Cushion plants as soil engineers: the formation of islands of fertility differ between species in nival environments

Abstract

Aims

In extreme environments such as the nival vegetation belt, isolated cushion plants create islands of soil fertility within a mostly mineral environment. It, however, remains unknown whether different co-occurring cushion plant species engineer soils with contrasted properties, and to what extent plant functional traits mediate this process.

Methods

We sampled 117 individual cushions from seven species, together with 26 bare soil controls from 2600 to 3200m of elevation. Soil property (carbon and nitrogen content, soil C:N and available phosphorus) were compared between species, and the role of cushion plant traits was assessed using Structural Equation Modelling (SEM).

Results

The soils beneath cushion plants showed similar characteristics to those found 1000m below, where vegetation is continuously distributed. Isotopic signatures showed that the soil organic matter beneath cushions originated from the plant itself. Soil characteristics (C, N, C:N, P) differed among plant species. SEM revealed that soil characteristics were not driven by leaf C/N or phenolic content, but rather by cushion size and bedrock type.

Conclusions

Providing a unique overview of soil formation in nival ecosystems, our study provides novel insights into the mechanisms of ecosystem engineering by foundation plants and shows how interspecific variability contributes to this process.