The influence of competition from herbaceous vegetation and shade on simulated browsing tolerance of coniferous and deciduous saplings Charlotte Vandenberghe, Franc ¸ois Frele ´choux and Alexandre Buttler C. Vandenberghe (c.vandenberghe@macaulay.ac.uk), F. Frele´choux and A. Buttler, Swiss Federal Research Inst., WSL Station 2, EPFL, CH-1015 Lausanne, Switzerland. Present address for CV: The Macaulay Land Use Research Inst., Craigiebuckler, Aberdeen, Scotland, AB15 8QH. AB also at: Laboratory of Ecological Systems ECOS, Ecole Polytechnique Fe´de´rale de Lausanne EPFL, Station 2, CH-1015 Lausanne, Switzerland. The ability of saplings to tolerate browsing (i.e. the ability to persist with reduced biomass and to compensate for biomass loss) is influenced by the level of stress and their growth strategies. Ultimately, insight into species-specific responses of saplings to browsing, shade and competition from neighbours will help explain diversity, structure and function of grazed ecosystems such as the endangered wood-pasture systems. We measured the survival, whole-sapling biomass and compensatory growth responses of two coniferous (Picea abies and Abies alba) and two deciduous (Acer pseudoplatanus and Fagus sylvatica) tree species to simulated summer browsing (one single clipping event), shade (installation of a shade cloth) and neighbour removal (mowing surrounding vegetation to ground level) treatments and the interactions between them after two-growing seasons. For all species, there were interacting effects on growth of browsing and environmental condition (shade and neighbours). Simulated browsing resulted in relatively smaller growth losses when plants were growing slowly due to competitive conditions related to herbaceous neighbours. Although none of the clipped saplings could fully compensate for their biomass losses, the saplings were closer to compensation under high competitive conditions than under low competitive conditions. Survival of the clipped saplings remained relatively high and was only significantly reduced for Picea and Acer. Picea was least tolerant of competition and was the only species for which growth was not negatively affected by strong irradiance of a mountain pasture. Surprisingly, the tolerance of saplings to herbivory as browsing tolerance was enhanced under conditions that negatively affected sapling performance (i.e. survival and growth). Apparently, the relative impact of browsing at the early sapling stage is linked to tree life history characteristics such as competition and shade tolerance and will be lower in situations with intense competitive interactions and/or strong irradiance. Tree sapling survival and growth in grazed ecosystems such as wood-pastures is often negatively influenced by summer browsing disturbance from cattle (Bakker et al. 2004, Smit et al. 2006, Vandenberghe et al. 2007). The ability of saplings to tolerate browsing depends predominantly on the capacity to persist with reduced biomass and the ability to compensate for biomass loss through active regrowth (Haukioja and Koricheva 2000). Tolerance to herbivory is primarily determined by intrinsic factors that are genetically and/or developmentally determined by the plant itself (Rosenthal and Kotanen 1994, Strauss and Agrawal 1999). Compensatory growth responses are considered to be less developed for coniferous species because fast- growing deciduous species have more flexible growth patterns and can profit from their ability to mobilize stored energy reserves from stems and roots for refoliation (Bryant et al. 1983, Ayres et al. 2004, Hester et al. 2004). However, saplings in wood-pasture systems are not exposed solely to browsing, but to several environmental factors simulta- neously (House et al. 2003). Consequently, browsing tolerance is also affected by the level of stress imposed by extrinsic factors (Rosenthal and Kotanen 1994) such as shade (e.g. from adult trees) and competition from surrounding herbaceous neighbours. It is commonly assumed that plants have a greater tolerance to herbivory when growing in low-stress, resource-rich environments (i.e. compensatory continuum hypothesis, Belsky et al. 1993, Hja ¨lten et al. 1993). However, controversy exists concerning the compensatory continuum hypothesis and (over)compensation might also occur under high stress conditions (Hawkes and Sullivan 2001, Wise and Abra- hamson 2005). In general, shade and competition from surrounding neighbours reduce sapling performance (i.e. survival and growth) (Kozlowski and Pallardy 1997, Peltzer and Ko ¨chy 2001, Page `s et al. 2003) and, as consistent with the compensatory continuum hypothesis, might reduce browsing tolerance (McLaren 1996, Weltzin et al. 1998, Blundell and Peart 2001). However, sapling performance Oikos 117: 415423, 2008 doi: 10.1111/j.2007.0030-1299.16264.x, # The authors. Journal compilation # Oikos 2007 Subject Editor: Matt Ayres, Accepted 7 November 2007 415