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|  Trees regulate leaf temperature
Tree leaves hold a constant 21.4C when photosynthesizing From Canada To The Caribbean: Tree Leaves Control Their Own Temperature, Study Reveals Quote: |
From Canada To The Caribbean: Tree Leaves Control Their Own Temperature, Study Reveals
ScienceDaily (Jun. 11, 2008) — The temperature inside a healthy, photosynthesizing tree leaf is affected less by outside environmental temperature than originally believed, according to new research from biologists at the University of Pennsylvania.
Surveying 39 tree species ranging in location from subtropical to boreal climates, researchers found a nearly constant temperature in tree leaves. These findings provide new understanding of how tree branches and leaves maintain a homeostatic temperature considered ideal for photosynthesis and suggests that plant physiology and ecology are important factors to consider as biologists tap trees to investigate climate change.
Tree photosynthesis, according to the study, most likely occurs when leaf temperatures are about 21°C, with latitude or average growing-season temperature playing little, if any, role. This homeostasis of leaf temperature means that in colder climates leaf temperatures are elevated and in warmer climates tree leaves cool to reach optimal conditions for photosynthesis. Therefore, methods that assume leaf temperature is fixed to ambient air require new consideration.
"It is not surprising to think that a polar bear in northern Canada and a black bear in Florida have the same internal body temperature," Brent Helliker, professor of biology in the School of Arts and Sciences at Penn, said. "They are endothermic mammals like us ,and they generate their own heat. However, to think that a black spruce in Canada and a Caribbean pine in Puerto Rico have the same average leaf temperature is quite astonishing, particularly since trees are most definitely not endothermic. Our research suggests that they use a combination of purely physical phenomena -- like the cooling from water evaporation or the warming caused by packing a lot of leaves together -- to maintain what looks like leaf-temperature homeostasis."
Leaf temperature, cooled by the physiological and morphological techniques of evaporation, leaf angle or reflection and heated by a decrease in evaporation and an increase in the number of leaves per branch, can now be considered adaptations towards achieving homeostasis. Researchers do not suggest that tree canopies maintain a constant temperature through a day or a season, but rather that this ideal temperature is a long-term target value.
The study also presents a new hypothesis for why certain trees grow in certain climates and provides a new theory for how and why trees in the north will suffer from global warming, by overheating due to the mechanisms they have evolved to keep their leaves warm.
In addition, weather-forecasting models rely on accurate estimates of surface-water evaporation, much of which comes from tree leaves. Knowing the temperature of these leaves is crucial to an accurate prediction of future climate scenarios.
The research, published online in this week's Nature, contradicts the longstanding assumption that temperature and relative humidity in an actively photosynthesizing leaf are coupled to ambient air conditions. For decades, scientists studying climate change have measured the oxygen isotope ratio in tree-ring cellulose to determine the ambient temperature and relative humidity of past climates. The assumption in all of these studies was that tree leaf temperatures were equal to ambient temperatures.
Researchers at Penn, using measures of oxygen isotopes and current climate, determined a way to estimate leaf temperature in living trees and as a consequence showed this assumption to be incorrect.
This is an unfortunate finding for the potential to reconstruct climate through tree-ring isotope analysis but a boon to ecologists because it creates potential for the reconstruction of tree responses to both average climate and climate change over the last couple of centuries.
The team used their method to reconstruct tree canopy leaf temperatures in 39 species across 50 degrees of latitude and found that in warmer climates leaves were cooler than ambient temperatures and in cooler climates the opposite was true. Perhaps the more remarkable finding was that across such a large area and across so many types of tree that the leaves seemed to be operating at the same temperature, probably a result of natural selection acting to maintain optimal temperature for photosynthesis in the face of widely varying ambient climates.
The continental-scale dataset used in this research was part of a separate study done by Suzanna Richter, a contributor to this study and post-doctoral researcher in the Department of Earth and Environmental Science at Penn. "Multi-million-year-old wood that colleagues and I collected in Arctic Canada and Siberia was so well preserved that it both looked like modern wood and burned like modern wood. Its fantastic preservation brought about questions as to whether the chemistry of the wood could be used to determine the climate that the trees grew in millions of years ago."
Although there are few studies that compare tree-canopy temperatures throughout an entire growing season, a recent study using infrared thermal imaging of a mixed forest in Switzerland agreed with the current study: canopy temperature was 4-5 degrees (Celsius) higher than the cool, ambient air temperature of Switzerland.
Funding for this study was provided by the Department of Biology at the University of Pennsylvania and the Andrew W. Mellon Foundation.
| http://www.sciencenews.org/view/gene...ks_tree_leaves Quote:
Sweating in the heat or huddling in the cold keeps temperatures favorable
By Susan Milius
July 5th, 2008; Vol.174 #1
Tree leaves do a pretty good job of achieving temperatures that are just right for photosynthesis, even if it’s too hot or too cold where they live, a new study shows.
From roughly the top to the bottom of North America, across some 50 degrees of latitude, trees all do their photosynthesizing at leaf temperatures around 21.4° Celsius plus or minus 2.2 degrees, says physiological ecologist Brent Helliker of the University of Pennsylvania in Philadelphia. That conclusion was based on a broad survey of the ratios of two forms of oxygen that vary depending on the temperature and humidity of leaves. Those properties control evaporation and make a signature in the cellulose of the tree rings, Helliker and colleague Suzanna Richter report in an upcoming Nature.
Such temperature control undermines the assumption that the insides of leaves have the same temperature as the air, Helliker says. That’s an assumption underlying studies that check oxygen ratios in old tree tissue to reconstruct past climates, he says.
The tree-ring community is just starting to sort out what the finding means. “I, and I am sure my colleagues in isotope dendroclimatology, will welcome this paper because it improves our understanding of the complex relationship between climate and the stable isotope ratios in wood,” says Danny McCarroll of the University of Wales Swansea. However, he objects to Helliker’s claim that paleoclimatologists’ approaches have relied so heavily on whether leaf temperatures match those of the surrounding air.
Those paleoclimatology methods for using isotopes in tree rings to reconstruct climate have been validated by observations, says Jan Esper of the Swiss Federal Research Institute in Birmensdorf. “From this perspective, the findings by Helliker and Richter are indeed surprising, as I would have expected a closer association between leaf and surrounding air temperature,” he says.
Helliker says he has been bugged for years by the assumption that a tree leaf photosynthesized at whatever the local air temperature might be. Trees release water, and during hot times, that botanical sweat cools them down. And trees that grow in cold places tend to cluster their leaves. These tight formations can affect the rate at which leaves lose heat on cold days, just as fingers pressed together in mittens stay warmer than fingers separated by space in gloves.
Physiologists, of course, could measure the temperature on individual leaves, but measuring enough leaves to give a picture of the canopy has been difficult. Helliker estimates that scientists would need at least 140 leaves to get a valid reading for the temperature of photosynthesis of a single tree.
His colleague Richter, however, had collected tree ring data for another project, and Helliker realized it would be perfect to test his idea. Richter had not only recorded oxygen ratios in the tree rings, but had also collected data from nearby weather stations on relative humidity. Since she knew the humidity, the researchers could calculate what the leaf temperature must have been to produce particular ratios of oxygen isotopes. When the leaf is photosynthesizing, the sugars it produces include oxygen in the temperature-sensitive ratio. The cellulose in tree rings made from these sugars thus indicates the leaf temperature during photosynthesis.
“What I like about this paper is the fact that it highlights the need to account for actual life conditions,” says Christian Körner of the University of Basel in Switzerland.
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14th June 2008, 09:45 AM
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