Supervolcanoes Discovered in Utah: Evidence of Some of the Largest Eruptions in Earth’s History
Dec. 11, 2013 — Brigham Young University geologists found evidence of some of the largest volcanic eruptions in earth’s history right in their own backyard.
These supervolcanoes aren’t active today, but 30 million years ago more than 5,500 cubic kilometers of magma erupted during a one-week period near a place called Wah Wah Springs. By comparison, this eruption was about 5,000 times larger than the 1980 Mount St. Helens eruption.
“In southern Utah, deposits from this single eruption are 13,000 feet thick,” said Eric Christiansen, the lead author for the BYU study. “Imagine the devastation — it would have been catastrophic to anything living within hundreds of miles.”
Dinosaurs were already extinct during this time period, but what many people don’t know is that 25-30 million years ago, North America was home to rhinos, camels, tortoises and even palm trees. Evidence of the ancient flora and fauna was preserved by volcanic deposits.
The research group, headed by Christiansen and professor emeritus Myron Best, measured the thickness of the pyroclastic flow deposits. They used radiometric dating, X-ray fluorescence spectrometry, and chemical analysis of the minerals to verify that the volcanic ash was all from the same ancient super-eruption.
They found that the Wah Wah Springs eruption buried a vast region extending from central Utah to central Nevada and from Fillmore on the north to Cedar City on the south. They even found traces of ash as far away as Nebraska.
But this wasn’t an isolated event; the BYU geologists found evidence of fifteen super-eruptions and twenty large calderas. The scientific journal Geosphere recently published two of their papers detailing the discoveries.
Despite their enormous size, the supervolcanoes have been hidden in plain sight for millions of years.
“The ravages of erosion and later deformation have largely erased them from the landscape, but our careful work has revealed their details,” said Christiansen. “The sheer magnitude of this required years of work and involvement of dozens of students in putting this story together.”
Supervolcanoes are different from the more familiar stratovolcanoes — like Mount St. Helens — because they aren’t as obvious to the naked eye and they affect enormous areas.
“Supervolcanoes as we’ve seen are some of earth’s largest volcanic edifices, and yet they don’t stand as high cones,” said Christiansen. “At the heart of a supervolcano instead, is a large collapse.”
Those collapses in supervolcanoes occur with the eruption and form enormous holes in the ground in plateaus, known as calderas.
Not many people know that there are still active supervolcanoes today. Yellowstone National Park in Wyoming is home to one roughly the same size as the Wah Wah Springs caldera, which was about 25 miles across and 3 miles deep when it first formed.
More than a dozen undergraduate and graduate students made significant contributions to Best and Christiansen’s papers. Hundreds of other students were involved with the geologic mapping of the volcanic areas. The skills and experience each student gained along the way have opened doors to graduate schools, employers and entrepreneurship. Mentored learning is part of why BYU ranks in the Top 5 nationally in terms of where new Ph.D.’s received their undergraduate degrees.
Mapping the Demise of the Dinosaurs
Dec. 9, 2013 — About 65 million years ago, an asteroid or comet crashed into a shallow sea near what is now the Yucatán Peninsula of Mexico. The resulting firestorm and global dust cloud caused the extinction of many land plants and large animals, including most of the dinosaurs. At this week’s meeting of the American Geophysical Union (AGU) in San Francisco, MBARI researchers will present evidence that remnants from this devastating impact are exposed along the Campeche Escarpment — an immense underwater cliff in the southern Gulf of Mexico.
The ancient meteorite impact created a huge crater, over 160 kilometers across. Unfortunately for geologists, this crater is almost invisible today, buried under hundreds of meters of debris and almost a kilometer of marine sediments. Although fallout from the impact has been found in rocks around the world, surprisingly little research has been done on the rocks close to the impact site, in part because they are so deeply buried. All existing samples of impact deposits close to the crater have come from deep boreholes drilled on the Yucatán Peninsula.
In March 2013, an international team of researchers led by Charlie Paull of the Monterey Bay Aquarium Research Institute (MBARI) created the first detailed map of the Campeche Escarpment. The team used multi-beam sonars on the research vessel Falkor, operated by the Schmidt Ocean Institute. The resulting maps have recently been incorporated in Google Maps and Google Earth for viewing by researchers and the general public.
Paull has long suspected that rocks associated with the impact might be exposed along the Campeche Escarpment, a 600-kilometer-long underwater cliff just northwest of the Yucatán Peninsula. Nearly 4,000 meters tall, the Campeche Escarpment is one of the steepest and tallest underwater features on Earth. It is comparable to one wall of the Grand Canyon — except that it lies thousands of meters beneath the sea.
As in the walls of the Grand Canyon, sedimentary rock layers exposed on the face of the Campeche Escarpment provide a sequential record of the events that have occurred over millions of years. Based on the new maps, Paull believes that rocks formed before, during, and after the impact are all exposed along different parts of this underwater cliff.
Just as a geologist can walk the Grand Canyon, mapping layers of rock and collecting rock samples, Paull hopes to one day perform geologic “fieldwork” and collect samples along the Campeche Escarpment. Only a couple of decades ago, the idea of performing large-scale geological surveys thousands of meters below the ocean surface would have seemed a distant fantasy. Over the last eight years, however, such mapping has become almost routine for MBARI geologists using underwater robots.
The newly created maps of the Campeche Escarpment could open a new chapter in research about one of the largest extinction events in Earth’s history. Already researchers from MBARI and other institutions are using these maps to plan additional studies in this little-known area. Detailed analysis of the bathymetric data and eventual fieldwork on the escarpment will reveal fascinating new clues about what happened during the massive impact event that ended the age of the dinosaurs — clues that have been hidden beneath the waves for 65 million years.
In addition to the Schmidt Ocean Institute, Paull’s collaborators in this research included Jaime Urrutia-Fucugauchi from the Universidad Nacional Autónoma de Mexico and Mario Rebolledo- Vieyra of the Centro de Investigación Científica de Yucatán. Paull also worked closely with MBARI researchers, including geophysicist and software engineer Dave Caress, an expert on processing of multibeam sonar data, and geologist Roberto Gwiazda, who served as project manager and will be describing this research at the AGU meeting.
Functional Importance of Dinosaur Beaks Illuminated
Dec. 2, 2013 — Why beaks evolved in some theropod dinosaurs and what their function might have been is the subject of new research by an international team of palaeontologists published this week in PNAS (Proceedings of the National Academy of Sciences).
Beaks are a typical hallmark of modern birds and can be found in a huge variety of forms and shapes. However, it is less well known that keratin-covered beaks had already evolved in different groups of dinosaurs during the Cretaceous Period.
Employing high-resolution X-ray computed tomography (CT scanning) and computer simulations, Dr Stephan Lautenschlager and Dr Emily Rayfield of the University of Bristol with Dr Perle Altangerel (National University of Ulaanbaatar) and Professor Lawrence Witmer (Ohio University) used digital models to take a closer look at these dinosaur beaks.
The focus of the study was the skull of Erlikosaurus andrewsi, a 3-4m (10-13ft) large herbivorous dinosaur called a therizinosaur, which lived more than 90 million years ago during the Cretaceous Period in what is now Mongolia, and which shows evidence that part of its snout was covered by a keratinous beak.
This new study reveals that keratinous beaks played an important role in stabilizing the skeletal structure during feeding, making the skull less susceptible to bending and deformation.
Lead author Dr Stephan Lautenschlager of Bristol’s School of Earth Sciences said: “It has classically been assumed that beaks evolved to replace teeth and thus save weight, as a requirement for the evolution of flight. Our results, however, indicate that keratin beaks were in fact beneficial to enhance the stability of the skull during biting and feeding.”
Co-author Dr Emily Rayfield, Reader of Palaeobiology at Bristol said: “Using Finite Element Analysis, a computer modelling technique routinely used in engineering, we were able to deduce very accurately how bite and muscle forces affected the skull of Erlikosaurus during the feeding process. This further allowed us to identify the importance of soft-tissue structures, such as the keratinous beak, which are normally not preserved in fossils.”
Co-author Lawrence Witmer, Chang Professor of Paleontology at the Ohio University Heritage College of Osteopathic Medicine said: “Beaks evolved several times during the transitions from dinosaurs to modern birds, usually accompanied by the partial or complete loss of teeth and our study now shows that keratin-covered beaks represent a functional innovation during dinosaur evolution.”
This work was funded by a research fellowship to Stephan Lautenschlager from the German Volkswagen Foundation and grants from the National Science Foundation to Lawrence
Iron Preserves, Hides Ancient Tissues in Fossilized Remains
Nov. 26, 2013 — New research from North Carolina State University shows that iron may play a role in preserving ancient tissues within dinosaur fossils, but also may hide them from detection. The finding could open the door to the recovery of more ancient tissues from within fossils.
Mary Schweitzer, an NC State paleontologist with a joint appointment at the N. C. Museum of Natural Sciences, first announced the surprising preservation of soft tissues in a T. rex fossil in 2005. Her subsequent work identified proteins in the soft tissue that seemed to confirm that the tissue was indeed T. rex tissue that had been preserved for millions of years. But the findings remained controversial in part because no one understood the chemical processes behind such preservation.
Schweitzer’s latest research shows that the presence of hemoglobin — the iron-containing molecule that transports oxygen in red blood cells — may be the key to both preserving and concealing original ancient proteins within fossils. Her results appear in Proceedings of the Royal Society B.
“Iron is necessary for survival, but it’s also highly reactive and destructive in living tissues, which is why our bodies have proteins that transport iron molecules to where they are needed but protect us from unwanted reactions at the same time,” Schweitzer says. “When we die, that protective mechanism breaks down and the iron is turned loose on our tissues — and that destructive process can act in much the same way formaldehyde does to preserve the tissues and proteins.”
Hemoglobin seems to be the key. Both birds and crocodiles, the dinosaur’s closest living relatives, have large, nucleated red blood cells. Therefore they also have more hemoglobin per cell than mammals. If dinosaur blood cells were similar to either one of those species, which seems likely, then their blood cells would also contain much more hemoglobin than human cells, amplifying iron’s preservative effect on the tissues. If the hemoglobin were contained in a bone in a sandstone environment, keeping it dry and insulated from microbes, preservation becomes more likely.
Schweitzer and her team noticed that iron particles are intimately associated with the soft tissues preserved in dinosaurs. But when they chelated — or removed the iron from — soft tissues taken from a T. rex and a Brachyolophosaurus, the chelated tissues reacted much more strongly to antibodies that detect the presence of protein, suggesting that the iron may be masking their presence in these preserved tissues. They then tested the preservation hypothesis by using blood vessels and cells taken from modern ostrich bone. They soaked some of these vessels in hemoglobin taken from red blood cells, while placing other vessels in water. Two years later, the hemoglobin-treated soft vessels remained intact, while those soaked in water degraded in less than a week.
“We know that iron is always present in large quantities when we find well-preserved fossils, and we have found original vascular tissues within the bones of these animals, which would be a very hemoglobin-rich environment after they died,” Schweitzer says. “We also know that iron hinders just about every technique we have to detect proteins. So iron looks like it may be both the mechanism for preservation and the reason why we’ve had problems finding and analyzing proteins that are preserved.”
The research was funded by the National Science Foundation and the David and Lucile Packard Foundation. Research assistant Wenxia Zhang and former graduate student Timothy Cleland (now at Rensselaer Polytechnic Institute) contributed to the work, which was also done in collaboration with scientists at University of California-Berkeley, Lawrence Berkeley National Labs, and the Children’s Hospital Oakland Research Institute (CHORI).
CT and 3-D Printers Used to Recreate Dinosaur Fossils
Nov. 20, 2013 — Data from computed tomography (CT) scans can be used with three-dimensional (3-D) printers to make accurate copies of fossilized bones, according to new research published online in the journal Radiology.
Fossils are often stored in plaster casts, or jackets, to protect them from damage. Getting information about a fossil typically requires the removal of the plaster and all the sediment surrounding it, which can lead to loss of material or even destruction of the fossil itself.
German researchers studied the feasibility of using CT and 3-D printers to nondestructively separate fossilized bone from its surrounding sediment matrix and produce a 3-D print of the fossilized bone itself.
“The most important benefit of this method is that it is non-destructive, and the risk of harming the fossil is minimal,” said study author Ahi Sema Issever, M.D., from the Department of Radiology at Charité Campus Mitte in Berlin. “Also, it is not as time-consuming as conventional preparation.”
Dr. Issever and colleagues applied the method to an unidentified fossil from the Museum für Naturkunde, a major natural history museum in Berlin. The fossil and others like it were buried under rubble in the basement of the museum after a World War II bombing raid. Since then, museum staff members have had difficulty sorting and identifying some of the plaster jackets.
Researchers performed CT on the unidentified fossil with a 320-slice multi-detector system. The different attenuation, or absorption of radiation, through the bone compared with the surrounding matrix enabled clear depiction of a fossilized vertebral body.
After studying the CT scan and comparing it to old excavation drawings, the researchers were able to trace the fossil’s origin to the Halberstadt excavation, a major dig from 1910 to 1927 in a clay pit south of Halberstadt, Germany. In addition, the CT study provided valuable information about the condition and integrity of the fossil, showing multiple fractures and destruction of the front rim of the vertebral body.
Furthermore, the CT dataset helped the researchers build an accurate reconstruction of the fossil with selective laser sintering, a technology that uses a high-powered laser to fuse together materials to make a 3-D object.
Dr. Issever noted that the findings come at a time when advances in technology and cheaper availability of 3-D printers are making them more common as a tool for research. Digital models of the objects can be transferred rapidly among researchers, and endless numbers of exact copies may be produced and distributed, greatly advancing scientific exchange, Dr. Issever said. The technology also potentially enables a global interchange of unique fossils with museums, schools and other settings.
“The digital dataset and, ultimately, reproductions of the 3-D print may easily be shared, and other research facilities could thus gain valuable informational access to rare fossils, which otherwise would have been restricted,” Dr. Issever said. “Just like Gutenberg’s printing press opened the world of books to the public, digital datasets and 3-D prints of fossils may now be distributed more broadly, while protecting the original intact fossil.”
Rising Mountains Dried out Central Asia
Dec. 11, 2013 — The uplift of two mountain ranges in Central Asia beginning 30 million years ago expanded the Gobi Desert and set Central Asia on its path to extreme aridity, a Stanford study suggests.
Stanford researchers visited the Nemegt Basin in Mongolia’s Gobi Desert in search of clues to Central Asia’s extreme aridity.
A record of ancient rainfall teased from long-buried sediments in Mongolia is challenging the popular idea that the arid conditions prevalent in Central Asia today were caused by the ancient uplift of the Himalayas and the Tibetan Plateau.
Instead, Stanford scientists say the formation of two lesser mountain ranges, the Hangay and the Altai, may have been the dominant drivers of climate in the region, leading to the expansion of Asia’s largest desert, the Gobi. The findings will be presented on Thursday, Dec. 12, at the annual meeting of the American Geophysical Union (AGU) in San Francisco.
“These results have major implications for understanding the dominant factors behind modern-day Central Asia’s extremely arid climate and the role of mountain ranges in altering regional climate,” said Page Chamberlain, a professor of environmental Earth system science at Stanford.
Scientists previously thought that the formation of the Himalayan mountain range and the Tibetan plateau around 45 million years ago shaped Asia’s driest environments.
“The traditional explanation has been that the uplift of the Himalayas blocked air from the Indian Ocean from reaching central Asia,” said Jeremy Caves, a doctoral student in Chamberlain’s terrestrial paleoclimate research group who was involved in the study.
This process was thought to have created a distinct rain shadow that led to wetter climates in India and Nepal and drier climates in Central Asia. Similarly, the elevation of the Tibetan Plateau was thought to have triggered an atmospheric process called subsidence, in which a mass of air heated by a high elevation slowly sinks into Central Asia.
“The falling air suppresses convective systems such as thunderstorms, and the result is you get really dry environments,” Caves said.
This long-accepted model of how Central Asia’s arid environments were created mostly ignores, however, the existence of the Altai and Hangay, two northern mountain ranges.
Searching for answers
To investigate the effects of the smaller ranges on the regional climate, Caves and his colleagues from Stanford and Rocky Mountain College in Montana traveled to Mongolia in 2011 and 2012 and collected samples of ancient soil, as well as stream and lake sediments from remote sites in the central, southwestern and western parts of the country.
The team carefully chose its sites by scouring the scientific literature for studies of the region conducted by pioneering researchers in past decades.
“A lot of the papers were by Polish and Russian scientists who went there to look for dinosaur fossils,” said Hari Mix, a doctoral student at Stanford who also participated in the research. “Indeed, at many of the sites we visited, there were dinosaur fossils just lying around.”
The earlier researchers recorded the ages and locations of the rocks they excavated as part of their own investigations; Caves and his team used those age estimates to select the most promising sites for their own study.
At each site, the team bagged sediment samples that were later analyzed to determine their carbon isotope content. The relative level of carbon isotopes present in a soil sample is related to the productivity of plants growing in the soil, which is itself dependent on the annual rainfall. Thus, by measuring carbon isotope amounts from different sediment samples of different ages, the team was able to reconstruct past precipitation levels.
An ancient wet period
The new data suggest that rainfall in central and southwestern Mongolia had decreased by 50 to 90 percent in the last several tens of million of years.
“Right now, precipitation in Mongolia is about 5 inches annually,” Caves said. “To explain our data, rainfall had to decrease from 10 inches a year or more to its current value over the last 10 to 30 million years.”
That means that much of Mongolia and Central Asia were still relatively wet even after the formation of the Himalayas and the Tibetan Plateau 45 million years ago. The data show that it wasn’t until about 30 million years ago, when the Hangay Mountains first formed, that rainfall started to decrease. The region began drying out even faster about 5 million to 10 million years ago, when the Altai Mountains began to rise.
The scientists hypothesize that once they formed, the Hangay and Altai ranges created rain shadows of their own that blocked moisture from entering Central Asia.
“As a result, the northern and western sides of these ranges are wet, while the southern and eastern sides are dry,” Caves said.
The team is not discounting the effect of the Himalayas and the Tibetan Plateau entirely, because portions of the Gobi Desert likely already existed before the Hangay or Altai began forming.
“What these smaller mountains did was expand the Gobi north and west into Mongolia,” Caves said.
The uplift of the Hangay and Altai may have had other, more far-reaching implications as well, Caves said. For example, westerly winds in Asia slam up against the Altai today, creating strong cyclonic winds in the process. Under the right conditions, the cyclones pick up large amounts of dust as they snake across the Gobi Desert. That dust can be lofted across the Pacific Ocean and even reach California, where it serves as microscopic seeds for developing raindrops.
The origins of these cyclonic winds, as well as substantial dust storms in China today, may correlate with uplift of the Altai, Caves said. His team plans to return to Mongolia and Kazakhstan neStanford researchers visited the Nemegt Basin in Mongolia’s Gobi Desert in search of clues to Central Asia’s extreme aridity.
A record of ancient rainfall teased from long-buried sediments in Mongolia is challenging the popular idea that the arid conditions prevalent in Central Asia today were caused by the ancient uplift of the Himalayas and the Tibetan Plateau.
Instead, Stanford scientists say the formation of two lesser mountain ranges, the Hangay and the Altai, may have been the dominant drivers of climate in the region, leading to the expansion of Asia’s largest desert, the Gobi. The findings will be presented on Thursday, Dec. 12, at the annual meeting of the American Geophysical Union (AGU) in San Francisco.
“These results have major implications for understanding the dominant factors behind modern-day Central Asia’s extremely arid climate and the role of mountain ranges in altering regional climate,” said Page Chamberlain, a professor of environmental Earth system science at Stanford.
Scientists previously thought that the formation of the Himalayan mountain range and the Tibetan plateau around 45 million years ago shaped Asia’s driest environments.
“The traditional explanation has been that the uplift of the Himalayas blocked air from the Indian Ocean from reaching central Asia,” said Jeremy Caves, a doctoral student in Chamberlain’s terrestrial paleoclimate research group who was involved in the study.
This process was thought to have created a distinct rain shadow that led to wetter climates in India and Nepal and drier climates in Central Asia. Similarly, the elevation of the Tibetan Plateau was thought to have triggered an atmospheric process called subsidence, in which a mass of air heated by a high elevation slowly sinks into Central Asia.
“The falling air suppresses convective systems such as thunderstorms, and the result is you get really dry environments,” Caves said.
This long-accepted model of how Central Asia’s arid environments were created mostly ignores, however, the existence of the Altai and Hangay, two northern mountain ranges.
Searching for answers
To investigate the effects of the smaller ranges on the regional climate, Caves and his colleagues from Stanford and Rocky Mountain College in Montana traveled to Mongolia in 2011 and 2012 and collected samples of ancient soil, as well as stream and lake sediments from remote sites in the central, southwestern and western parts of the country.
The team carefully chose its sites by scouring the scientific literature for studies of the region conducted by pioneering researchers in past decades.
“A lot of the papers were by Polish and Russian scientists who went there to look for dinosaur fossils,” said Hari Mix, a doctoral student at Stanford who also participated in the research. “Indeed, at many of the sites we visited, there were dinosaur fossils just lying around.”
The earlier researchers recorded the ages and locations of the rocks they excavated as part of their own investigations; Caves and his team used those age estimates to select the most promising sites for their own study.
At each site, the team bagged sediment samples that were later analyzed to determine their carbon isotope content. The relative level of carbon isotopes present in a soil sample is related to the productivity of plants growing in the soil, which is itself dependent on the annual rainfall. Thus, by measuring carbon isotope amounts from different sediment samples of different ages, the team was able to reconstruct past precipitation levels.
An ancient wet period
The new data suggest that rainfall in central and southwestern Mongolia had decreased by 50 to 90 percent in the last several tens of million of years.
“Right now, precipitation in Mongolia is about 5 inches annually,” Caves said. “To explain our data, rainfall had to decrease from 10 inches a year or more to its current value over the last 10 to 30 million years.”
That means that much of Mongolia and Central Asia were still relatively wet even after the formation of the Himalayas and the Tibetan Plateau 45 million years ago. The data show that it wasn’t until about 30 million years ago, when the Hangay Mountains first formed, that rainfall started to decrease. The region began drying out even faster about 5 million to 10 million years ago, when the Altai Mountains began to rise.
The scientists hypothesize that once they formed, the Hangay and Altai ranges created rain shadows of their own that blocked moisture from entering Central Asia.
“As a result, the northern and western sides of these ranges are wet, while the southern and eastern sides are dry,” Caves said.
The team is not discounting the effect of the Himalayas and the Tibetan Plateau entirely, because portions of the Gobi Desert likely already existed before the Hangay or Altai began forming.
“What these smaller mountains did was expand the Gobi north and west into Mongolia,” Caves said.
The uplift of the Hangay and Altai may have had other, more far-reaching implications as well, Caves said. For example, westerly winds in Asia slam up against the Altai today, creating strong cyclonic winds in the process. Under the right conditions, the cyclones pick up large amounts of dust as they snake across the Gobi Desert. That dust can be lofted across the Pacific Ocean and even reach California, where it serves as microscopic seeds for developing raindrops.
The origins of these cyclonic winds, as well as substantial dust storms in China today, may correlate with uplift of the Altai, Caves said. His team plans to return to Mongolia and Kazakhstan next summer to collect more samples and to use climate models to test whether the Altai are responsible for the start of the large dust storms.
“If the Altai are a key part of regulating Central Asia’s climate, we can go and look for evidence of it in the past,” Caves said.
xt summer to collect more samples and to use climate models to test whether the Altai are responsible for the start of the large dust storms.
“If the Altai are a key part of regulating Central Asia’s climate, we can go and look for evidence of it in the past,” Caves said.
Clues to How Plants Evolved to Cope With Cold
Dec. 22, 2013 — Researchers have found new clues to how plants evolved to withstand wintry weather. In a study to appear in the December 22 issue of the journal Nature, the team constructed an evolutionary tree of more than 32,000 species of flowering plants — the largest time-scaled evolutionary tree to date. By combining their tree with freezing exposure records and leaf and stem data for thousands of species, the researchers were able to reconstruct how plants evolved to cope with cold as they spread across the globe. The results suggest that many plants acquired characteristics that helped them thrive in colder climates — such as dying back to the roots in winter — long before they first encountered freezing.
Fossil evidence and reconstructions of past climatic conditions suggest that early flowering plants lived in warm tropical environments, explained co-author Jeremy Beaulieu at the National Institute for Mathematical & Biological Synthesis (NIMBioS) at the University of Tennessee.
As plants spread to higher latitudes and elevations, they evolved in ways that helped them deal with cold conditions. Plants that live in the tundra, such as Arctic cinquefoil and three-toothed saxifrage, can withstand winter temperatures below minus 15 degrees Celsius.
Unlike animals, most plants can’t move to escape the cold or generate heat to keep them warm. It’s not so much the cold but the ice that poses problems for plants. For instance, freezing and thawing cause air bubbles to form in the plant’s internal water transport system.
“Think about the air bubbles you see suspended in the ice cubes,” said co-author Amy Zanne of the George Washington University. “If enough of these air bubbles come together as water thaws they can block the flow of water from the roots to the leaves and kill the plant.”
The researchers identified three traits that help plants get around these problems.
Some plants, such as hickories and oaks, avoid freezing damage by dropping their leaves before the winter chill sets in — effectively shutting off the flow of water between roots and leaves — and growing new leaves and water transport cells when warmer weather returns.
Other plants, such as birches and poplars, also protect themselves by having narrower water transport cells, which makes the parts of the plant that deliver water less susceptible to blockage during freezing and thawing.
Still others die back to the ground in winter and re-sprout from their roots, or start growing as new plants from seeds when conditions are right.
To compile the plant trait data for their study, the researchers spent hundreds of hours scouring and merging multiple large plant databases containing tens of thousands of species, largely with the support of the National Evolutionary Synthesis Center in North Carolina and Macquarie University in Australia.
When they mapped their collected leaf and stem data onto their evolutionary tree for flowering plants, they found that many plants were well equipped for icy climates even before cold conditions hit.
Plants that die back to the ground in winter, for example, acquired the ability to die and come back when conditions improve long before they first experienced freezing. Similarly, species with narrow water transport cells acquired a finer circulatory system well before they confronted cold climates.
“This suggests that some other environmental pressure — possibly drought — caused these plants to evolve this way, and it happened to work really well for freezing tolerance too,” said Zanne.
The only exceptions were plants that shed and replace their leaves seasonally — these plant groups didn’t gain the ability to drop their leaves during winter until after they encountered freezing, Beaulieu added.
As a next step, the researchers plan to use their evolutionary tree to find out how plants evolved to withstand other environmental stresses in addition to freezing, such as drought and heat.