The role of ecotypic variation for plant facilitation in a metal-polluted system: Stress-intolerant target ecotypes are the best beneficiaries and stress-tolerant nurse ecotypes the best benefactors

Abstract

Disentangling competitive-response and -effect abilities has strongly improved our understanding of the role of competition for the diversity and composition of plant communities. Much less is known about the relative importance of facilitative-effect and -response abilities in harsh ecosystems. Here, we aim to fill this gap by simultaneously assessing the facilitative-response and -effect abilities of different species and ecotypes in former mining sites in the French Pyrenees, both in naturally occurring communities and in a common-garden designed on a slag heap. The response of two ecotypes of the target species Festuca rubra with contrasting metal-stress tolerances and the facilitative effects of two ecotypes with contrasting metal-stress tolerances of four different metallicolous nurse species were assessed. The results revealed that the response of the Festuca ecotype with lower metal-stress tolerance shifted from competitive (RII = -0.24) to facilitative (RII = 0.29) as pollution increased, consistently with the stress-gradient-hypothesis. The Festuca ecotype with high metal-stress tolerance did not show any facilitative response. Regarding facilitative effect ability assessed in the common-garden, nurse ecotypes from highly polluted habitats (RII = 0.04) had a significantly higher facilitative effects than ecotypes from less polluted habitats (RII = -0.05). Metal-intolerant target ecotypes of Festuca rubra were the most sensitive to the positive effects of neighbours, while metal-tolerant nurse ecotypes were the best benefactors. Facilitative-response ability appeared to be driven by a trade-off between stress-tolerance and facilitative response of target ecotypes. In contrast, facilitative-effect ability was positively correlated to the stress-tolerance of nurse plants. The results of this study show that the highest restoration success of highly metal-stressed systems should be found when highly stress-tolerant nurse ecotypes are associated with less stress-tolerant target ecotypes.

Keywords: Ecological restoration; Ecotypes; Facilitative effects; Facilitative responses; Heavy metals; Stress-tolerance.