Coping with changing plant–plant interactions in restoration ecology: Effect of species, site, and individual variation

[Question]: Nurse–beneficiary plant interactions are often used to restore degraded habitats. However, whether and how shifts in plant–plant interactions along the facilitation–competition continuum alter revegetation success has been seldom considered. To test whether and how shifts in plant–plant interactions (due to woody species identity, study site, early life stage, and individual nurse) might alter plant recruitment and thus the success of revegetation projects, we chose a system comprising the Mediterranean dwarf palm (Chamaerops humilis) and seven common woody plant species. [Location]: Two human-degraded sites within Doñana National Park (southwestern Spain). [Methods]: We carried out several well-replicated field experiments to compare plant performance (seed survival, seedling emergence, seedling survival, seedling recruitment) in the presence and absence of Chamaerops humilis. [Results]: Chamaerops humilis had marked effects on the performance of woody species that, however, changed among life stages. Depending on woody species identity, seed survival was up to 193 times greater in adjacent open spaces than beneath Chamaerops humilis. Conversely, seedling survival and recruitment were up to 19 times greater beneath Chamaerops humilis than in open spaces. Importantly, none of the studied woody species showed greater accumulated recruitment in open spaces than beneath Chamaerops humilis. Interestingly, we found strong inter-individual palm variation in the sign and strength of their effect on woody plant performance. [Conclusions]: We found strong seed–seedling conflicts the strength of which was species-specific. The strong inter-individual palm variation depicts a facilitation–competition continuum with important implications for restoration. We propose several management recommendations across different hierarchical levels (i.e., from individuals to communities) that may increase plant recruitment and therefore the success of revegetation projects. Our results are particularly relevant for restoring arid, semi-arid and alpine landscapes worldwide where the nurse–beneficiary plant interactions are critical to ameliorating stressful conditions.This work was supported by the Portuguese Foundation for Science and Technology to PJG (SFRH/BD/130527/2017) and ARC (SFRH/BPD/115781/2016); and by a grant of the Spanish Ministry of Education and Science to JMF (PGC2018-094808-B- I00)