Aching cold: How do plants survive it?
Pick your cliché: the cold across the Northeast, Midwest and even the South is bone-chilling. It’s brutal. It’s life-threatening. It’s Arctic — even Antarctic!
This winter is hard enough for people and animals to survive — but what about plants? They don’t burrow or crank up the thermostat. They don’t fly to Mexico or huddle like penguins. They don’t build themselves houses – or hot tubs.
Perennial plants — the kind that don’t last out the winter as a seed — just stand there and take it – half-buried in frozen soil, and half exposed to frozen wind.
So how do perennial plants survive the winter? Where and when did they “learn” the tricks they needed? We started our pursuit by talking to scientists who have completed a study of the genetics of cold tolerance.
Their research, published Dec. 22 in Nature1, was based on a new evolutionary tree showing the genetic relationships among more than 32,000 species of flowering plants. Having built the tree, they used it to examine three traits used by perennial plants to avoid cold damage, especially caused when water crystallizes into ice:
Dying back to the underground portions, in herbaceous species like milkweed and goldenrod, avoids much of the desiccation and freeze-thaw cycles that occur above ground
Having narrow circulatory vessels, to prevent air bubbles from forming in frozen sap and blocking fluid movement
Dropping leaves before winter, a trait that thousands of deciduous shrubs and trees use to reduce the pressure to draw fluid through their circulatory systems
The giant evolutionary tree showed that two of the three traits were present when flowering plants evolved at least 130 million years ago. That was something of a surprise, says co-author Douglas Soltis, a professor of biology at the University of Florida. “In most cases, they already had these adaptations, and the plants just used those to adapt to the cold.”
You might figure that plants would evolve or die, but for many plants, “the features were around, and they used them for other reasons.”
Dropping leaves was the only one of the three tactics that did not go back to the root of flowering plantdom, Soltis said.
“When you look at these shifts across evolutionary history, changes are not that common,” says his co-author and wife Pamela Soltis, also a professor of biology at Florida.
The logical conclusion is that plants survived a future they could not anticipate by adapting structures and traits that evolved against threats like fire, wind, heat and drought. “Some plants probably occurred in areas with seasonal drought, such as dry tropical forest, and would have adapted for that,” says Doug. “Maybe a small water-conduit cell is an advantage. You don’t want a big water-conducting cell when not a lot of water is available, and this was later used for response to cold.”
But he concedes, “We are starting to arm-wave here.”
We wondered whether these features could have evolved in the cold. “Without superimposing ancient climate history onto our phylogeny, we don’t know necessarily under what conditions these traits were present” in the past, says Pamela Soltis. “Potentially they could have evolved in earlier cold periods, but we can’t address that with available data.”
When ice is not nice
In the Arctic, scientists are seeing the fastest warming on the planet that is feeding, among other things, a rapid increase in rain falling on snow. That, like the increase in freeze-thaw cycles during the warmer winters in temperate zones, can coat plants with ice.
Ice shells on can crush or break a plant, but even a thinner coating can block movement of oxygen, slowing metabolism and possibly leading to cell death. Ice also conducts heat better than snow, so the plant is cooler and subject to frost damage.
A study of shrubs in Sweden2, looked at flowering, bud timing, shoot growth and leaf damage. When shrubs were deliberately encased in ice in March, one species was seriously harmed, while another was largely unaffected. The take-home? “…plant community structure in the longer term could change if winters continue to see a greater frequency of icing events,” the authors wrote.
Different strokes for different folks
Plants have several other tactics3 to survive freezing conditions.
The flower and leaf buds of woody plants display extraorgan freezing — ice forms outside the bud and draws water from the bud to a harmless location.
Many herbaceous plants, including crops like wheat, barley and spinach, display a different wrinkle on that theme, called extracellular freezing: Ice forms outside their cells, drawing water from the cells to a safe freezing location.
Furthermore, in late summer, a plant’s internal chemistry may
synthesize compounds that stabilize critical cellular components, such as membranes, proteins and nucleic acids
create substances that prevent water from freezing
slow the production of chemicals that ease ice-crystal formation.
Heat, not cold, the biggest challenge today
What with the sub-zero forecasts in unlikely places like Little Rock and Washington, DC, we’ve inevitably focused on cold, and we have read the canard that one cold spell proves the hoaxiness of global warming. But the critical problem facing plants right now is warming, not cooling.
Average date of first flowering
Ecologists have begun using “climate chaos” to discuss concatenations of odd weather: Rainfall in Wisconsin in January, late-fall hurricanes like Sandy, or the record early spring in 2012 that was a harbinger of a record hot summer in much of the United States.
Phenology, the study of timing in nature, is one way to assess the long-term effects of changing climate. Typically, phenology records the first flowering of plants or the first arrival of spring migrant birds.
The PNAS 2013 study compared the modern timing of flowering to data kept by author-scientists Henry David Thoreau and Aldo Leopold. 2010 and 2012 were record warm years in the Northeast, where Thoreau started keeping data at Walden Pond. 2012 was a record in Wisconsin, where Leopold began documenting events in what he called “Sand County,” in 1935.
In 2010 and 2012, some Massachusetts flowers bloomed 11 days earlier than in Thoreau’s time. In Wisconsin, the advance reached almost a month in 2012. Although those advances – over such a short interval – were dramatic, even shocking — the study also demonstrated that plants did not exhaust their adaptive ability, even though 2012 was “off the charts, an unprecedented warm spring,” says study co-author Stanley Temple, a professor emeritus of wildlife ecology at the University of Wisconsin-Madison.
End of the road?
But with climate models forecasting ever more warming and ever-earlier springs, “This can’t go on forever,” says Temple. At stake is the process of vernalization, which prepares plants for blooming in the spring.
Consider: if temperate-zone plants responded robot-like to any old warm spell, they could bloom in a warm fall, only to have the flowers destroyed in winter. And thus there is a strong evolutionary pressure to delay flowering (and sprouting of seeds) until after the passage of a certain cold period.
Eventually, Temple predicts, winters will be so warm that some plants will no longer even flower. “The specter is that as winter arrives later, and spring comes earlier, eventually we are going to get to the point where some North-temperate zone plants that have evolved a need to accumulate quite a few chill units might not accumulate enough because the winter is too short.”
Plenty of factors affect how plants will fare as warming continues, Temple says. “It’s very hard to figure out because there are very few data sets like Leopold and Thoreau that cover a large number of native species for a long period. There may be information on the plants, but not on the insects, or information on only some plants, usually the ones with the most horticultural interest.”
Climate chaos portends other dangers. In 2012, one of the warmest springs on record, pollinating bees were scarce some Wisconsin apple orchards. Those early-developing blossoms were then ruined by frost — another disruption that could be linked to the bizarre weather.
That example is only a suggestion of the many ways that climate chaos could affect plants, Temple says. Warming temperatures are:
accelerating the arrival of spring, even though photoperiod, the lengthening daylight in the spring that also affects the timing of flowering, is unchanged
hastening the migration of competing plants and pathogens from the south
changing precipitation, likely leading to more floods, droughts and wildfires
When the many interactions are taken together, the result is a complex and ominous picture of the ecology of a warming world, where plants, and entire ecosystems, struggle to adapt.
Warning about warming
That question of adaptation brought us back to the genetic study of cold tolerance in flowering plants, which found that two of the three studied characteristics were present all along. That result “probably shows how it may be difficult for plants to respond to a rapid change in temperature,” says co-author Douglas Soltis. “Even for something as important as cold adaptation, plants didn’t adapt with new innovations, they just used something they had.”
As they face warming, Soltis continues, “Plants can’t respond with new traits, innovation is not that fast. It’s going to be hard for a lot of plant lines to make the adaptation [to a warming climate] if change happens really quickly.”
What about ecological communities, the interdependent suites of plants, animals and microbes found in nature? “A number of species may have a predisposition to survive increased temperature as well, but to think the plant communities that represent different evolutionary histories will remain intact is pretty naïve,” says Pamela Soltis.
Indeed, a 2008 study4 shows that the species that disappeared over a 150-year span were those that could not adjust their flowering time to climate change. And because that ability is genetically determined, it tends to be present or absent in related groups. “Who you are related to makes a difference,” says Douglas Soltis. “Some plants don’t have the necessary underlying machinery to make the switch, so we will lose chunks of things that are closely related.”
In terms of adjusting to climate, we WhyFilers have a special regard for the plants we eat. Crops may be especially vulnerable to climate change since breeding has already reduced their genetic diversity. For breeding crops to adapt to rapid change climate, “it’s not like there is one or a few traits you are trying to add to the repertoire,” says Douglas Soltis. “For a lot of crops, response to drought and increasing temperature are not traits they have, and they will not be easy to breed in.”
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Three keys to the radiation of angiosperms into freezing environments, Amy E. Zanne, et al, Nature (2013), doi:10.1038/nature12872 ↩
- Impacts of winter icing events on the growth, phenology and physiology of sub-arctic dwarf shrubs; Catherine Preece et al, Physiologia Plantarum 146: 460–472. 2012 ↩
- Plant strategies for survival in changing environment; Matsuo Uemuraa & Jean-Francois Hausman, Physiologia Plantarum 147: 1–3. 2013 ↩
- Phylogenetic patterns of species loss in Thoreau’s woods are driven by climate change; Charles G. Willis et al, PNAS, 2008 ↩
- Bees, Plants Waking Up Earlier Each Spring ↩
- Tree Trunk Biology – Basic Wood Structure ↩
- [Video] Why Do Autumn Leaves Change Color? ↩
- Why Do Leaves Fall from Trees in Fall? ↩
- Maple Syrup Takes Turn Toward Technology ↩