Contrasting soil- and canopy-nurse effects in metalliferous systems may be explained by dominant plant functional strategies.
Plant-plant interaction studies in metalliferous systems have focused either on the role of facilitation or on negative effects of elemental allelopathy. However, no studies have investigated both of these effects in the same system, and their relationships with the functional strategy of the nurse species, although this is crucial for the ecological restoration of polluted sites. We assessed the effects of two dominant nurse species, with apparent contrasting strategies, on two target species on a slag heap in the Pyrenees (France). We quantified both long-term soil-engineering and short-term canopy effects on the growth of two target species. We also measured morphological traits and leaf metal concentration of the two nurse species and their effects on air moisture, temperature and vapour pressure deficit. Nurse functional strategies, as inferred from morphological traits and leaf metal concentration seem to drive their long-term soil engineering effects on target species: Gypsophila repens, the relative larger and more exploitative species with high leaf metal concentration, had negative long-term effects likely due to elemental allelopathy, whereas Minuartia verna, the relative smaller and more conservative species with lower amount of metals in leaves, had neutral long-term effects. Although Gypsophila repens had a slightly stronger positive effect on microclimate than Minuartia verna, this did not turn into higher short-term effects for the target species. Synthesis and applications: Our study highlights the diverse long and short-term nurse effects on companion species in metalliferous systems. Different nurse species can improve the performance of companion species by mitigation of climatic stress, and ultimately help speed-up the restoration of polluted environments. However, species status regarding leaf metal accumulation has to be verified. For species having a higher metal concentration in their leaves, positive microclimatic effects may be offset by local increase of soil contamination on the long-term.