These are deciduous maple leaves, recently senesced as winter approaches in Rock Creek Park, Washington, DC. Credit: Amy Zanne

A team of researchers studying plants has assembled the largest dated evolutionary tree, using it to show the order in which flowering plants evolved specific strategies, such as the seasonal shedding of leaves, to move into areas with cold winters. The researchers, including University of Minnesota professor Peter Reich, will publish their findings Sunday, Dec. 22 in the journal Nature.

Early flowering plants are thought to have been woody— maintaining a prominent stem above ground across years and changing weather conditions, such as maple trees—and restricted to warm, wet tropical environments. But they have since put down roots in chillier climates, dominating large swaths of the globe where freezing occurs. How they managed this expansion has long vexed researchers searching for plants' equivalent to the winter parka.

"Freezing is a challenge for plants. Their living tissues can be damaged. It's like a plant's equivalent to frostbite. Their water-conducting pipes can also be blocked by air bubbles as water freezes and thaws," said Amy Zanne, the study's lead author and an assistant professor of biology in the George Washington University's Columbian College of Arts and Sciences.

More than 25 scientists with a wide variety of expertise worked together on this study.

"We wanted to understand more about how plants came to have evolved the traits that allow them to withstand cold," Reich said.

The team of researchers identified three repeated evolutionary shifts they believe flowering plants made to fight the cold, Reich said. Plants either:

dropped their leaves seasonally, shutting down the pathways that would normally carry water between roots and leaves;

made skinnier water-conducting pathways, allowing them to keep their leaves (think of pines in winter) while reducing the risk of air bubbles developing during freezing and thawing, which would shut down those pathways (the fatter the pathways, the higher the risk); or

avoided the cold seasons altogether as herbs, losing aboveground stems and leaves and retreating as seeds or storage organs underground, such as tulips or tomatoes.

The researchers also identified the order of evolutionary events. Most often woody plants became herbs or developed skinnier water-conducting pipes before moving into freezing climates. In contrast, plants usually began dropping their leaves after moving into freezing climates.

Identifying these evolutionary adaptations and likely paths to them, required the team to build two robust sets of data. First, researchers created a database of 49,064 species, detailing whether each species maintains a stem above ground over time, whether it loses or keeps its leaves and the width of its water-carrying pathways. To these they added whether it is ever exposed to freezing, using resources from the Global Biodiversity Information Facility and a global climate database. Then, researchers took that information and combined it with an unprecedented dated evolutionary tree with 32,223 species of plants, allowing them to model the evolution of species' traits and climate surroundings. This "timetree," which can be viewed at OneZoom here, is the most comprehensive view yet into the evolutionary history of flowering plants.

"Until now, we haven't had a compelling narrative about how leaf and stem traits have evolved to tolerate cold temperatures," Zanne said. "Our research gives us this insight, showing us the whens, hows and whys behind plant species' trait evolution and movements around the globe."

To build on these findings, researchers will use the massive tree to explore other aspects of the evolutionary history of plants, especially to examine how plants respond to additional environmental pressures besides just freezing.

Researchers will use information from the findings in countless other ways as well, Reich says.

"In the near term – say in 10 to 20 years – this kind of information can help us build better models of what's going to happen with vegetation in the future as the climate changes," Reich said.

There could be other possibilities in the longer term, Reich said.

"It may be possible in 50 to 100 years that people can breed cold tolerance in a different way than we do today," Reich said. "We already do breed for cold tolerance and have for some time, but by understanding the evolutionary pathways and advantages of the different traits, it might be possible to use that information at some future date to improve the kind of vegetation we have here."

The team will make available at Dryad the data and tools developed for this study for other researchers' use. The National Evolutionary Synthesis Center, National Science Foundation (grant number EF-0905606) and Australia-based Macquarie University's Genes to Geoscience Research Centre funded this study.

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More information: Three keys to the radiation of angiosperms into freezing environments, Journal information: Nature Three keys to the radiation of angiosperms into freezing environments, DOI: 10.1038/nature12872