Adaptation and diversification of bluebells <i>Mertensia</i> spp., Boranginaceae

Abstract

Flowering phenology marks the transition from vegetative growth to reproduction and exposes plants to seasonally varying selective environments during flowering. Life-history trade- offs between time and size at reproduction suggest that early-flowering species should generally be smaller than late-flowering species. Furthermore, consistent seasonal variation in environmental conditions may generate seasonally varying selection on morphological traits (such as vegetative size and floral traits), potentially causing correlations between morphological traits and flowering phenology. Similarly, trait correlations may be driven by multiple traits responding in parallel to environmental gradients; for example, low air temperature and high wind velocity in high-altitude habitats might favour smaller species that also flower rapidly after snowmelt. To assess interspecific correlations between flowering phenology and other traits, I focused on 12 species of Mertensia (Boraginaceae) that have similar vegetative and floral morphology but differ greatly in size, flowering phenology, and altitudinal distribution. I developed two novel techniques to estimate flowering phenology of each species based on a single day’s survey, measured size and reproductive traits of each species, and used phylogenetic comparative methods to test for interspecific correlations among flowering phenology, size, floral traits, and altitude. Early-flowering Mertensia species were shorter in stature, produced fewer flowers, and occurred at higher altitudes than late-flowering species. However, traits of individual flowers (stigma–anther separation and floral volume) were not strongly associated with variation in flowering phenology or plant size. The evolution of flowering phenology within Mertensia has been associated with changes in plant size but not floral traits. Early-flowering species may be smaller because they have less time for growth before reproduction, and because short stature and early flowering are both adaptations to high-altitude environments. 9

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