Early flowering plants are thought to have been woody species restricted to warm habitats1, 2, 3. This lineage has since radiated into almost every climate, with manifold growth forms4. 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 conduits5 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 labile6, 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 seasonally shedding their leaves only after exposure to freezing (that is, the climate occupancy evolved before the trait). For angiosperms to inhabit novel cold environments they had to gain new structural and functional trait solutions; our results suggest that many of these solutions were probably acquired before their foray into the cold.
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Extended data figures and tables
Extended Data Figures
- Extended Data Figure 1: Examples of the definition of ‘woody’. (1,581 KB)
a–d, We defined ‘woody’ as having a prominent aboveground stem that is persistent over time and with changing environmental conditions. a, Liriodendron tulipifera (Magnoliaceae), Joyce Kilmer Memorial Forest, Robbinsville, North Carolina, USA. b, Carnegiea giganteana (Cactaceae), Biosphere II, Tucson, Arizona, USA, c, Rhopalostylis sapida (Arecaceae) and Cyathea sp. (Cyatheaceae), Punakaiki, South Island, New Zealand. d, Pandanus sp. (Pandanaceae), Moreton Bay Research Station, North Stradbroke Island, Queensland, Australia (photographs by A.E.Z.).
- Extended Data Figure 2: Reference timetree used for congruification analyses. (455 KB)
Results of the divergence time estimation of 639 taxa of seed plants from the reanalysis of a previously described10 phylogeny. Fossil calibrations are indicated at the nodes with green circles, and numbers correspond to fossils described in Supplementary Table 2. Concentric dashed circles represent 100-Myr intervals as indicated by the scale bar.
Extended Data Tables
- Supplementary Information (2.8 MB)
This file contains Supplementary Methods, Supplementary References and Supplementary Tables 1-4.