Old growth giants limited by water-pulli...( CORVALLIS Ore. The Douglas-fir stat...)

CORVALLIS, Ore. The Douglas-fir, state tree of Oregon, towering king of old-growth forests and one of the tallest tree species on Earth, finally stops growing taller because it just can't pull water any higher, a new study concludes.

This limit on height is somewhere above 350 feet, or taller than a 35-story building, and is a physiological tradeoff between two factors in the tree's wood - a balance between efficiency and safety in transporting water to the uppermost leaves.

The findings are being published this week in Proceedings of the National Academy of Sciences, by a team of scientists from Oregon State University and the U.S.D.A. Forest Service. The research was funded by grants from the U.S. Department of Agriculture and the Forest Service.

"People have always been fascinated by how some trees, such as Douglas-fir or redwoods, can grow so tall," said Barb Lachenbruch, a professor of wood science at Oregon State University. "This is not an easy thing to do. Think about trying to drink water through a narrow, 350-foot-long straw. It takes a lot of suction."

Douglas-fir wood consists mostly of dead cells called "tracheids" that function in water transport and physical support, the researchers said. These tracheids have pits on their sides that function as valves, allowing water to go from one tracheid to the next, and the pits have a membrane with an impermeable middle. Normally, water flows through the porous edges of the membrane, but if there's an air bubble in one tracheid, the membrane moves to the side and blocks off the pit so air bubbles can't spread.

Although it's important to allow water to pass efficiently from one wood cell to the next, air bubbles would block water movement altogether. Because water is pulled through a tree by the forces of evaporation from the leaf surfaces, the water is in "tension," like a pulled rubber band. If an air bubble gets in, it's like the rubber band breaking
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