After a millenarian history of overexploitation, most forests in the Mediterranean Basin have disappeared, leaving many degraded landscapes that have been recolonized by early successional shrub‐dominated communities. Common reforestation techniques treat these shrubs as competitors against newly planted tree seedlings; thus shrubs are cleared before tree plantation. However, empirical studies and theory governing plant– plant interactions suggest that, in stress‐prone Mediterranean environments, shrubs can have a net positive effect on recruitment of other species. Between 1997 and 2001, we carried out experimental reforestations in the Sierra Nevada Protected Area (southeast Spain) with the aim of comparing the survival and growth of seedlings planted in open areas (the current reforestation technique) with seedlings planted under the canopy of preexisting shrub species. Over 18 000 seedlings of 11 woody species were planted under 16 different nurse shrubs throughout a broad geographical area. We sought to explore variation in the sign and magnitude of interactions along spatial gradients defined by altitude and aspect. In the present work, we report the results of a meta‐analysis conducted with seedling survival and growth data for the first summer following planting, the most critical period for reforestation success in Mediterranean areas. The facilitative effect was consistent in all environmental situations explored (grand mean effect size d+ = 0.89 for survival and 0.27 for growth). However, there were differences in the magnitude of the interaction, depending on the seedling species planted as well as the nurse shrub species involved. Additionally, nurse shrubs had a stronger facilitative effect on seedling survival and growth at low altitudes and sunny, drier slopes than at high altitudes or shady, wetter slopes. Facilitation in the dry years proved higher than in the one wet year. Our results show that pioneer shrubs facilitate the establishment of woody, late‐successional Mediterranean species and thus can positively affect reforestation success in many different ecological settings.

dar, J. A. 2006. Conditional outcomes in plant Á/ herbivore interactions: neighbours matter. Á/ Oikos 113: 148 Á/156.Spatial distribution of palatable and unpalatable plants can influence the foraging behaviour of herbivores, thereby changing plant-damage probabilities. Moreover, the immediate proximity to certain plants can benefit other plants that grow below them, where toxicity or spines act as a physical barrier or concealment against herbivores. This paper presents the results of a multi-scale experiment performed to test the effect of shrubs as protectors of tree saplings against herbivores and the mechanism involved in Mediterranean ecosystems. We performed a factorial design in two mountain ranges, similar in physiognomy and vegetation, planting saplings of a palatable tree, the maple (Acer opalus subsp. granatense) , and an unpalatable tree, the black pine (Pinus nigra) , under three different types of shrubs. We considered four experimental microhabitats: highly palatable shrub (Amelanchier ovalis) , palatable but spiny shrub (Crataegus monogyna or Prunus ramburii ), unpalatable spiny shrub (Berberis vulgaris subsp. australis ) and control (gaps of bare soil without shrubs). Three main factors were found to determine the probability of sapling attack: sapling palatability, experimental microhabitat and plot. Palatable saplings (maples) were browsed much more than unpalatable ones (pines). The degree of protection provided by the shrub proved greater as its palatability decreased with respect to sapling palatability, the unpalatable spiny shrub being the safest microhabitat for palatable saplings and bare soil for unpalatable ones. The differences found in number of attacked saplings between plots may be attributable to differences in herbivore pressure. The community context in which interaction takes place, namely the characteristics of the neighbours and the intensity of herbivore pressure, are determining factors for understanding and predicting the damage undergone by a target plant species. The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs. It is necessary to introduce this neighbour effect in theoretical models and food-web approaches that analyse the plant Á/herbivore relationships, since it can strongly determine not only the intensity of the interaction, but also the spatial distribution and diversity of the plant community.Herbivory has traditionally been viewed as a binary interaction, focusing on a simple pair of interacting elements (one plant vs one herbivore; reviewed by Zamora et al. 1999). Consequently, most of the classical plant Á/herbivore theories have been founded on the way in which intrinsic plant traits influence herbivore preference, i.e. plant appearance, nutritional quality or effectiveness and quantity of defences (Rhoades and Cates 1976, Bryant et al. 1983, Coley et al. 1985, Herms and Matson 1992. This species-to-species view of plant Á/herbivore interactions has been challenged by an increasing body...

Many biotic and abiotic factors affect seedling establishment in woody plants. In Mediterranean environments, the major factors affecting tree regeneration are light, water, and herbivory. We investigated the response of some morphological and chemical traits of Quercus pyrenaica Willd. seedlings to simulated herbivory (hand removal of 50% of the aerial mass) and two levels of light (sun vs. shade) and water (one vs. two waterings). Water had no appreciable direct effect on morphological or chemical traits. Shaded seedlings grew less but had greater total leaf area. Simulated herbivory decreased the total leaf area, and root and aerial mass. Among the chemical characteristics, shaded seedlings had higher levels of nitrogen and lower levels of condensed tannins. In colorimetric assays of tannins, clipped seedlings had lower absorbances than did unclipped plants, and this effect was more pronounced in the sun than in the shade. Our experiment shows that light availability and herbivory affect the development and defence of Q. pyrenaica seedlings. Although Q. pyrenaica tolerated shade and simulated herbivory, both factors decreased biomass and chemical defence, which could affect the seedlings' future performance.Key words: environmental context, light, mammal herbivory, oak seedlings, Quercus pyrenaica, secondary compounds.

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