Contrasting effects of two phenotypes of an alpine cushion plant on understory species drive community assembly

In alpine systems, cushion plants act as foundation species by ameliorating local environmental conditions. Empirical studies indicate that contrasting phenotypes of alpine cushion species have different effects on understory plant spe-cies, either facilitative or competitive. Furthermore, dependent species within each community type might also exhibit different responses to each cushion phenotype, which can be clustered into several response groups. Additionally, these species-groups specific responses to alpine cushion species phenotypes could alter community assembly. How-ever, very few studies have assessed responses of dependent communities at species-group levels, in particular for both above-and below-ground communities. Here, we selected a loose and a tight phenotype of the alpine cushion spe-cies Thylacospermum caespitosum in two sites of northwest China, and use the relative intensity of interactions index to quantify cushion plant effects on subordinate communities of plants and soil fungi and bacteria. We assessed variations in responses of both above-and below-ground organisms to cushion plant effects at species-group level. Species-group level analyses showed that the effects of the phenotype varied among groups of each of the three community types, and different species-groups were composed by unique taxa. Additionally, we found that loose cushions enhanced stochas-tic processes in community assembly, for plants and soil fungi but not for soil bacteria. These variations of phenotypic effects on different species-group induced contrasting taxonomic composition between groups, and alter community assembly thereby. Our study highlights the occurrence of contrasting effects of two phenotypes of a foundation cushion plant on understory plants, soil fungi and bacteria community composition, but not necessarily on their richness. We also showed that assessing responses of understory species at the species-group level allows a more realistic and mech-anistic understanding of biotic interactions both for above-and below-ground communities.

Automated summary

In alpine systems, cushion plants play a crucial role in modifying local environmental conditions. Different phenotypes of alpine cushion species have varying effects on understory plant species, either promoting or inhibiting their growth. Moreover, dependent species within each community type exhibit divergent responses to different cushion phenotypes, which can be categorized into specific response groups. However, only a limited number of studies have investigated the responses of dependent communities at the species-group level, particularly for both above-ground and below-ground communities.