Study: Trees suffer 'drought' in winter

July 25, 2011 at 8:35 PM
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CORVALLIS, Ore., July 25 (UPI) -- Researchers say conifers of the U.S. Pacific Northwest face greater water stress in the region's wet winters than in summer when weeks can pass without rain.

Scientists from Oregon State University and the U.S. Forest Service say their studies show freeze-thaw cycles in winter can disrupt water flow by causing air bubbles to form in the conductive xylem, the layer of transport tissue, of the region's tall conifer trees, leaving them seriously stressed for water when they are practically standing in a lake of it.

"Everyone thinks that summer is the most stressful season for these trees, but in terms of water, winter can be even more stressful," said Katherine McCulloh, a research assistant professor in the OSU Department of Forest Ecosystems and Society.

"We've seen trees in standing water, at a site that gets more than 2 meters [more than 6 feet] of rain a year, yet the xylem in the small branches at the tops of these trees can't transport as much water as during the summer," McCulloh said.

Trees such as Douglas-fir actually do better dealing with water issues during summer when they simply close down the pores in their leaves and stems to conserve water and reduce their photosynthesis and growth rate, the researchers said.

Studies such as this are important, the scientists said, to better understand how forests might respond to a warmer or drier climate of the future, an OSU release reported Monday.

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