Do co-occurring plant species adapt to one another? The response of Bromus erectus to the presence of different Thymus vulgaris chemotypes

Abstract

Local modification of the soil environment by individual plants may affect the performance and composition 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 modifications mediated by the local thyme chemotypes. Reproductive 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 variation may be related to environmental factors which also contribute to the spatial differentiation of thyme chemotypes. 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.

Intraspecific genetic variation and species coexistence in plant communities

“…Some monoterpenes are of a phenolic type and others of a non-phenolic type. The competitively strong perennial grass Bromus erectus shows adaptation to growing with local nonphenolic thymes, but not with its phenolic type [43]. In a large vegetation survey [22] comprising several communities dominated by thyme that varied in chemical phenotype, the presence and abundance of B. erectus was significantly lower around phenolic thyme, i.e.…”

An allelopathic plant facilitates species richness in the Mediterranean garrigue

“…The phenolic monoterpenes, in general, have a more inhibiting effect on the germination and growth of plants than non-phenolic ones (Tarayre et al 1995). However, reciprocal transplant experiments showed that associated plants can adapt to their local thyme chemotype (Ehlers & Thompson 2004;Grøndahl & Ehlers 2008;Jensen & Ehlers 2010) performing better on soil containing their local monoterpene. Monoterpenes are also a dominant component of the essential oil in many other aromatic plants common to the Mediterranean flora (Thompson 2005); therefore, the effect of these compounds on the associated community may not only be restricted to thyme.…”

“…In thyme communities, genetic variations in associated plant species for the sensitivity to thyme monoterpene have been found (Jensen & Ehlers 2010), and reciprocal transplant experiments on a number of associated species found that plants showed an adaptative response to their local thyme chemotype (Ehlers & Thompson 2004;Grøndahl & Ehlers 2008;Jensen & Ehlers 2010). Although individual plant species are able to adapt to their local thyme chemotype, and some species even performing significantly better on soil with than on soil without thyme monoterpene (Ehlers & Thompson 2004;Jensen & Ehlers 2010), it is less clear how this chemotype variation, and the potential for other species to adapt to local chemotypes, may impact upon the diversity of the entire community. We found in the present study that the abundance of B. erectus was reduced in thyme microsites where thyme produced carvacrol and that this was correlated with an increase in species richness.…”

“…In soil from six different thyme sites collected under thyme plants, and at microsites devoid of thyme at a distance of 1-a few metres away from the nearest thyme plant, the soil under thyme plants had a significantly higher amount of both organic matter and available nitrogen (Ehlers & Thompson 2004). Provided that negative allelopathic effects of monoterpenes on germination and growth are not too large, or that associated plants have adapted to overcome this, leaching of thyme monoterpenes to soil may create more favourable microsites for plants due to an elevated amount of nutrient.…”

Coevolution between invasive and native plants driven by chemical competition and soil biota

“…Additionally, several studies of chemically divergent Thymus species have suggested that populations of cooccurring plant species may adapt to the particular Thymus chemotype with which they interact (20)(21)(22). Again, this adaptation may be partially mediated by soil properties or communities (20,23).…”