Researchers Investigate How Plants Evolved to Weather the Cold

Amy Zanne and colleagues have created largest evolutionary “timetree” of land plants.
Amy Zanne
Amy Zanne is an assistant professor of biology in the Columbian College of Arts and Sciences.
December 20, 2013

In just a few months, plants around campus that are now barren and frost-covered will be dotted with flowers. Ancestors of these plants were once, however, restricted to tropical environments and never would have endured a freezing, mid-Atlantic winter.

How flowering plants managed to put down roots in chillier locations across the globe is a question that has long vexed researchers searching for the leafy equivalent to the winter parka.

Amy Zanne, who is an assistant professor in George Washington University’s Department of Biological Sciences, and a team of researchers have 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 research was published in the journal Nature on Monday.

“If we look around today, everywhere we go we see angiosperms. So somehow, they’ve been able to adjust to these conditions,” Dr. Zanne said. “What we wanted to ask was how did they move into these different environments? What were the characteristics that they needed to evolve to make it in these places?”

Dr. Zanne and her team identified three repeated evolutionary shifts they believe flowering plants made to fight the cold. 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 while ensuring freezing-induced air bubbles wouldn’t develop and shut down those pathways; or
  • avoided the cold seasons altogether as herbs, losing above-ground tissue and retreating as a seeds or storage organs underground.

The researchers also identified the order of evolutionary events. Most often woody plants—trees, shrubs or lianas—became herbs or developed skinnier pathways 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, they 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 they constructed containing 32,223 species of plants, allowing them to model the evolution of species’ traits and climate surroundings. This “timetree” is the most comprehensive view yet into the evolutionary history of flowering plants.

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

“Until now, we haven’t had a compelling narrative about how leaf and stem traits have evolved to tolerate cold temperatures,” Dr. 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.”

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