Higher facilitation for stress‐intolerant ecotypes along a metal pollution gradient are due to a decrease in performance in absence of neighbours

Abstract

The study of variation in plant-plant interactions along metal-pollution gradient is in its infancy, although this is worth to be assessed for both restoration and theoretical perspectives. Additionally, the mechanisms of facilitation at stake in these particular stressed conditions are poorly known. We aim at understanding the importance of species and ecotypes stress-tolerance in competitive and facilitative responses to neighbours along metal-pollution gradients. We addressed this goal in a field experiment conducted in a former mining area located in a subalpine grassland of the French pyrenees. Two ecotypes of Armeria muelleri (a highly tolerant species to metal stress) and Agrostis capillaris (a less stress-tolerant species) were harvested at the highest and lowest levels of pollution within their distribution range and transplanted with and without neighbours along a strong metal-pollution gradient. The relative interaction index (RII) was used to assess both the effect of neighbours at different stress levels and the effect of metallic stress with and without neighbours. With increasing pollution, plant-plant interactions shifted from negative to positive for Agrostis, but did not vary significantly for Armeria. At high pollution level, the unpolluted Agrostis ecotype was more facilitated than the polluted one, and in benign habitats the polluted Armeria ecotype was more sensitive to competition than the less polluted ecotype. Interestingly, the increase in facilitation with increasing metal stress for the stress-intolerant Agrostis ecotype was primarily due to a decrease in performance without neighbours, whereas the increase in competition with decreasing metal stress for the stress-tolerant Agrostis ecotype was primarily due to an increase in performance with neighbours. More generally, these results suggest that the high size-plasticity of competitive (and stress-intolerant) species or ecotypes may explain both their competitive effects in benign habitats through neighbour-trait effects and facilitative responses in stressed habitats in the context of environmental-severity effects.