Direct and indirect interactions co-determine species composition in nurse plant systems

Facilitation by nurse plants plays an important role in determining community composition in severe environments. Although the unidirectional effect of nurses on beneficiary species has received considerable research interest, nurse-mediated interactions among beneficiary species (so-called indirect interactions) are less known. Consequently, community composition in nurse plant systems is generally considered as a simple consequence of the facilitative effect of the nurse even though beneficiary species may significantly contribute to community assembly and modulate the direct nurse effects on the community. In an observational study we assessed nurse effects and nurse-mediated beneficiary interactions in two contrasting nurse plant systems in dry environments using a newly developed framework. We quantified plant-plant interaction intensity using the relative interaction index (RII) at the community and species level for three Retama sphaerocarpa shrub size-classes in a semiarid shrubland and four Arenaria tetraquetra cushion plant communities differing in aspect and elevation in dry alpine gravel habitats. The observed RII was split into nurse and beneficiary effects, and related to individual mass, species frequency and abundance using generalized linear mixed models. Results showed predominantly positive nurse effects and negative beneficiary interactions. The effect size of nurse plants, however, was significantly higher than the effect size of beneficiary species in both systems. Individual plant mass and abundance of species was dependent on the combined effects of nurse and beneficiary species whereas species occurrence was related to nurse effects only. Despite evident differences, the semiarid and alpine nurse plant systems showed strong functional parallelisms. We found interdependence between the effects of nurse and beneficiary species on beneficiary plant assemblages emphasizing their combined role on community assembly in both systems. Our results highlight the need to consider indirect interactions to understand fully plant community dynamics.