Comparative floral ecology of bicolor and concolor morphs of Viola pedata (Violaceae) following controlled burns

Abstract

We compared pollination and seed set of bicolor and concolor morphs in self-incompatible, Viola pedata over two seasons in two populations of unequal sizes.  One population grew on a wooded slope (CR) and the second on an exposed glade (SNR).  Both were burned in 2014. The number of flowers produced by concolor plants at SNR was higher in 2014 while the number of flowering bicolors increased at CR in 2015. Petal temperatures, regardless of site, showed that dark purple, posterior petals of bicolors were consistently warmer than their own mauve-lilac, anterior (lip) petals and the all mauve petals of concolors. Major pollen vectors were female bees (Andrenidae, Apidae and Halictidae) but polylectic, Andrena carlinii dominated both sites.  Bees foraged on flowers upside down or right side up but neither mode correlated with either morph. Bees foraged preferentially on concolor at both sites.  Pistils containing pollen tubes were higher in concolor pistils at both sites with a marginally greater number of  tubes penetrating concolor ovules regardless of site or year.  While both populations produced more seeds in 2014 SNR plants always produced more seeds than CR plants.  The increasing numbers of bicolor plants at CR in 2015 suggested that bicolors may equal or outnumber concolors when dark petals offer additional warmth to ectothermic pollinators in a shady (cooler) forest vs. an open, sunny glade. Subtle environmental factors may give a floral trait a selective advantage influencing fitness in an unbalanced polymorphism persisting in localized populations.