Tree roots anchor soil and reduce landslide risk: case studies in Indonesia Kurniatun Hairiah(a), Widianto(b), Didik Suprayogo(c), Bruno Verbist(d), Meine van Noordwijk(e)
(a,b,c)Brawijaya University, Faculty of Agriculture, Jl Veteran, Malang 65145, Indonesia; (d) Department of Earth and Environmental Sciences, K.U.Leuven, Geo-Institute, Celestijnenlaan 200E, 3001 Leuven, Belgium; (e) World Agroforestry Centre, ICRAF S.E. Asia, Bogor, Indonesia
Abstract
Tree root systems can reduce landslide risk and stabilize river banks. As part of a broad evaluation of the role of agroforestry in maintaining watershed services and reducing natural hazards in the humid tropics, case studies in two catchments in Indonesia included individual root, rooted soil volume and tree-root-system level effects. The major part (81%) of variation in root tensile (breaking) strength at standard root diameter across 5 species was accounted for by the model: Lignin content plus 0.76 times Nitrogen minus 0.07 times Polyphenol content. Tree root length density (Lrv) was weakly correlated with soil shear strength (SS), but it improved a mixed model with soil texture parameters. Overall, bamboo plots had the largest soil shear strength and the highest root length density. An index of root anchoring (IRA: ratio of cross sectional area of vertical roots and stem) was assessed across 25 tree species (5 year old) in agroforestry system; the highest IRA (7.7) was observed in non-pruned coffee, while pruning stimulates formation of roots in the surface layer. Three other tree species (Artocarpus elasticus, Parkia speciosa and Durio zibethinus) had a high IRA value (>1.0), and tend to grow to larger dbh values, providing more anchoring than coffee on a per tree basis. The common shade trees in coffee agroforestry system i.e. Gliricidia sepium and Erythina subumbrans and the tree most frequently used for government reforestation programs in the past, Calliandra calothyrsus, have low IRA values, indicating little soil anchoring; other timber and fruit trees had intermediate IRA values. Based on these data, a mix of tree species with different pattern of rooting characteristics grown to mature size will provide a good protection of the soil surface and also increase river bank and hill slope stability.
Keywords: Agroforestry, Index of Root Anchoring, Slope Stability, Soil shear strength, Root strength, Root length density
