Revolutionary solar cells double as lasers

Latest research finds that the trailblazing ‘perovskite’ material used in solar cells can double up as a laser, strongly suggesting the astonishing efficiency levels already achieved in these cells is only part of the journey.Commercial silicon-based solar cells — such as those seen on the roofs of houses across the country — operate at about 20% efficiency for converting the Sun’s rays into electrical energy. It’s taken over 20 years to achieve that rate of efficiency.A relatively new type of solar cell based on a perovskite material — named for scientist Lev Perovski, who first discovered materials with this structure in the Ural Mountains in the 19th century — was recently pioneered by an Oxford research team led by Professor Henry Snaith.Perovskite solar cells, the source of huge excitement in the research community, already lie just a fraction behind commercial silicon, having reached a remarkable 17% efficiency after a mere two years of research — transforming prospects for cheap large-area solar energy generation.Now, researchers from Professor Sir Richard Friend’s group at Cambridge’s Cavendish Laboratory — working with Snaith’s Oxford group — have demonstrated that perovskite cells excel not just at absorbing light but also at emitting it. The new findings, recently published online in the Journal of Physical Chemistry Letters, show that these ‘wonder cells’ can also produce cheap lasers.By sandwiching a thin layer of the lead halide perovskite between two mirrors, the team produced an optically driven laser which proves these cells “show very efficient luminescence” — with up to 70% of absorbed light re-emitted.The researchers point to the fundamental relationship, first established by Shockley and Queisser in 1961, between the generation of electrical charges following light absorption and the process of ‘recombination’ of these charges to emit light.Essentially, if a material is good at converting light to electricity, then it will be good at converting electricity to light. The lasing properties in these materials raise expectations for even higher solar cell efficiencies, say the Oxbridge team, which — given that perovskite cells are about to overtake commercial cells in terms of efficiency after just two years of development — is a thrilling prospect.”This first demonstration of lasing in these cheap solution-processed semiconductors opens up a range of new applications,” said lead author Dr Felix Deschler of the Cavendish Laboratory. “Our findings demonstrate potential uses for this material in telecommunications and for light emitting devices.”Most commercial solar cell materials need expensive processing to achieve a very low level of impurities before they show good luminescence and performance. Surprisingly these new materials work well even when very simply prepared as thin films using cheap scalable solution processing.The researchers found that upon light absorption in the perovskite two charges (electron and hole) are formed very quickly — within 1 picosecond — but then take anywhere up to a few microseconds to recombine. This is long enough for chemical defects to have ceased the light emission in most other semiconductors, such as silicon or gallium arsenide. “These long carrier lifetimes together with exceptionally high luminescence are unprecedented in such simply prepared inorganic semiconductors,” said Dr Sam Stranks, co-author from the Oxford University team.”We were surprised to find such high luminescence efficiency in such easily prepared materials. This has great implications for improvements in solar cell efficiency,” said Michael Price, co-author from the group in Cambridge.Added Snaith: “This luminescent behaviour is an excellent test for solar cell performance — poorer luminescence (as in amorphous silicon solar cells) reduces both the quantum efficiency (current collected) and also the cell voltage.”Scientists say that this new paper sets expectations for yet higher solar cell performance from this class of perovskite semiconductors. Solar cells are being scaled up for commercial deployment by the Oxford spin-out, Oxford PV Ltd. …

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Salamanders shrinking as their mountain havens heat up

Wild salamanders living in some of North America’s best salamander habitat are getting smaller as their surroundings get warmer and drier, forcing them to burn more energy in a changing climate.That’s the key finding of a new study, published March 25 in the journal Global Change Biology, that examined museum specimens caught in the Appalachian Mountains from 1957 to 2007 and wild salamanders measured at the same sites in 2011-2012. The salamanders studied from 1980 onward were, on average, 8% smaller than their counterparts from earlier decades. The changes were most marked in the Southern Appalachians and at low elevations — settings where detailed weather records showed the climate has warmed and dried out most.Scientists have predicted that some animals will get smaller in response to climate change, and this is strongest confirmation of that prediction.”This is one of the largest and fastest rates of change ever recorded in any animal,” said Karen R. Lips, an associate professor of biology at the University of Maryland and the study’s senior author. “We don’t know exactly how or why it’s happening, but our data show it is clearly correlated with climate change.” And it’s happening at a time when salamanders and other amphibians are in distress, with some species going extinct and others dwindling in number.”We don’t know if this is a genetic change or a sign that the animals are flexible enough to adjust to new conditions,” Lips said. “If these animals are adjusting, it gives us hope that some species are going to be able to keep up with climate change.”The study was prompted by the work of University of Maryland Prof. Emeritus Richard Highton, who began collecting salamanders in the Appalachian Mountains in 1957. The geologically ancient mountain range’s moist forests and long evolutionary history make it a global hot spot for a variety of salamander species. Highton collected hundreds of thousands of salamanders, now preserved in jars at the Smithsonian Institution’s Museum Service Center in Suitland, MD.But Highton’s records show a mysterious decline in the region’s salamander populations beginning in the 1980s. Lips, an amphibian expert, saw a similar decline in the frogs she studied in Central America, and tracked it to a lethal fungal disease. …

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Asbestos is The Way of Life for Russian Mountain Population

It is unbelievable that there is a place in today’s world where a person’s daily routine could involve shaking asbestos dust off laundry hanging on a clothesline or sweeping asbestos dust out of a window sill to let in the morning light. In the eastern slopes of Russia’s Ural Mountains, such a place does in fact exist.In the recent New York Times article, City in Russia Unable to Kick Asbestos Habit, author Andrew Kramer gives a detailed description of life in the mountain city of Asbest. With a population of 70,000, Asbest is home to the largest open pit asbestos mine in the world. The mine it is about half the size of Manhattan and descends about 1,000 feet down into the earth. The city’s anthem is, “Asbestos, …

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Mysterious ancient human crossed Wallace’s Line

Oct. 17, 2013 — Scientists have proposed that the most recently discovered ancient human relatives — the Denisovans – somehow managed to cross one of the world’s most prominent marine barriers in Indonesia, and later interbred with modern humans moving through the area on the way to Australia and New Guinea.Three years ago the genetic analysis of a little finger bone from Denisova cave in the Altai Mountains in northern Asia led to a complete genome sequence of a new line of the human family tree — the Denisovans. Since then, genetic evidence pointing to their hybridisation with modern human populations has been detected, but only in Indigenous populations in Australia, New Guinea and surrounding areas. In contrast, Denisovan DNA appears to be absent or at very low levels in current populations on mainland Asia, even though this is where the fossil was found.Published today in a Science opinion article, scientists Professor Alan Cooper of the University of Adelaide in Australia and Professor Chris Stringer of the Natural History Museum in the UK say that this pattern can be explained if the Denisovans had succeeded in crossing the famous Wallace’s Line, one of the world’s biggest biogeographic barriers which is formed by a powerful marine current along the east coast of Borneo. Wallace’s Line marks the division between European and Asian mammals to the west from marsupial-dominated Australasia to the east.”In mainland Asia, neither ancient human specimens, nor geographically isolated modern Indigenous populations have Denisovan DNA of any note, indicating that there has never been a genetic signal of Denisovan interbreeding in the area,” says Professor Cooper, Director of the University of Adelaide’s Australian Centre for Ancient DNA. “The only place where such a genetic signal exists appears to be in areas east of Wallace’s Line and that is where we think interbreeding took place — even though it means that the Denisovans must have somehow made that marine crossing.””The recent discovery of another enigmatic ancient human species Homo floresiensis, the so-called Hobbits, in Flores, Indonesia, confirms that the diversity of archaic human relatives in this area was much higher than we’d thought,” says Professor Stringer, Research Leader in Human Origins, Natural History Museum, in London. “The morphology of the Hobbits shows they are different from the Denisovans, meaning we now have at least two, and potentially more, unexpected groups in the area.”The conclusions we’ve drawn are very important for our knowledge of early human evolution and culture. Knowing that the Denisovans spread beyond this significant sea barrier opens up all sorts of questions about the behaviours and capabilities of this group, and how far they could have spread.””The key questions now are where and when the ancestors of current humans, who were on their way to colonise New Guinea and Australia around 50,000 years ago, met and interacted with the Denisovans,” says Professor Cooper.”Intriguingly, the genetic data suggest that male Denisovans interbred with modern human females, indicating the potential nature of the interactions as small numbers of modern humans first crossed Wallace’s Line and entered Denisovan territory.”

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Calculating the true cost of a ton of mountaintop coal

Sep. 11, 2013 — To meet current U.S. coal demand through surface mining, an area of the Central Appalachians the size of Washington, D.C., would need to be mined every 81 days.That’s about 68 square miles — or roughly an area equal to 10 city blocks mined every hour.A one-year supply of coal would require converting about 310 square miles of the region’s mountains into surface mines, according to a new analysis by scientists at Duke University, Kent State University and the Cary Institute for Ecosystem Studies.Creating 310 square miles of mountaintop mine would pollute about 2,300 kilometers of Appalachian streams and cause the loss of carbon sequestration by trees and soils equal to the greenhouse gases produced in a year by 33,600 average U.S. single-family homes, the study found.The study, published today in the peer-reviewed online journal PLOS ONE, is “the first to put an environmental price tag on mountaintop removal coal,” said Brian D. Lutz, assistant professor of biogeochemistry at Kent State, who began the analysis as a postdoctoral research associate at Duke’s Nicholas School of the Environment last year.While many studies have documented the severity of surface mining’s impacts on local ecosystems, few have quantified the region-wide extent of the damage and provided the metrics needed to weigh the environmental costs of mountaintop mining against its economic benefits, Lutz said.”This is a critical shortcoming,” Lutz said, “since even the most severe impacts may be tolerated if we believe they are sufficiently limited in extent.”To help fill the data gap, the study’s authors used satellite images and historical county-by-county coal production data to measure the total area of land mined and coal removed in the Central Appalachian coalfields between 1985 and 2005.They found that cumulative coal production during the 20-year period totaled 1.93 billion tons, or about two years’ worth of current U.S. coal demand. To access the coal, nearly 2,000 square kilometers of land was mined — an area similar in size to the Great Smoky Mountains National Park.The team calculated the average per-ton environmental costs of this activity by using previously reported assessments of the extent of stream impairment and loss of carbon sequestration potential associated with every hectare of land mined.”Given 11,500 tons of coal was produced for every hectare of land disturbed, we estimate 0.25 centimeters of stream length was impaired and 193 grams of potential carbon sequestration was lost for every ton of coal extracted,” said Emily S. Bernhardt, associate professor of biogeochemistry at Duke’s Nicholas School of the Environment.That doesn’t sound like much until you put it in perspective, she stressed.”Based on the average carbon sequestration potential of formerly forested mine sites that have been reclaimed into predominantly grassland ecosystems, we calculate it would take around 5,000 years for any given hectare of reclaimed mine land to capture the same amount of carbon that is released when the coal extracted from it is burned for energy,” she said.”Even on those rare former surface mines where forest regrowth is achieved, it would still take about 2,150 years for the carbon sequestration deficit to be erased,” said Lutz, who earned his PhD from Duke in 2011.”This analysis shows that the extent of environmental impacts of surface mining practices is staggering, particularly in terms of the relatively small amount of coal that is produced,” said William H. Schlesinger, president of the Cary Institute of Ecosystem Studies in Millbrook, N.Y. Schlesinger is James B. …

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An unprecedented threat to Peru’s cloud forests

Sep. 11, 2013 — Peru’s cloud forests are some of the most biologically diverse ecosystems in the world. A profusion of tree and plant species as well as one third of Peru’s mammal, bird and frog species make their home in these perennially wet regions, located along the eastern slopes of the Andes Mountains. The high elevation (6,500-11,000 feet), and remote location of these areas makes them some of the hardest to reach and therefore hardest to study ecosystems in the world. To date, scientists only believe a fraction of cloud forest tree and plant species have been discovered.This massive array of underexplored biodiversity will face an unprecedented threat before the end of the century.Now, researchers at Wake Forest University in Winston-Salem, N.C. have pieced together startling new evidence that shows rapid 21st century warming may spell doom for tree species in Peruvian cloud forests, with species losing 53-96 percent of their populations.Stuck in a Hot PlaceThe habitats of most Andean plants-and therefore the habitats of the organisms that use them for food and shelter- are determined largely by temperature. Temperatures change quickly on the slopes of the Andes due to the region’s steep terrain. This means the vast majority of trees and plants only can live in a range that extends a few hundred meters.”I could be standing among a group of one tree species and throw a rock completely across their ranges,” says David Lutz, the paper’s lead author and a former postdoctoral associate at Wake Forest University. Lutz, who is now a post-doctoral research associate at Dartmouth College in New Hampshire, says this means cloud forest trees are particularly sensitive to climate change.Historically, Andean cloud forest seedlings sprout higher in elevation during periods of global warming. However, an unprecedented rate of projected temperature gain in the region over the next century, 5 degrees Celsius, will have them going upslope faster than ever before, says Miles Silman, professor of Biology at Wake Forest University. …

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Valley networks suggest ancient snowfall on Mars

July 23, 2013 — Researchers at Brown University have shown that some Martian valleys appear to have been caused by runoff from orographic precipitation — moisture carried part of the way up a mountain and deposited on the slopes. Valley networks branching across the Martian surface leave little doubt that water once flowed on the Red Planet. But where that ancient water came from — whether it bubbled up from underground or fell as rain or snow — is still debated by scientists. A new study by researchers at Brown University puts a new check mark in the precipitation column.The study finds that water-carved valleys at four different locations on Mars appear to have been caused by runoff from orographic precipitation — snow or rain that falls when moist prevailing winds are pushed upward by mountain ridges. The new findings are the most detailed evidence yet of an orographic effect on ancient Mars and could shed new light on the planet’s early climate and atmosphere.A paper describing the work has been accepted by Geophysical Research Letters and published online in June.Kat Scanlon, a geological sciences graduate student at Brown, led the research and is well-acquainted with the orographic effect. She did graduate work in meteorology in Hawaii, which is home to a quintessential orographic pattern. Moist tropical winds from the east are pushed upward when they hit the mountains of Hawaii’s big island. The winds lack the kinetic energy to reach the mountain summit, so they dump their moisture on the eastern side of the island, making parts of it a tropical jungle. The western side, in contrast, is nearly a desert because it sits in a rain shadow cast by the mountain peak.Additional modeling might determine how fast Martian snow could have melted and whether snowmelt alone could have carved the valleys.Scanlon thought similar orographic patterns might have been at play on early Mars and that the valley networks might be an indicator. “That’s what immediately came to mind in trying to figure out if these valleys on Mars are precipitation related,” she said.The researchers, including Jim Head, professor of geological sciences, started by identifying four locations where valley networks were found along tall mountain ridges or raised crater rims. …

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Evolution too slow to keep up with climate change

July 9, 2013 — Many vertebrate species would have to evolve about 10,000 times faster than they have in the past to adapt to the rapid climate change expected in the next 100 years, a study led by a University of Arizona ecologist has found.Scientists analyzed how quickly species adapted to different climates in the past, using data from 540 living species from all major groups of terrestrial vertebrates, including amphibians, reptiles, birds and mammals. They then compared their rates of evolution to rates of climate change projected for the end of this century. This is the first study to compare past rates of adaption to future rates of climate change.The results, published online in the journal Ecology Letters, show that terrestrial vertebrate species appear to evolve too slowly to be able to adapt to the dramatically warmer climate expected by 2100. The researchers suggested that many species may face extinction if they are unable to move or acclimate.”Every species has a climatic niche which is the set of temperature and precipitation conditions in the area where it lives and where it can survive,” explained John J. Wiens, a professor in UA’s department of ecology and evolutionary biology in the College of Science. “For example, some species are found only in tropical areas, some only in cooler temperate areas, some live high in the mountains, and some live in the deserts.”Wiens conducted the research together with Ignacio Quintero, a postgraduate research assistant at Yale University.”We found that on average, species usually adapt to different climatic conditions at a rate of only by about 1 degree Celsius per million years,” Wiens explained. “But if global temperatures are going to rise by about 4 degrees over the next hundred years as predicted by the Intergovernmental Panel of Climate Change, that is where you get a huge difference in rates. What that suggests overall is that simply evolving to match these conditions may not be an option for many species.”For their analysis, Quintero and Wiens studied phylogenies — essentially evolutionary family trees showing how species are related to each other — based on genetic data. These trees reveal how long ago species split from each other. The sampling covered 17 families representing the major living groups of terrestrial vertebrates, including frogs, salamanders, lizards, snakes, crocodilians, birds and mammals.They then combined these evolutionary trees with data on the climatic niche of each species to estimate how quickly climatic niches evolve among species, using climatic data such as annual mean temperature and annual precipitation as well as high and low extremes.”Basically, we figured out how much species changed in their climatic niche on a given branch, and if we know how old a species is, we can estimate how quickly the climatic niche changes over time,” Wiens explained. …

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Farming started in several places at once: Origins of agriculture in the Fertile Crescent

July 5, 2013 — For decades archaeologists have been searching for the origins of agriculture. Their findings indicated that early plant domestication took place in the western and northern Fertile Crescent. In the July 5 edition of the journal Science, researchers from the University of Tübingen, the Tübingen Senckenberg Center for Human Evolution and Paleoenvironment, and the Iranian Center for Archaeological Research demonstrate that the foothills of the Zagros Mountains of Iran in the eastern Fertile Crescent also served as a key center for early domestication.Archaeologists Nicholas Conard and Mohsen Zeidi from Tübingen led excavations at the aceramic tell site of Chogha Golan in 2009 and 2010. They documented an 8 meter thick sequence of exclusively aceramic Neolithic deposits dating from 11,700 to 9,800 years ago. These excavations produced a wealth of architectural remains, stone tools, depictions of humans and animals, bone tools, animal bones, and — perhaps most importantly — the richest deposits of charred plant remains ever recovered from the Pre-Pottery Neolithic of the Near East.Simone Riehl, head of the archaeobotany laboratory in Tübingen, analyzed over 30,000 plant remains of 75 taxa from Chogha Golan, spanning a period of more than 2,000 years. Her results show that the origins of agriculture in the Near East can be attributed to multiple centers rather than a single core area and that the eastern Fertile Crescent played a key role in the process of domestication.Many pre-pottery Neolithic sites preserve comparatively short sequences of occupation, making the long sequence form Chogha Golan particularly valuable for reconstructing the development of new patterns of human subsistence. The most numerous species from Chogha Golan are wild barley, goat-grass and lentil, which are all wild ancestors of modern crops. These and many other species are present in large numbers starting in the lowest deposits, horizon XI, dating to the end of the last Ice Age roughly 11,700 years ago. In horizon II dating to 9.800 years ago, domesticated emmer wheat appears.The plant remains from Chogha Golan represent a unique, long-term record of cultivation of wild plant species in the eastern Fertile Crescent. Over a period of two millennia the economy of the site shifted toward the domesticated species that formed the economic basis for the rise of village life and subsequent civilizations in the Near East. …

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Was prehistoric rock art strategically placed to reveal a cosmological puzzle?

June 19, 2013 — It is likely some of the most widespread and oldest art in the United States. Pieces of rock art dot the Appalachian Mountains, and research by University of Tennessee, Knoxville, anthropology professor Jan Simek finds each engraving or drawing is strategically placed to reveal a cosmological puzzle.Recently, the discoveries of prehistoric rock art have become more common. With these discoveries comes a single giant one — all these drawing and engravings map the prehistoric peoples’ cosmological world.The research led by Simek, president emeritus of the UT system and a distinguished professor of science, is published in this month’s edition of the journal Antiquity. The paper is co-authored by Nick Herrmann of Mississippi State University, Alan Cressler of the U.S. Geological Survey and Sarah Sherwood of The University of the South.The researchers proposed that rock art changed the natural landscape to reflect a three-dimensional universe central to the religion of the prehistoric Mississippian period.”Our findings provide a window into what Native American societies were like beginning more than 6,000 years ago,” said Simek. “They tell us that the prehistoric peoples in the Cumberland Plateau, a section of the Appalachian Mountains, used the rather distinctive upland environment to map their conceptual universe onto the natural world in which they lived.”Simek and his team analyzed 44 open- air art sites where the art is exposed to light and 50 cave art sites in the Cumberland Plateau using nondestructive, high-tech tools, such as a high-resolution laser scanner. Through analysis of the depictions, colors, and spatial organization, they found that the sites mimic the Southeastern native people’s cosmological principles.”The cosmological divisions of the universe were mapped onto the physical landscape using the relief of the Cumberland Plateau as a topographic canvas,” said Simek.The “upper world” included celestial bodies and weather forces personified in mythic characters that exerted influences on the human situation. Mostly open-air art sites located in high elevations touched by the sun and stars feature these images. Many of the images are drawn in the color red, which was associated with life.The “middle world” represented the natural world. A mixture of open air and cave art sites hug the middle of the plateau and feature images of people, plants and animals of mostly secular character.The “lower world” was characterized by darkness and danger, and was associated with death, transformation and renewal. …

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New species of marine fish from 408 million years ago discovered in Teruel

June 3, 2013 — Researchers from the University of Valencia and the Natural History Museum of Berlin have studied the fossilised remains of scales and bones found in Teruel and the south of Zaragoza, ascertaining that they belong to a new fish species called Machaeracanthus goujeti that lived in that area of the peninsula during the Devonian period. The fossils are part of the collection housed in the Palaeontology Museum of Zaragoza.In the journal Geodiversitas, a research team led by the University of Valencia describes a new species of spiny shark (Acanthodii), a primitive type of fish that shared characteristics with sharks and bony fish. Remains of scales, bones and scapular joint bones were found in Devonian (approximately 408 million years ago) in Teruel and the south of Zaragoza.The paper also includes an analysis of fossils of a fragmented spine and isolated scales from the Lower Devonian found in northern Spain (Palencia and Cantabrian Mountains) and western France (Saint-Céneré commune), originally attributed to the Machaeracanthus sp species.”The discovery of this new species, which we call Machaeracanthus goujeti and belongs to the Acanthodii group -of which very little is known-, expands our knowledge of the biodiversity that existed on the peninsula 480 million years ago, when the modern-day region of Teruel was covered by the sea,” Héctor Botella, professor in the palaeontology unit in the University of Valencia and the study’s lead author, explained.The Acanthodii group of fish are also known as ‘spiny sharks’ owing to their appearance and, from what we know to date, they only lived during the Palaeozoic Era and reached their maximum level of diversity in the Devonic period. However, the bones typically found in the Acanthodii group grow differently to the bones found, therefore this type could be even more similar to sharks and would date from the very early stages of the radiation of jawed vertebrates (gnathostomata).A fish fossil no more than one metre in lengthThe majority of the samples found by the researchers are juveniles. Based on the fossilised remains, the researchers estimate that the largest fish in this species would not reach one metre in length. “This is just an estimation because there are animals that can have large bones and be small, and vice versa,” Botella stated.For their part, the fossils found in the sediment layers of the Iberian mountain range must surely have belonged to fish that swam close to the coast. “In other words, they must have lived in an epicontinental sea -an extensive but shallow salt water mass-, and it is therefore possible that this area was used as a breeding ground,” he concludes. Larger fossils were found in sediment layers a little further down.The fossils form part of the collection housed in the Palaeontology Museum of Zaragoza.

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Loss of eastern hemlock will affect forest water use

May 9, 2013 — The loss of eastern hemlock from forests in the Southern Appalachian region of the United States could permanently change the area’s hydrologic cycle, reports a new study by U.S. Forest Service scientists at the Coweeta Hydrologic Laboratory (Coweeta) located in Otto, North Carolina, published online in the journal Ecological Applications and available now in preprint format.

“The hemlock woolly adelgid, an exotic invasive insect, has caused widespread hemlock mortality,” says Steven Brantley, a post-doctoral researcher at Coweeta and lead author of the paper. “Hemlock decline is expected to have a major impact on forest processes, including transpiration.”

Transpiration describes the loss of water from plant leaves or needles. Coweeta researchers estimated changes in transpiration at the forest-level since hemlock woolly adelgid infestation by monitoring tree water use and changes in forest composition from 2004 to 2011. The four studied stands were once dominated by eastern hemlock trees, and are located in the Coweeta watersheds.

Because of its dense evergreen foliage and dominance in riparian and cove habitats, eastern hemlock plays an important role in the area’s water cycle, regulating stream flow year round. The loss of hemlock from southern Appalachian forests can be compared to the loss of American chestnut from eastern forests, which became functionally extinct after the introduction of an exotic fungus in the early 20th century. Changes in local forest hydrology from the loss of eastern hemlock will largely depend on which species replace it.

Rhododendron, a woody evergreen shrub common in southern Appalachian forests, is one of the species replacing eastern hemlock trees. Although rhododendron is evergreen, it has lower leaf area than hemlock, and thus transpiration in rhododendron-dominated forest stands is lower than in previously-healthy hemlock forests. Most of the other species replacing eastern hemlock trees are deciduous, such as sweet birch, which unlike the evergreen rhododendron and eastern hemlock, do not transpire during the winter. Sweet birch trees also have a much higher transpiration rate than eastern hemlock trees during the growing season.

“The cumulative effect of these species changes will probably mean permanent changes in seasonal transpiration patterns,” says Brantley. “In the growing season, transpiration rates will likely rise, leading to lower stream flow in the summer. However, transpiration rates in the winter will be reduced, which could cause increased winter stream discharge.” Whatever species eventually replace eastern hemlock, there will be important long-term implications for riparian habitats beyond stream discharge. Without the shade provided by eastern hemlock, stream temperatures could rise, threatening aquatic animals like eastern brook trout that require cold water for survival. The loss of eastern hemlock will not only affect the animal and plant communities in riparian habitats, but ecosystem function throughout these areas.

The study was conducted at the U.S. Forest Service Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina. Coweeta is one of the oldest continuous environmental studies in North America. Since 1934, precipitation, temperature, and stream flow have been continuously recorded at Coweeta, a U.S. Forest Service Southern Research Station facility.

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