Washington State University

Small Farms Team

Converting Christmas trees to forests takes gradual thinning

Tiffany Woods
The Capital Press Newspaper, April, 19, 2002

Oregon City, Ore. - What do you do with out-of-control Christmas trees that are too big to be harvested? "One option is to sell the property," said woodland manager Gilbert Shibley.

Because not everyone wants to take such a drastic step, he also suggested owners convert the trees into a forest for timber production. But it is not as easy as thinning a few trees and then doing nothing for the next 40 years, said Shibley, who lives on a tree farm near Estacada, OR.

Thinning too quickly can shock the trees and make them vulnerable to sunburns, wildfires and wind, he said. First, however, you have to decide if the soil is adequate and if the trees are healthy. Ideally, they should come from a local seed source, he told a class at the Tree School Seminar at Clackamas Community College on April 6.

Tree farmers typically plant 1,500 Christmas trees per acre, but forests should have only 300 to 500 trees per acre because they need more light, water and nutrients, said Shibley, who works part-time for Oregon State University.

"Get those 1,500 ex-Christmas trees down to 500 or less by age 15 if possible," he said. As a rule of thumb for spacing, measure the trees' average diameter at breast height in inches and add four to it, he said. That tells you how many feet should be between each tree. For example, an 8-inch diameter plus 4 means that the trees should be spaced every 12 feet. Shibley's calculations are for Douglas firs.

He described several conversion schedules. If you start thinning when the trees are 10 years old, you should cut every third row, leaving 1,000 trees. Then when they are 15 years old, select out the worst third. Do the same at years 19 and 22 so that you end up with 297 trees, he said.

A more aggressive option is to cut every third row and the third worst in the remaining rows at year 10, leaving 667 trees, he said. Then at year 15 select out the worst third and in year 20 thin the worst half to finish with 224 trees remaining, he said.

If you get a late start and begin thinning when the trees are 15 years old, Shibley recommended cutting every third row, leaving 1,000 trees. Then when they are 18 years old, remove the worst third. Four years later, remove the worst half and do that again when the trees are 27 years old, to wind up with 167 trees, he said.

While you wait, you can sell some of the thinned trees.

A quicker approach is to cut every third row and the worst 33 percent to 50 percent of the remaining rows when the trees are 15 years old. Then at 20 years old, remove the worst half of the remaining trees, leaving you with 250 to 333 trees.

A last-ditch effort would be to start thinning the trees at 20 years old, he said, cutting every second row. Three years later, select out the worst third. Finally, when the trees are 26 years old, thin the worst half, leaving 250 trees.
Amore aggressive approach is to cull every third row and the worst remaining half at year 20 so that you have 500 trees standing. When they are 24 years old, thin the worst third and repeat at year 28, leaving 224 trees, he said.

Shibley said the schedules can be modified to fit an individual's goals. But one thing to keep in mind is to make sure that the top approximately 40 percent of a tree stays green, he said.

Trees are ready to be harvested for saw logs when they are 9 inches to 10 inches in diameter at breast height, he said.

Turning to the economics of producing Douglas fir for timber, Shibley said, "It's reasonable to think you can earn as much per acre per year with forestry as with Christmas trees. The difference is you have to wait 20 years."

The good news is that while you wait, you can sell some of the thinned trees, he said. By year 20 to 25, a thinning can return $800 to $1,600 per acre, he said. When the trees are 70 years old, an acre can net $20,000 to $24,000, he said. That figure includes profits from thinnings at 10-year intervals, he said. Averaged over the long term, a 70-year rotation would net up to about $350 per acre annually, he said. His estimates were based on log prices of $600 to $700 per 1,000 board feet.

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