THE EFFECTS OF PLANT ROOTS ON SOIL EROSION BY CONCENTRATED FLOW
De Baets, S.1* and Poesen, J.1
(1) Dep. Earth and Environmental Sciences, K.U. Leuven, Celestijnenlaan 200E, B-3001
Heverlee, Belgium
* corresponding author e-mail: sarah.debaets@ees.kuleuven.be
Keywords: vegetation, rill and gully erosion, below-ground biomass, soil conservation
Abstract
Many studies attribute the effects of vegetation in reducing soil erosion rates to the effects of the
above-ground biomass. The effects of the below-ground biomass on flow erosivity and topsoil
resistance to concentrated flow erosion are much less studied. However, roots play an important role
in controlling soil erosion rates when the above-ground biomass disappears (e.g. due to fire, drought,
harvest, grazing) and particularly when incisive processes are concerned. Roots affect properties of
the soil, such as infiltration rate, aggregate stability, moisture content, shear strength and organic
matter content, all of which control soil erodibility to various degrees. It is generally recognized that
plant roots contribute to the overall cohesion of the soil. Moreover, roots were also reported to have an
additional flow-retarding effect, although these effects were not quantified yet. The main objectives of
this study are to quantify the mechanical and hydraulic effects of plant roots during concentrated flow
erosion in order to better predict soil erosion rates.
Therefore, concentrated flow experiments were conducted using a hydraulic flume. Both bare and
root-permeated topsoil samples were tested and their effects on soil cohesion and water flow velocity
were studied.
The approach for quantifying additional root cohesion is based on a comparison of measured soil
detachment rates for bare and for root-permeated topsoil samples with predicted erosion rates under
the same flow conditions using the erosion equation of EUROSEM. Through backwards calculation,
soil cohesion values could be assessed for bare and root-permeated topsoils respectively.
The methodology to correct for the flow-retarding effects of plant roots is based on calculating the
effective soil shear strength (part of the flow that is responsible for soil detachment) and a corrected
soil detachment rate by making corrections for the proportion of soil that is occupied by plant roots.
The results are promising and show that the contribution of roots to soil cohesion is very important for
preventing soil loss and reducing runoff volume. Moreover, grass roots provide a larger increase in soil
cohesion as compared with tap-rooted species. On the other hand, tap roots provide a small flowretarding effect, whereas fine- branched grass roots prevent incision but do not retard water flow
velocity.