Three keys to the radiation of angiosperms into freezing environments

This large comparative phylogenetic study across angiosperms shows that species that are herbaceous or have small conduits evolved these traits before colonizing environments with freezing conditions, whereas deciduous species changed their climate niche before becoming deciduous. Cold comfort for early angiosperms The earliest flowering plants or angiosperms were probably woody evergreen trees in warm tropical environments. If they were to colonize environments that experience freezing conditions, one of several changes was required. They needed either to become deciduous, to become herbaceous, or to reduce the size of their water conduits. Amy Zanne et al . present a large phylogeographic study of 49,000 angiosperms which shows that species that are herbaceous and/or have small conduits evolved these traits before colonizing freezing conditions, whereas deciduous species changed their climate niche before becoming deciduous. Early flowering plants are thought to have been woody species restricted to warm habitats 1 , 2 , 3 . This lineage has since radiated into almost every climate, with manifold growth forms 4 . As angiosperms spread and climate changed, they evolved mechanisms to cope with episodic freezing. To explore the evolution of traits underpinning the ability to persist in freezing conditions, we assembled a large species-level database of growth habit (woody or herbaceous; 49,064 species), as well as leaf phenology (evergreen or deciduous), diameter of hydraulic conduits (that is, xylem vessels and tracheids) and climate occupancies (exposure to freezing). To model the evolution of species’ traits and climate occupancies, we combined these data with an unparalleled dated molecular phylogeny (32,223 species) for land plants. Here we show that woody clades successfully moved into freezing-prone environments by either possessing transport networks of small safe conduits 5 and/or shutting down hydraulic function by dropping leaves during freezing. Herbaceous species largely avoided freezing periods by senescing cheaply constructed aboveground tissue. Growth habit has long been considered labile 6 , but we find that growth habit was less labile than climate occupancy. Additionally, freezing environments were largely filled by lineages that had already become herbs or, when remaining woody, already had small conduits (that is, the trait evolved before the climate occupancy). By contrast, most deciduous woody lineages had an evolutionary shift to sea