Tunicate bulb size variation in monocots explained by temperature and phenology

Abstract

Plant bulbs are modified shoot systems comprised of short internodes with apical bud(s) surrounded by layers of leaf bases. Bulb diameters can vary greatly, with overall bulb size playing a role in flower formation and resource allocation. Despite the importance of bulb size to the overall fitness of an individual, evolutionary and ecological aspects of this trait have been almost completely neglected. Examining over 2,500 herbarium vouchers for 115 selected species, we analyzed monocot tunicate bulb size within a phylogenetic context in order to investigate its evolutionary significance. We recorded two bulb diameter optima and observed that as bulb size increases taxa inhabit warmer areas with less temperature seasonality. Furthermore, we found that hysteranthous taxa, a habit where leaves emerge separately from flowers, exhibit overall larger bulbs potentially due to reliance upon belowground stored resources to flower rather than on current environmental inputs. This work highlights the importance of including the belowground portion of plants into ecological and evolutionary studies in order to gain a more complete understanding of the evolution of plant forms and functions.