Scientists have created the largest-ever evolutionary tree of more than 32,000 species of flowering plants, which sheds light on how plants evolved to withstand wintry weather.
By combining their tree with freezing exposure records and leaf and stem data for thousands of species, researchers were able to reconstruct how plants evolved to cope with cold as they spread across the globe.
The results suggest that many plants acquired characteristics that helped them thrive in colder climates - such as dying back to the roots in winter - long before they first encountered freezing.
Fossil evidence and reconstructions of past climatic conditions suggest that early flowering plants lived in warm tropical environments, said co-author Jeremy Beaulieu at the National Institute for Mathematical & Biological Synthesis (NIMBioS) at the University of Tennessee.
As plants spread to higher latitudes and elevations, they evolved in ways that helped them deal with cold conditions.
Plants that live in the tundra, such as Arctic cinquefoil and three-toothed saxifrage, can withstand winter temperatures below minus 15 degrees Celsius.
Unlike animals, most plants can't move to escape the cold or generate heat to keep them warm. It's not so much the cold but the ice that poses problems for plants. For instance, freezing and thawing cause air bubbles to form in the plant's internal water transport system.
The researchers identified three traits that help plants get around these problems.
Some plants, such as hickories and oaks, avoid freezing damage by dropping their leaves before the winter chill sets in - effectively shutting off the flow of water between roots and leaves - and growing new leaves and water transport cells when warmer weather returns.
To compile the plant trait data for their study, the researchers spent hundreds of hours scouring and merging multiple large plant databases containing tens of thousands of species.
When they mapped their collected leaf and stem data onto their evolutionary tree for flowering plants, they found that many plants were well equipped for icy climates even before cold conditions hit.
Plants that die back to the ground in winter, for example, acquired the ability to die and come back when conditions improve long before they first experienced freezing.
Similarly, species with narrow water transport cells acquired a finer circulatory system well before they confronted cold climates.
"This suggests that some other environmental pressure - possibly drought - caused these plants to evolve this way, and it happened to work really well for freezing tolerance too," said Zanne.
The study was published in the journal Nature.