An allelopathic plant facilitates species richness in the Mediterranean garrigue

Abstract

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.

Competitor relatedness, indirect soil effects and plant coexistence

“…This local dominance possibly arose via a combination of positive nurse effect and intraspecific facilitation among A. sterilis plants (Schöb, Armas & Pugnaire ). Thyme soil is richer in nitrogen and organic matter than no‐thyme soil (Ehlers & Thompson ) and harbours a more species‐rich plant community (Ehlers, Charpentier & Grøndahl ). Several results suggest that competition is more severe on thyme soil.…”

“…; Pakeman et al . ; Cavieres & Badano ; Ehlers, Charpentier & Grøndahl ). The engineer species may indirectly promote coexistence of each competitor by locally modifying the environment and generating a mosaic of microhabitats.…”

Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi‐arid ecosystem

“…In particular, we expected that in drylands, where competition for space is not a dominant process, the allelopathic species would have a negative effect on diversity, in contrast to results found in mesic Mediterranean areas (Ehlers et al. ). Also, we hypothesized that if allelopathic compounds have determinant effects on germination and survival of other species present in community, only species adapted to allelochemicals will be able to coexist with the allelopathic species.…”

“…Ehlers et al. () found that the allelopathic Mediterranean species, Thymus vulgaris L., acts as diversity accumulator because it suppresses a superior competitor. In other words, allelopathy had positive net effects on diversity because of indirect facilitation (Brooker et al.…”

Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

“…Thus, I conclude that direct facilitation of CAM plants by trees occurs simultaneously with the indirect effect in situations of a low-to-moderate root overlap between trees and CAM plants. (Knipe and Herbel, 1966;Callaway, 2007;Ehlers et al, 2014). In contrast, other studies indicate that the indirect positive effect can be outweighed by the direct negative effect (Pagès et al, 2003 andPagès andMichalet, 2003), thus precluding the occurrence of indirect facilitation (Levin, 1999;Brooker et al, 2008).…”