Plant roots reduce rill detachment and shallow instability in forest topsoils

The competition between tree and grass roots for water and nutrients under the canopies of forest species may reduce the grass cover and thus increase rill erosion and shallow instability up to the values that are typical of the bare soils. This study has carried out flume experiments at different soil slopes and water flow rates, in order to evaluate rill detachment capacity (Dc) and erodibility (Kr) as well as the stability factors of hillslopes with Gleditsia caspica L. (a Fabaceae species, commonly called 'Persian honeylocust', a local endemic tree of Northern Iran) in comparison to bare soils. The variability of Dc has been associated to soil aggregate stability and plant root characteristics as key descriptors of rill erosion and surface stability. Dc was significantly lower (-41%) in the soil under the canopies compared to the bare sites. This was due to the higher soil aggregate stability (+83%) as well as to the denser and more extended plant root system, as confirmed by the negative correlations between Dc and soil and root parameters including root total length, mass density and specific root length. Kr was instead similar for the two soil conditions. The root system of the surface soil layer also played a beneficial action for slope stabilization, increasing the mean safety factor between soils with Gleditsia caspica and bare soils to 1.52 (well over the threshold of 1.3 identifying possible shallow instability). However, this safety factor was the highest at the lower slopes (1.63), and decreased with slope down to 1.39 in the steepest soils. Overall, this study provides indications to land managers on how to contrast soil erosion and shallow instability in delicate forestlands under semi-arid conditions.