"Higher and higher in the tree, the valves are able to withstand more pulling force from the long heavy column of water before air bubbles can be sucked through," Lachenbruch said. "But the problem is that the valves become less efficient at letting water pass. The height at which no water would pass at all, according to our models, coincides the tallest records for Douglas-fir, about 350 to 400 feet."
Trees of that height were discovered in Washington and British Columbia in the late 19th and early 20th centuries. The tallest Douglas-fir today is a 326-foot-tall tree in Coos County, Oregon.
"As you go higher and higher in a Douglas-fir tree, it's almost like experiencing a drought," said Rick Meinzer, a Forest Service scientist at the Pacific Northwest Research Station. "And that's what we see at the tops of very tall trees. The foliage is struggling to get enough water and seems to be under drought stress. It's not unusual to see periodic die-back at the tops of very tall Douglas-fir trees that are near their height limits."
At a specific height determined by the physical structure of these pits and their membranes, the scientists discovered, the fierce resistance put up by the Douglas-fir to prevent any spread of air bubbles also prevents water from being pulled any higher. That is where it finally stops growing in height, no matter how favorable any other conditions might be, such as climate, soil or water availability.
The studies, Meinzer said, may improve our understanding of how trees grow in height and may be able to adapt to different environments, including their ability to deal with droughts or climate change.
Although height can be important in a competition for sunlight and photosynthesi
|Contact: Barb Lachenbruch|
Oregon State University