Soil under nurse plants is always better than outside: a survey on soil amelioration by a complete guild of nurse plants across a long environmental gradient

Soil under nurse plants is more fertile than in the harsh surroundings. This is a primary mechanism involved in plant to plant facilitation and it is critical in structuring plant communities under stressful conditions. However we do not know how this soil enrichment process varies along complex environmental gradients and among coexisting nurse plants. Soil properties related to structure, resource stocks and microbial activity, were compared among up to ten nurse plant species and adjacent barren soil areas, along a 1600 m elevation gradient above the treeline in central Chilean Andes. Shifts in Relative Interaction Index (RII) sensu Armas (Ecology 85: 2682–2686, 2004) and in coefficient of variation on soil properties were also modelled. Soil under nurse plants was always richer than on barren areas irrespective of altitude, except in the case of texture with more small particles in the intermediate altitude. β-glucosidase activity was higher under cushion plants than under nurse plants with other growth habit. Besides β-glucosidase and phosphatase activities were more variable at higher altitudes. Nitrogen was more variable under nurse plants than in barren areas and its RII values were lower at intermediate altitudes. Soil amelioration by nurse plants occurred all along the studied environmental gradient promoting islands of fertility and a general increase on soil niches heterogeneity.