Oecologia (2004) 141: 511–518 DOI 10.1007/s00442-004-1663-7 COMMUNITY ECOLOGY Bodil K. Ehlers . John Thompson Do co-occurring plant species adapt to one another? The response of Bromus erectus to the presence of different Thymus vulgaris chemotypes Received: 21 October 2003 / Accepted: 22 June 2004 / Published online: 11 August 2004 # Springer-Verlag 2004 Abstract Local modification of the soil environment by individual plants may affect the performance and compo- sition of associated plant species. The aromatic plant Thymus vulgaris has the potential to modify the soil through leaching of water-soluble compounds from leaves and litter decomposition. In southern France, six different thyme chemotypes can be distinguished based on the dominant monoterpene in the essential oil, which is either phenolic or non-phenolic in structure. We examine how soils from within and away from thyme patches in sites dominated by either phenolic or non-phenolic chemotypes affect germination, growth and reproduction of the associated grass species Bromus erectus. To do so, we collected seeds of B. erectus from three phenolic and three non-phenolic sites. Seeds and seedlings were grown on soils from these sites in a reciprocal transplant type experiment in the glasshouse. Brome of non-phenolic origin performed significantly better on its home soil than on soil from a different non-phenolic or a phenolic site. This response to local chemotypes was only observed on soil collected directly underneath thyme plants and not on soil in the same site (<5 m away) but where no thyme plants were present. This is preliminary evidence that brome plants show an adaptive response to soil modifica- tions mediated by the local thyme chemotypes. Repro- ductive effort was consistently higher in brome of phenolic origin than in brome of non-phenolic origin (on both thyme- and grass-soil), indicating that life-history varia- tion may be related to environmental factors which also contribute to the spatial differentiation of thyme chemo- types. Moreover, we found that brome growing on thyme- soil in general was heavier than when growing on grass- soil, regardless of the origin of the brome plants. This is concordant with thyme-soil containing higher amounts of organic matter and nitrogen than grass-soil. Our results indicate that patterns of genetic differentiation and local adaptation may modify competitive interactions and possible facilitation effects in natural communities. Keywords Chemical polymorphism . Community structure . Facilitation . Local adaptation Introduction Individual plant species may modify soil texture, nutrient level, water availability and chemistry and thereby affect the composition of local communities of associated species (Zinke 1962; Charley and West 1975; Boettcher and Kalisz 1990). Indeed, the leaching of volatile water soluble compounds from aerial parts into the soil can markedly affect seed germination and seedling establish- ment of associated species, and thereby modify competi- tive interactions within and between species and poten- tially impact on the structure of natural plant communities (McPherson and Muller 1969; Friedman and Orshan 1975; Heisey and Delwiche 1985; Callaway 1995). The importance attached to such “allelopathic” interactions and the role they play in community dynamics has however a checkered history (Rice 1979; Williamson 1990) due to the problem of decoupling confounding factors. A range of both negative inhibitory and positive effects can occur due to plants modifying the soil chemical environment in ways which may stimulate, retard or inhibit germination and establishment success (Preston and Baldwin 1999). An interesting component of such positive and negative effects concerns how plant species respond to spatial heterogeneity of the soil environment associated with spatial variation in the presence of other species. In the case of two-species interactions, such interactions may also depend on whether particular species B. K. Ehlers (*) Department of Ecology and Genetics, University of Aarhus, Ny Munkegade, build. 540, 8000 Århus C, Denmark e-mail: bodil.ehlers@biology.au.dk Fax: +45-86-127191 J. Thompson Centre d‘Ecologie Fonctionnelle et Evolutive, CNRS, 1919 Route de Mende, 34293 Montpellier, cedex 5, France