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Christmas Tree Blight (video)
December 13, 2001

Also on ScienCentral News

Rings of Fire (video) - According to those who can read trees like history books, wildfires raging across the country should bring considerations of a new wildland management policy based on more controlled burning. (9/6/01)

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With with the holiday season in full swing, many people are heading out to get their annual Christmas tree.

But as this ScienCentral News video reports, one of the most popular kinds of tree is being threatened in its natural environment.

The secret of evergreens

For Christmas tree aficionados, there are several desirable traits to look for when choosing a real tree: a pleasant aroma, natural shape, attractive color, strong branches, and perhaps most importantly, the ability to hold onto its needles. One species that has all of these traits is the Fraser fir, which has become one of the premier varieties of Christmas tree.

"It has excellent post harvest needle retention, so that allows us to be able to cut Fraser fir very early—as early as October—and ship it across the country," says John Frampton, associate professor in the department of forestry at North Carolina State University. "And it still retains its needles very well and is fresh when it arrives in peoples homes—through Christmas and on past New Years."

But needle retention is largely a family trait that has given trees like firs, spruce and pines the nickname of evergreen. One species of pine, the Bristlecone (which counts among it’s ranks what is believed to be the oldest living thing), hangs onto its needles for as long as 30 years. Why do evergreens or coniferous trees hold their needles through the winter while deciduous trees lose their leaves? A dispatch from ScienCentral News’s Q&A; archive has the answer.

According to Roger C. Funk, plant physiologist, vice president and general manager of The Davey Institute:

Evergreen leaves actually remain on for two to three years, in general, and then in the fourth year they usually drop off. Other leaves, like deciduous leaves, fall off at the end of the every year.

When you’re talking about a tree, the annual rings that make up the trunk of the tree are actually an accumulation of the tubes that make up the veins of the leaves in that particular year. In other words, if you collect all of those tubes that make up the veins of the leaves that are produced this year, they come out of the leaves, onto the branch, down to the trunk, and they circle around last year’s trunk and make the new annual ring. So the leaves that are produced this year have a direct connection down to the roots. There is a direct pipeline between the roots and the new crop of leaves each year.

With the deciduous tree, typically you only have that annual ring functional for one year. So as the annual ring begins to close down and no longer supports the transportation from the roots to the leaves and vice versa, then the leaves must also shut down. There’s other things involved too—photo periodism, the effect of light, hormones—but basically what it comes down to is a deciduous tree will only have leaves on the tree one year because the annual ring is only functional for one year. With your evergreens, your annual rings remain functional for two to three years, therefore they can support the leaves for that period of time. So evergreens actually lose leaves every year.

If you look at an evergreen, a lot of them, not all of them, produce growth in whirls. If you look at pine for example, go back to the trunk and as you come out from it you produce a length of branch that is that year’s growth and at the end of the branch tip it produces a terminal bud. Next year’s growth comes from that terminal bud, so the next year’s length will start from that point, ending in another terminal bud. And then the next year the growth from that terminal bud starts. And when you get out about four lengths, the oldest needles—which are on the ones inside, nearest the trunk—drop, because they’re no longer supported by annual rings. And then next year it produces another one to the outside, and the next one out drops. So as long as the needles are dropping from the inside, that’s normal. But if you watch the needles drop on the outside, there’s something wrong with the tree. Sometimes they’ll drop it suddenly if the tree is under stress, and people will notice and they’ll think that something’s really, really wrong. But again, as long as it’s only dropping the needles on the inside of the tree, the oldest needles first, the tree may be stressed but there’s nothing really wrong with it.

That’s why you see kind of a bed of needles around the trunk. If you look you can see how far back in, and you’ll see about two or usually three years of growth where the needles are attached and then all the older growth needles have fallen off one year at a time.

Obviously, this is all in the genetic makeup of the tree. The genetics control the hormones of the tree, the longevity of the annual rings, and that in turn controls how long the leaves can stay on.

—intro by Curt Epstein; interview by Jill Max


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