Gender and abiotic stress affect community-scale intensity of facilitation and its costs

1. Facilitation allows some plant species to occupy environments that they are otherwise unable to inhabit, potentially leading to greater fitness, greater productivity or abundance, and range expansions. However, we know little about the costs incurred by facilitators or how variation in resource allocation by facilitators influences effects and costs. 2. Gynodioecy provides an opportunity to explore the effects of facilitation and potential costs because females and hermaphrodites differ in resource allocation patterns. We explored whether environmental stress and gender-specific facilitation influence species interactions in an alpine plant community. We investigated the degree of facilitation and correlative associated costs for the gynodioecious alpine plant, Silene acaulis, at two elevations (2317 and 2560 m) in the northern Rocky Mountains of Montana, USA. 3. Hermaphroditic Silene individuals supported a greater number of plant species and individual plants than female Silene (hermaphrodites = 4.2 +/- 0.3, females = 3.5 +/- 0.2; hermaphrodites = 11.5 +/- 1.0, females = 9.1 +/- 1.1). Facilitative effects on species richness significantly increased with elevation (2.1 +/- 1.6 species at the low site vs. 3.2 +/- 1.8 species at the high site), but abundance and percentage cover did not. 4. Silene fitness at the high elevation site (2560 m) was reduced compared to the lower site (2317 m) as measured by flower and seed production. Female flower production decreased by 40%, the number of seeds per fruit by 11.6% and leaf size by 24%. 5. Increases in the percentage cover of beneficiaries reduced the number of flowers per Silene plant, indicating a cost of facilitating other species, and this cost was slightly greater for females. However, seed size for hermaphrodites and leaf length for females increased with the percentage cover of beneficiaries, suggesting a mutual benefit of harbouring other species. 6. Synthesis. Our results show that gender can affect the balance between competitive and facilitative interactions. Stronger facilitation by hermaphroditic Silene acaulis, coupled with a lower cost of harbouring beneficiaries, suggests that life-history traits and related patterns of resource allocation can influence the facilitative effects of a species.