How plants weather the cold
A team of researchers has used data on nearly 30,000 species, shared by hundreds of institutions through GBIF, to cast new light on how plants colonized colder regions.
The study published in Nature journal assembled the largest evolutionary “timetree” to show the order in which flowering plants evolved strategies such as the shedding of leaves to move into areas with cold winters.
The international research team used more than 47 million occurrence records accessed via GBIF to determine the distributions of over 27,000 plant species. From these records, they were able to extract minimum temperatures from the Worldclim climate database, to flag which species are exposed to freezing across their ranges.
“Without GBIF the study absolutely would not have been possible as we had no other way to access distributions for so many species. These locations allowed us then to determine whether species were exposed to freezing,” said Amy Zanne, the study’s lead author and an assistant professor of biology at George Washington University’s Columbian College of Arts and Sciences.
The executive secretary of GBIF, Donald Hobern, added: “This is an outstanding example of the return on investment by governments and hundreds of data-holding institutions worldwide, enabling groundbreaking research that is only possible with a common data publishing infrastructure, and a culture of opening up biodiversity data for science and society.”
Early flowering plants are thought to have been woody, maintaining a prominent stem above ground across years and changing weather conditions, 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.
Amy Zanne noted: “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. So over time, if plants moved into colder climates, they’ve had to figure out how to get around these problems.”
The research 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 narrower water-conducting pathways, allowing them to keep their leaves while reducing the risk of air bubbles developing during freezing and thawing; or
- avoided the cold seasons altogether as herbs, losing above-ground 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 narrower 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, Amy 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 each species is ever exposed to freezing, using the occurrence data from GBIF 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” 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 continued. “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, 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 other environmental pressures.
Among the co-authors of the paper was Nathan Swenson of Michigan State University, who was the 2012 winner of the GBIF Ebbe Nielsen Prize.
The team will make available all the data and tools developed for this study for other researchers’ use, through the Dryad digital repository.
The study was funded by The National Evolutionary Synthesis Center, National Science Foundation and Macquarie University’s Genes to Geoscience Research Centre.
For more information:
(For Amy Zanne)
George Washington University
Photo: Sunrise through a deciduous oak-hickory forest canopy at the Tyson Research Center at Washington University in St. Louis, Eureka, MO, USA. By Jonathan Myers.
December 23rd, 2013
December 23rd, 2013