Local adaptation to biotic factors: reciprocal transplants of four species associated with aromatic Thymus pulegioides and T. serpyllum

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 b-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 b-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.