Effects of indirect facilitation on functional diversity, dominance and niche differentiation in tropical alpine communities

Positive interactions between plants have well‐known effects on community species richness and biomass via micro‐environment improvements. However, the effects of indirect facilitation performed by a nurse on an associated plant community, occurring e.g., through protection against grazing, have not attracted much attention in the literature so far, in particular regarding the functional traits of the associated plant communities. Because characteristics of trait distributions can reveal selective pressures at play, they offer valuable information for the study of grazing pressure. Here, we tested the extent to which indirect facilitation affects the amount of distinct trait combinations (H1), trait dominance (H2) and niche differentiation between species (H3) of associated plant communities living under nurse plants. We set up a grazing exclusion experiment over two dominant nurse cushion species: one providing indirect facilitation under grazing pressure and the other not. We measured three functional traits in the associated communities, which are known to vary depending on the grazing level: LDMC, leaf thickness and maximum height. We assessed the amount of distinct trait combinations by computing the volume of the phenotypic space (H1). The variation in trait dominance was quantified with kurtosis and skewness of the trait distributions (H2). The variation in niche differentiation was evaluated using mean intra‐population trait variance relative to intra‐community trait variance (H3). We did not find a significant effect of grazing and indirect facilitation on the volume of the phenotypic space (H1). However, our study revealed a significant effect of indirect facilitation on dominance in the associated communities by maintaining the evenness of the trait distributions (maximum height and leaf thickness) in grazed compared to ungrazed plots (H2), and on niche differentiation by maintaining trait overlaps (LDMC and maximum height) between species in grazed compared to ungrazed contexts (H3). Our results suggest that indirect facilitation promotes the co‐existence of contrasting functional strategies (H2) and species niches (H3) in associated plant communities subject to grazing. These results reflect the buffering effect of indirect facilitation on the grazing selective pressures exerted on associated plant communities living under the protection of nurses.