What nurse shrubs can do for barren soils: rapid productivity shifts associated with a 40 years ontogenetic gradient

We postulated that soil productivity shifts associated with the development of facilitation-driven plant patches in abiotically stressful ecosystems can be faster than currently assumed. This might be due to the effect of the nurse plant by promoting resource accumulation both directly and indirectly through facilitating a plant community underneath its canopy. We analysed a 40 year-ontogenetic gradient of Ononis tridentata, a colonizer shrub of barren gypsum soils in drylands. Soil fertility (chemical variables) and microbial productivity (microbial biomass, respiration and enzymatic activities) were measured along the gradient. The contribution of the nurse plant and the facilitated community to the soil fertility and microbial productivity shifts were separated and quantified by regression commonality analysis. Soil chemical fertility and microbial productivity rapidly increased during the first 20 years, with total organic carbon and microbial parameters rising six-fold along the Ononis ontogenetic gradient. This fast development of soil fertility was mostly explained by the unique effect of Ononis age (37.5 %) with an important contribution of the abundance of facilitated species (15.6 %). Facilitative interactions counterbalance the negative effect of harsh abiotic stress on the time rate and intensity at which plant and soil development occur in drylands.