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 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 (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.

See the University of Minnesota's news release at http://www1.umn.edu/news/news-releases/2013/UR_CONTENT_466215.html.