Floral traits and pollination of Solidago altissima: mechanisms of local adaptation among and within biomes

Abstract

Solidago altissima is an herbaceous, clonal plant that has differentiated into two subspecies in the prairie and forest biomes in Minnesota. The mechanisms that have driven the divergent evolution of these subspecies are not well understood. The evolution of floral traits is influenced by the trade-off between sexual and asexual reproduction. Floral traits can evolve rapidly, and this often occurs in response to interactions with pollinators. I found differences in floral traits between plants from the two biomes, and these differences strongly affected pollinator abundance. Forest plants allocate proportionally more resources to flowering than to vegetative reproduction via rhizomes compared to plants in the prairie. I hypothesize that there is stronger competition among plants for resources in the prairie, where selection favors greater allocation of resources to vegetative reproduction. I also tested the hypotheses that pollinator abundance is influenced by differences among plant genotypes and the genotypes of neighboring plants. I conducted an experiment and found that the number of pollinators on a plant was influenced by the genotype of a plant, but not by the genetic diversity of neighboring plants. I also found that the abundance of neighboring flowers affected pollinator abundance. Plant genotypes varied in floral size, flowering time, and nectar quantity. Floral size of the individual stem had the strongest effect on pollinator abundance. The variation in floral traits among genotypes may be a result of selection to optimize the tradeoff between vegetative growth and flowering, which can vary spatially and temporally.