Contrasting soil- and canopy-nurse effects on dependent species 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 the 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 relatively larger and more exploitative species
with high leaf metal concentration, had negative long-term effects likely
due to elemental allelopathy, whereas Minuartia verna, the relatively
smaller and more conservative species with a 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.
Our study highlights the possible role of the nurse’s functional
strategies in driving the relative importance of long and short-term
effects on target species in metalliferous systems. This knowledge is key
to increasing our understanding of plant-plant interaction outcomes in
metalliferous systems and our ability to restore polluted environments.