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Study finds how Christmas trees stay green so long
Scripps Howard News Service


December 23, 2005

Every December, some millions of pine trees and other evergreens are plopped into stands full of water in hopes that the needles stay on until St. Nick has come and gone.

Usually, a fresh-cut or even not-so-fresh-cut tree gulps up the water at a remarkable pace - a feat even more impressive to botanists and biologists, who know about the plumbing structure of conifers compared to the trunks of flowering trees such as oaks or maples.




Water moving up the trunk of a pine has to move through minute conduits that are 10 times shorter than those found in flowering trees, a trait left over from primitive ancestors that arose some 400 million years ago.

The Tannenbaum family gets around this handicap by having microscopic valves at the ends of those water-carrying pipes that are up to 59 times more efficient than in flowering trees, according to a new study published Friday.

Without this water-transfer efficiency, conifers might have been competed out of existence millions of years ago, the researchers say.

"When you are sitting around admiring your Christmas tree, consider that it owes its existence in part to this clever microscopic valve,'' said John Sperry, a University of Utah biology professor who headed the research team. The findings appear in the Dec. 23 issue of the journal Science.

"Without the valves, conifers could be much less common than they are, and conceivably their survival might be marginal,'' Sperry added.

Instead, conifers thrive and actually dominate forests in many parts of the Earth. The tallest trees, redwoods and sequoias, are conifers, as are the oldest trees, bristlecone pines.

Although scientists were aware that the valves between water tubes were different in conifers from those of flowering trees, also called angiosperms, they didn't know how that difference affected how easily water flowed through them.

The study was designed as part of a doctoral thesis by Jarmila Pitterman, now a postdoctoral fellow at the University of California. She, Sperry and colleagues set up a test to measure water flow through twigs from 18 species of conifers and 29 species of angiosperms.

They connected both ends of each twig to plastic tubing, used an elevated reservoir to force water into one end, and then used an electronic scale to weigh the water that dripped out the other end in a set time.

Then, based on the number of tubes and valves in the twigs, and their known dimensions, they could calculate the resistance to water flow in each structure.

They found that for conduits of the same diameter, resistance to water flow was only 1.2 times greater in the pines than in flowering trees - virtually the same. And even though flowering trees have many more valves at the end of each tube, water flow was actually better in the pines and cedars than in the oaks, willows and ash that they tested, in terms of resistance to flow per unit area of wood.

While conifers have shorter pipes and fewer valves, "they compensate for that because each individual valve is so much more efficient,'' Sperry said. "The flow resistance through a valve of a given size is 59 times lower in a Christmas tree than in an oak tree."

So even if vacuuming up the needles next week is a pain, consider how hard that tree worked to stay green as long as it did.


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Contact Lee Bowman at BowmanL(at)

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