QLK5-CT-2001-00289 ECO-SLOPES: Eco-engineering and conservation of slopes for long-term protection from erosion, landslides and storms

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Quality of Life - 5.3.1 Multifunctional Management of Forests : Wood (Lignocellulose)

Contract No:	QLK5-CT-2001-00289
Source:	Final Report November 2004

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Final Report - November 2004 - Abstract

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A multidisciplinary project was carried out to determine the influence of vegetation on different types of unstable slopes. Sites were chosen from throughout Europe, each one demonstrating a different instability problem: shallow landslides in Italy and the UK, rockfall in the French Alps, erosion after wildfires in Greece and Spain, abandoned land in Spain and stand instability of forested slopes in Scotland. Each site was characterised using protocol developed in the project, and experiments carried out to determine the influence of vegetation on these sites.

A major component of soil fixation by plants is the root-soil interaction. This interaction is determined by several factors including root tensile strength and architecture, soil mechanical, physical and hydrological properties. These parameters were determined for a large number of species and slopes in the project. Further experiments were carried out to estimate forest tree resistance to rockfall and windthrow on slopes, as well as tree root anchorage. Results showed that root anchorage differs between species, although the relationship with architecture is not entirely clear and merits further research. Root tensile strength depends largely on root chemical properties. Forest trees on slopes were shown to be more resistant to overturning than those growing on flat ground.

The most resistant species to rockfall were determined, as well as how to better manage forest plantations subjected to rockfall activity. Both the effects of wildfires on Mediterranean plant communities and how best to manage a forested slope after a fire, were investigated. Experiments were carried out whereby plots were subjected to repeated fires of different intensities.

Results showed that the incidence of repeated fires, of any intensity is determinant in zones not totally recovered from a previous fire. The degree of vegetation recovery and the intensity and readiness of rains after the fire are key factors in the effect on soil of water erosion processes, especially in fragile ecosystems. Once a wildfire has occurred, remediation works should take place immediately. Natural regeneration is an ecological factor adapted well to wildfires and protects soil from erosion and floods. The removal of burnt trees is not necessary to ensure seedling germination and re-establishment as removal increases seedling mortality.

With regards to abandoned land in Spain, studies were also carried out at the catchment scale. Results showed that vegetation succession progressively decreases erosive surface processes whereas mass wasting processes become more prominent. Together with results from soil mechanical tests and rooting experiments, we concluded that rooting is contributing to the soil strength, but only to the upper 0.6 m of the soil. Most failures however, occur at greater depths (1.00-1.20m) as anchoring by deeper roots is not sufficient or absent. The increase of mass wasting processes after land abandonment can therefore be explained the limited contribution of root anchorage at the potential slip plane and the potential fast wetting of the slip planes through macro-pores.

Data from the project were used as input to models of forest tree and stand stability, root anchorage and slope stability. Several models were developed and tested within the project, with huge advances in this area. These models helped us to determine which would be the most efficient root morphological type to resist uprooting, and how best to manage forest stands on a slope subjected to mechanical abiotic stresses. Different slope stability models were tested and compared, incorporating soil physical, mechanical and hydrological factors. By using these models, we were able to determine more precisely the role of vegetation on an unstable slope. Results showed that static representations of root reinforcement overestimate the mechanical influence of vegetation on slope stability and that the effect of saturation is more profound than root reinforcement. The combined effects of root reinforcement, soil properties and hydrology are also extremely complex and require further research.

Results from the project were included into a Slopes Decision Support System (SDSS), adapted specifically for the Eco-Slopes project. This SDSS is designed for the end-user, forester, engineer and ecologist who seeks help with regards to planting and management of a slope with any of the above instability problems. A manual was also written to be used hand-in-hand with the SDSS.

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