Abstract: As climate change advances and environments diverge from historical patterns, species will increasingly experience a lack of the resources to which they are adapted. We expect competition for such limiting resources to be stronger between more closely related species with similar traits. Conversely, more distantly related species with more dissimilar traits should compete less, and as a result, opportunities for facilitation among species are more likely. The relationships between facilitation, competition, and phylogeny predict that communities where facilitation is more critical to fitness should be more phylogenetically overdispersed. Co-flowering is one mechanism of facilitation that has been shown to increase pollinator visits to plants and their subsequent reproductive success, which may be especially important with climate change-driven pollinator decline. Among closely related species, sharing pollinators in time and space may lead to reduced fitness through increased resource competition (due to niche overlap) and/or reproductive interference. The benefits of co-flowering may be stronger between more distantly related species. We tested 1) whether facilitation via co-flowering increases plant fitness, 2) how the benefits of facilitation change over an elevational gradient, and 3) whether the benefits correlate with phylogenetic relatedness and functional similarity. We tracked flowering phenology and fruit and seed output over ten weeks from June to August at the Rocky Mountain Biological Laboratory (RMBL) for two summers. Each week, we marked newly opened flowers with colored yarn, and then counted fruits and seeds as they formed. We then used a phylogeny and functional trait data from the BIEN database to test the functional and phylogenetic similarity of co-flowering species. We found that the fitness benefit of co-flowering significantly increased with elevation, but there was no difference in phylogenetic distance between groups of co-flowering and non co-flowering species. Overall, we show that co-flowering is an important mechanism of facilitation that may be most influential at higher elevations, and that these interactions are not strongly shaped by relatedness among species. Our work highlights that the impact of interactions may be context dependent, but that facilitation has a significant impact on plant fitness in high elevation communities. This finding is particularly important as climate change is driving significant changes in plant phenology and pollinator populations in this plant community and worldwide.
