Morfología Fluvial.-Erosion, water availability and plant characteristics control plant colonisation on semiarid eroded slopes

Abstract

ABSTRACT The objective of this study was to understand the mechanisms that control spontaneous plant colonisation on severely eroded slopes in a semiarid badland area of East Spain. More specifically, we aimed at (1) determining topographic thresholds for plant colonisation, (2) identifying the soil properties that limit plant establishment and (3) identifying plant traits that enable colonising species to cope with these limitations. We used slope angle and aspect as surrogates of erosion rate and water availability respectively. Since soil erosion and water availability can limit plant establishment and both interact in the landscape, we analysed variations in colonisation success with slope angle and aspect. Vegetation success was measured in terms of total vegetation cover on 156 different slopes. After determining slope angle thresholds for plant colonisation, soil was sampled on slopes just above and just below the threshold values for soil analysis in order to test for differences in soil properties related to plant establishment and development. Plant traits related to plant colonising capacity were analysed in two different groups of species: the group of species colonising the steep slopes near the threshold and the group of noncolonising species present on more gentle slopes but unable to colonise the slopes just below the threshold. The identified slope angle threshold values for plant colonisation clearly decreased from North to South. No differences were found in soil properties neither among slope aspects at the slope angle threshold values nor between slope positions (just below and above the threshold) within slope aspect classes. Long-distance dispersal mechanisms and ability of seeds to segregate mucilage in contact with water were more frequent characteristics in colonising species than in non-colonising ones. It is concluded that water availability for plants which, in turn, is controlled by the solar radiation can explain the differences in the slope angle threshold values for plant colonisation among slope aspect classes. Some important implications of these results in the context of ecological restoration of these severely eroded areas are also discussed.�L /