Complementarity in nurse plant systems: soil drives community composition while microclimate enhances productivity and diversity

Aims Nurse plants facilitate the establishment of other species under their canopies through improvement of microclimate and soil conditions beneath their canopies. We aimed to test whether the effect of nurse soil on understorey communities was independent of the canopy effect, and to what extent soil properties (i.e., nutrients, pH, moisture, temperature, aggregates) can predict understorey biomass and diversity. Methods In a field experiment, we extracted three soil blocks (20 x 20 x 15 cm) under the canopy of eighteen Retama sphaerocarpa shrubs --grouped into small, medium, and large canopy sizes (n = 6). Soil blocks were distributed in a stratified-random design, so that each shrub received back three soil blocks (one from under each Retama canopy size). At the end of the growing season plant biomass, abundance, diversity and soil properties were recorded. Results Canopy and soil independently affected understorey community structure. Biomass and diversity increased under large Retama canopies irrespective of soil origin, whereas biomass was higher -and plant diversity lower- in soils from large Retama shrubs irrespective of canopy size. Biomass and species abundance were influenced more by soil than by canopy. There was, however, complementarity between these factors, as biomass increased in soils with high moisture and low temperature, while diversity was highly influenced by canopy size. Conclusions Soil, rather than microclimate amelioration by the nurse, was the main driver of understorey community structure, soil moisture playing a pivotal role. This is highly relevant as drought is expected to increase in drylands worldwide.