008) and Giadrossich et al (2017Giadrossich et al ( , 2019. However, root tensile strength for small diameter classes is similar for data obtained from the laboratory or in the field, thus when merging the two datasets, the model parameters do not change significantly from those obtained considering data only from the field pullout tests.…”

The nurse-plant effect under the dislodgement stress of landslides

“…The nurse-plant effect of large trees should be incorporated into the management of landslide-prone forests. Due to the facts that root reinforcement is mainly contributed by large roots (Giadrossich et al 2019) and larger trees develop more large roots than do small trees, large trees survived landslides better than did small trees (Fig. 2, Fig.…”