Additive effects of nurse and facilitated plants on ecosystem functions

Abstract

Abstract Nurse plants drive the assembly of facilitated communities and commonly promote plant–soil feedbacks, and are thus recognized as key engineers in abiotically stressful ecosystems. The literature neglects; however, the role of the communities which benefit from the presence of the nurse as contributors to soil ecosystem functions. We hypothesized that the nurse and its beneficiaries synergistically enhance essential ecosystem functions mediated by soil microbiota. To track how plant–plant facilitation impacts plant–soil feedbacks, we selected three nurse species in semi‐arid mine tailings and defined three microsites (open space, nurse canopy and nurse + facilitated canopy). In each microsite, we quantified 18 abiotic and biotic variables associated with four functions: reduction in climatic stress, reduction in edaphic stress, soil fertility and soil microbial productivity (decomposition and nutrient cycling). Litter biomass increased from open spaces to the microsite beneath the nurses, and further beneath the nurses and their beneficiaries. Litter biomass was a good predictor of both the reduction in climatic stress and increase in edaphic stress (likely owing to metal bioaccumulation). We attributed increments in soil organics and heterotrophic respiration beneath the nurses and their beneficiaries, compared to nurses alone, to biomass effects through increased litter deposition. Variation in fertility and microbial productivity among microsites shaped by the nurses and their facilitated communities was attributed to both diversity and biomass effects. In particular, fertility was promoted beneath phenotypically diverse facilitated communities, as inferred from ten above‐ and below‐ground traits. However, microbial productivity increased at low levels of root biomass likely due to reduced plant–microbe competition for nutrients. Synthesis. Our results show that facilitated plant communities sheltered by nurse species relieve local abiotic stress and promote plant–microbe interactions, both through biomass and biodiversity effects. These observations shift the conception of facilitated species from simple beneficiaries of the nurse's effects to co‐drivers of essential ecosystem functions.