A spatial and temporal model of root cohesion in forest soils

Abstract

Root cohesion is an important parameter governing slope stability in steep forested terrain. Forest harvesting impacts root cohesion, and although the temporal effects have been noted, this dynamic parameter is often assumed to be spatially uniform. A model was developed to simulate the variation in root cohesion on a hillslope resulting from various forest management treatments. The model combines physical data on the horizontal rooting distribution of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) together with a temporal relation of root cohesion decay. Harvesting methods examined include clear-cutting, single-tree selection cutting, and strip-cutting. Model outputs are analysed qualitatively for regions of root cohesion minima and quantitatively for the average root cohesion within the simulated hillslope. A selection cutting simulation maintained the highest average root cohesion value, decreasing to only 81% of the preharvest condition. In contrast, the minimum root cohesion following clear-cutting declined to 38% of the preharvest value. Selection and strip-cutting scenarios resulted in smaller areas of reduced root cohesion that were adjacent to areas with high root cohesion. Such partial cutting methods shorten the period of reduced root cohesion following timber harvesting compared with clear-cutting.

Mechanisms of vegetation uprooting by flow in alluvial non-cohesive sediment

“…From a mechanical viewpoint the reinforcement of soil by plant roots has been quite extensively studied mainly for steep soils and river banks (Dunaway et al, 1994;Pearce et al, 1998;Millar, 2000;Abernethy and Rutherford, 1998;Micheli and Kirchner, 2002;Sakals and Sidle, 2004;Dupuy et al, 2005;Pollen and Simon, 2005;Pollen, 2007;Eaton, 2006). Yet, knowledge of the role of below-ground biomass in stabilizing sediments and soil is limited (Gyssels and Poesen, 2003;Gyssels et al, 2005;De Baets et al, 2006) and practically unexplored for naturally non-cohesive material such as gravel and sand found on river bars and vegetated islands.…”

Root reinforcement dynamics in subalpine spruce forests following timber harvest: a case study in Canton Schwyz, Switzerland

“…Sakals and Sidle (2004) observed that the temporal dynamics of root reinforcement is usually assumed to be spatially uniform, which is, according to the authors, 4 inappropriate because root distribution varies spatially and it has been proved that this variability can be huge as function of forest structure and patterns (Genet et al, 2010;Hales et al, 2009;Mao et al, 2012;Schmidt et al, 2001;Schwarz et al, 2010b, Vergani et al, 2014b. Nowadays, one of the main challenges is to consider both the spatial and temporal dynamics of root reinforcement, as this can be the key to understand landslide triggering mechanisms (Hales et al, 2013;Schmidt et al, 2001;Schwarz et al, 2012).…”

“…In these latter works there is an effort to consider the variation of roots number after felling, but the lack of information regarding the spatial variability of root distribution does not allow the upscale of results to the stand scale and then to the hillslope scale. Sakals and Sidle (2004) proposed a model for the spatial and temporal variability of root reinforcement, based on literature data and field data for a Pseudotsuga menziesii forest stand but they didn't consider the effect of the decay of the single elements that contribute to root reinforcement, i.e. strength and root density by class diameter.…”

Quantifying lateral root reinforcement in steep slopes – from a bundle of roots to tree stands

“…In the presence of signifi cant lateral root reinforcement, the area that must be destabilized in order to trigger a landslide increases (Reneau and Dietrich, 1987). Notwithstanding preliminary attempts to quantify lateral root reinforcement (Krogstad, 1995;Zhou et al, 1998;Roering et al, 2003;Sakals and Sidle, 2004) a comprehensive modeling framework for spatial distribution of lateral root reinforcement is urgently needed.…”