Abstract

1. Several plant species have the potential to facilitate the presence of other plant species in the community, and yet most of our knowledge on this phenomenon comes from studies considering only one facilitator species. What happens when there are two facilitator species in the community? Are the facilitation effects by one species being altered by the presence of a potentially second facilitator species and vice-versa (i.e. bidirectional facilitation)? 2. Knowing that cushion plants ameliorate stress and improve soil fertility and that cluster root-bearing species mobilize soil phosphorus (P), we investigated the potential occurrence of bidirectional facilitation via stress amelioration and nutrient exchange in the cushion plant Acaena integerrima (Rosaceae) and the cluster root-bearing tree species Embothrium coccineum (Proteaceae). 3. In southern Chile, where both species naturally occur, we compared aspects of the vegetative and reproductive performance of each species growing in association or not. We also determined if Embothrium alters the nurse effects of Acaena cushions on other species by evaluating species richness, frequency, and abundance in cushion plants with and without Embothrium. To infer the existence of nutrient exchange, we determined soil and leaf nutrient levels of both species. 4. Species richness, frequency and abundance were higher within than outside Acaena cushions. Although the presence of Embothrium did not enhance the overall positive effects of Acaena on species composition, nine species occurred more frequently when Embothrium was present. Embothrium occurred more frequently and had a higher survival and leaf nitrogen (N) concentration when growing with Acaena than without it, although its growth and leaf P concentration were lower. In contrast, Acaena had a lower number of and mass of fruits per inflorescence and a higher leaf P concentration when growing with versus growing without Embothrium. 5. Synthesis. The cushion plant Acaena integerrima facilitates the occurrence of many species in the community, including the cluster root-bearing species Embothrium coccineum, which in turn enhances the facilitation effect of Acaena on certain species. When Embothrium and Acaena occur together, the latter improves its nutritional status but reduces its fitness, implying a lack of bidirectional facilitation.