Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland Estela Nadal-Romero, 1 * Kristien Petrlic, 2 Els Verachtert, 2 Esther Bochet 3 and Jean Poesen 2 1 Department of Geography, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain 2 Department of Earth and Environmental Sciences, Division of Geography, KU Leuven, Heverlee, Belgium 3 Centro de Investigaciones sobre Desertificación (CSIC, UV, GV), Moncada, Valencia, Spain Received 16 April 2013; Revised 5 February 2014; Accepted 5 February 2014 *Correspondence to: E. Nadal-Romero, Department of Geography, University of Zaragoza, Pedro Cerbuna 12, 50009, Zaragoza, Spain. E-mail: estelanr@unizar.es ABSTRACT: Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands oc- cupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi-arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south-facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant col- onization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south-facing slopes, which contrasts with the results obtained in semi-arid badlands. North-facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope-aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd. KEYWORDS: erosion; vegetation; topography; climate; freeze–thaw; colonization; threshold; Central Pyrenees Introduction Badlands are peculiar landscape features that have received a lot of attention. According to Bryan and Yair (1982) badlands are intensely dissected natural landscapes where vegetation is sparse or absent and which are useless for agriculture. Fluvial processes (water erosion and associated mass movements) are mainly responsible for the geomorphic origin of badlands. Since badlands usually develop on bedrock, weathering processes are a prerequisite for erosion to take place (Dickie and Parsons, 2012; Zhao et al., 2013). Both wetting–drying and freezing–thawing processes contribute to the formation of a weathering layer called regolith (Poesen and Hooke, 1997; Gallart et al., 2002). With ongoing maturation of the unconsol- idated slope material, drainage densities can become very high. The growing interest in badland dynamics is justified by the significant contribution of these dissected landscapes to runoff and sediment production (Nadal-Romero et al., 2011, 2012) and to the fact that badlands are very sensitive to climate and human-induced changes. Badlands are also laboratories for geomorphologic and hydrologic research, and they explain the initial processes and rates of soil formation (Cerdà, 1999). These highly erodible landscapes mainly appear in semi-arid environments (de Ploey, 1989). However, they are also present in humid environments, usually mountainous in character (Gallart et al., 2002; Nadal-Romero and Regüés, 2010). Nadal-Romero and Regüés (2010) provided a review of the main experimental studies investigating humid badland dynamics. Though scientific interest in badlands of humid envi- ronments has been increasing over the last two decades (e.g. Regüés et al., 1995; Guàrdia et al., 2000; Esteves et al., 2005; Mathys et al., 2005; Nadal-Romero and Regüés, 2010; Nadal- Romero et al., 2013), studies about semi-arid badlands are still more abundant (Gallart et al., 2013). This justifies an improved understanding of the intense geomorphic processes operating in humid badlands. The main focus of this study is to assess how erosion processes affect the spatial patterns of vegetation on humid badland slopes. EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2014) Copyright © 2014 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3549