Positive interactions between plants are common and their effects on community
richness via environment improvement are well known. However, the effects of
indirect facilitation by a nurse through protection against grazing on its
associated plant community have attracted less attention, in particular
regarding functional traits. As the characteristics of trait distribution can
reveal selective pressure, they offer valuable information for the study of
grazing pressure. We tested to what extent indirect facilitation affects the
amount of trait combinations (H1), the trait dominance (H2) and the niche
differentiation (H3) of associated communities.

We set up a grazing exclusion experiment over two nurse cushion species: one
providing indirect facilitation through grazing to its associated community and
the other not. We measured on the associated communities three functional
traits, which are known to vary according to grazing: 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 the kurtosis and skewness of the trait
distributions (H2). The variation in niche differentiation was evaluated by
using the community-wide overlap of intraspecific trait variation (H3).

We did not find a significant effect of grazing or indirect facilitation on the
volume of the phenotypic space. However, our study revealed a significant effect
of indirect facilitation on dominance in the associated community by maintaining
the evenness of the trait distributions (H2) and on niche differentiation by
maintaining trait overlaps between species in grazed compared to ungrazed
contexts (H3). The effects of indirect facilitation were however found to depend
on the trait considered. Our results highlight that indirect facilitation
promotes evenness of trait distributions and niche differentiation between
species, suggesting that it buffers the selective pressures of grazing on plant
communities.