How plants evolved to weather the cold

23 December 2013

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 results have been published in the journal Nature.

Early flowering plants are thought to have been woody—which maintain 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.

“So over time, if plants moved into colder climates, they’ve had to figure out how to get around these problems.”

Dr. Zanne and a team of researchers 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 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 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, Dr. Zanne and colleagues 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, 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,” 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.”

To build on these findings, Dr. Zanne and others 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.

“Understanding the processes that shaped biodiversity millions of years ago is hard because we can’t run experiments or even directly observe what plants and communities looked like then,” said Macquarie University researcher, Richard Fitzjohn.

“The new evolutionary tree and statistical approaches we developed allow us some insight into this deep past.”

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.

Filed under: Featured Science & nature

Deciduous sugar maple leaves, soon to senesce as winter approaches at the Gateway Arch, St. Louis, MO, USA. Photo by: Amy Zanne

Deciduous sugar maple leaves, soon to senesce as winter approaches at the Gateway Arch, St. Louis, MO, USA. Photo by: Amy Zanne

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