Phylogenetic diversity of flowering plants declines across the growing season in Rocky Mountain wildflower communities

Phylogenetic diversity is an important axis of biodiversity associated with many ecosystem functions, but its variation over time in temperate communities has rarely been explored. Plants’ flowering phenology is key for reproduction: it determines synchrony among plants, mutualists, antagonists, and favorable environmental conditions. Individual species’ flowering phenologies combine to determine which species’ flowers co-occur in time and space to form co-flowering communities. We recorded plants’ flowering phenology across elevations in the Colorado Rocky Mountains and predicted: 1) flowering species will form subsets of co-flowering species, 2) groups of co-flowering species will be more distantly related than the broader community, 3) marginal early and late season conditions will constrain flowering to species groups that are less phylogenetically diverse and 4) the entire community will have higher phylogenetic diversity at higher elevations. We identified co-flowering groups using a network analysis and quantified their phylogenetic diversity. Co-flowering communities had higher phylogenetic diversity early in the season and at higher elevations, but became less diverse by all metrics at the end of the season across elevations. Our results indicate that phylogenetic diversity varies consistently across the season and that conditions in the late season and at low elevations might constrain diversity.