Phylogenetic diversity of flowering plants declines throughout the growing season across a subalpine elevational gradient

Abstract

Co-flowering is a potential facilitative mechanism for plants to overcome pollinator declines and phenological mismatches by attracting more pollinators to a multi-species patch. Due to associated competition, facilitation is expected to be more beneficial among more distantly related plants that compete less, and more prevalent in stressful conditions where benefits outweigh potential costs. We test a novel set of predictions that follow from these hypotheses, including that modules of co-flowering species in montane plant communities will be associated with i) increased fitness, ii) phylogenetic overdispersion, and ii) higher elevations. We quantified flowering phenology and fitness of plants across an elevational gradient in the Colorado Rocky Mountains, identified co-flowering modules using a network approach, and analyzed the phylogenetic distribution of co-flowering modules. We found that fitness correlated positively with co-flowering across elevations, but the correlation was strongest at high-elevation sites. Phylogenetic dispersion increased with elevation but declined throughout the growing season. Late-season modules were more underdispersed at all elevations than early-season modules. By combining analyses of phenological timing, network structure, environmental stress, fitness, and phylogenetic patterns, our results reveal that plant communities may lose phylogenetic diversity as conditions become hotter and drier and climate change alters flowering phenology and pollination networks.