Summary 1.A plant producing secondary compounds may affect the fitness of other plants in the vicinity, and, likewise, associated plants may evolve adaptation to the presence of their 'chemical neighbour'. Species of the genus Thymus are aromatic plants, well known for their production of aromatic oils whose constitution is dominated by mono-or sesquiterpenes. A polymorphism for the production of the dominant terpene in the oil exists both within and between thyme species. 2. Here we examine the effects of two different terpenes produced by Thymus pulegioides and T. serpyllum on the performance of four associated plant species: Achillea millefolium , Agrostis capillaris , Galium verum and Plantago lanceolata . In a reciprocal transplant experiment we studied how plants naturally occurring together with thyme producing either carvacrol or β -caryophyllene perform on soil treated with these compounds. 3. We found evidence of local adaptation to the 'home' terpene. Plants originating from sites where they grow together with carvacrol-producing thyme plants also perform better on soil treated with carvacrol. One of the associated species ( A. millefolium ) also showed evidence of local adaptation to the sesquiterpene β -caryophyllene . 4. Seed germination and root biomass showed an adaptive response to soil treatment. Vegetation analysis supported the results of the reciprocal transplant experiment. When the associated species performed best on 'home' soil, thyme and the associated species also showed a positive spatial association at natural sites of origin. Moreover, coefficients of variation in plant traits were significantly lower on 'home' soil compared to other soils for both A. capillaris and A. millefolium , but higher for G. verum . 5. Synthesis . Our results show that plant species can adapt to the presence of neighbour plants that produce specific chemical compounds. This supports the idea that local plant communities may be a lot more co-evolved than was previously thought.

Summary1. Positive plant-plant interactions are known to increase species richness in stressful and poor habitats that are often species poor, but the role of facilitative interactions in species-rich communities is less well understood. It has been proposed that allelopathic plants may create non-transitive species interactions, which increase species coexistence, and that such indirect facilitation may be important in species-rich communities. 2. We examined species richness in 12 different plant communities all dominated by the aromatic Thymus vulgaris that produces monoterpenes known to inhibit germination and growth of other plants. 3. We found consistently, and across communities, higher species richness in microsites with thyme than without. Species richness in microsites with, respectively without, two other perennial plants did not differ, suggesting that increased species richness in thyme microsites is due to the presence of thyme. We found a more similar species composition among thyme microsites and positive estimates of thyme on landscape richness enhancement, indicating that thyme also affect richness at the community level. However, across communities, we did not find species consistently confined to thyme microsites, albeit within communities some plants were exclusive to thyme. 4. Abundance of a dominant grass was reduced in microsites where thyme produces the monoterpene carvacrol, suggesting that one mechanism by which thyme facilitates species richness was by the repression of a superior competitor that could allow other species to persist. However, this does not explain higher species richness in microsites of thyme producing other monoterpenes. We discuss how chemical variation in thyme and adaptation of associated species to local monoterpenes may affect richness and community diversity. 5. Synthesis. Allelopathic plants are generally believed to negatively impact upon the performance of associated species. However, allelopathic plants may be important determinants of species richness at the community level by creating microenvironments where species-specific interactions differ. Our finding shows that thyme increases species richness both locally and at the community level by creating a mosaic of thyme-modified and unmodified microsites differing in richness and composition. We suggest that this may also apply to other aromatic plants common in Mediterranean vegetation.

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The long-term maintenance of specialized mutualisms remains an evolutionary puzzle. Recent focus has been on factors governing the stability of these mutualisms, including sanctions by the host, partner choice, and coevolutionary constraint, that is, the genetic correlation (rG) between fitness of both partners. So far these studies have been typically carried out in a single environment. Here, we ask if the genetic correlation between fitness of the host plant Medicago truncatula (Fabaceae) and its bacterial symbiont Sinorhizobium meliloti is affected by the presence/absence of a monoterpene (carvacrol) leached into the soil by Thymus vulgaris—a common plant of the Mediterranean vegetation, often co-occuring with Medicago. We show that the presence of carvacrol in the soil dramatically affects fitness of the rhizobial partner and increases the magnitude of rG between plant and rhizobia fitness (rG = 0.02 ± 0.05 vs. rG = 0.57 ± 0.02). This finding emphasizes the importance of heterogeneity in the biotic environment for understanding the evolution of species interactions.