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November 2, 2008
Tending the Forest Garden: Thinning
Master Gardener Column 11/1/08
An attractive part of our gardens, ponderosa pines need our care as well as the other plants. We don't live in a centuries-old, self-sustaining forest primeval. Once we intruded into the forest, the trees became our care. Sad to say, past land management practices have often resulted in ponderosa pine stands that are too dense with the result that our forests are prone to catastrophic wildfires, drought damage, and bark beetle outbreaks. In the Southwest, we are now faced with determining appropriate ways to care for the ponderosa and to prevent further deterioration.
The best way to reduce the threat of wildfires, damage by drought, and bark beetle attack is to thin the trees. An additional benefit is that remaining trees will grow more rapidly. Research has shown that tree diameters increase at a faster rate in stands with lower densities.
Thinning can also increase under-story plant density in forests. Herbaceous plant growth, both grasses and wildflowers, typically increase when additional light reaches the forest floor.
Some people believe that thinning may spoil the scenic beauty. Studies on scenic beauty in ponderosa pine stands have shown that less dense or more open stands have greater scenic value. The major reasons why people prefer less dense forests is the increase in herbaceous plant growth and the health of the remaining mature trees.
Although the last significant bark beetle outbreak in Arizona occurred five years ago, there are still beetles in the forest waiting for the right conditions. Poor sanitation practices, leaving refuse on the forest floor following thinning, can be just what bark beetles need to get going.
If you are thinking about thinning your ponderosa pines, autumn is a good time to begin because bark beetles are less active when the weather turns cold. When temperatures warm up in the spring, bark beetles can be most problematic and sanitation is most important.
Good sanitation is removing newly cut trees on the ground, not leaving them for more than 30 days. Within the first 30 days the material should be hauled off the property to a landfill where it will be buried or it can also be chipped on site. When chipping, don't pile the chips next to live trees as the chips may attract bark beetles. Keep chip piles in the open sun and as far from live trees as possible.
The small slash, limbs and tops less than 3 inches in diameter, can be used by adult beetles, but they won't produce much brood from it. The small material can be chipped or piled for burning. When piling, it's best to put the smallest material in the middle with the largest on the outside of the pile.
When trees are thinned, many people want to keep some of the material for firewood. If the thinning is done in the spring or early summer the wood should be covered with 6 mil plastic to keep the beetles from attacking it. Green firewood should never be stacked against live trees as the beetles will emerge from the wood and attack the live tree.
Thinning is dangerous work. Felling and removal of trees is best left to the professionals. The felling of trees is not recommended for amateurs.
After reading this article if you would like more information on this subject you can refer to the following websites which have bulletins published by the University of Arizona Forest Health Program http://cals.arizona.edu/pubs or http://cals.arizona.edu/extension/fh/. Some of the titles that go along with this article are: Guidelines for Thinning Ponderosa Pine for Improved Forest Health and Fire Prevention, Managing Slash to Minimize Colonization of Residual Trees by Ips and other Bark Beetle Species Following Thinning in Southwestern Ponderosa Pine, Pine Bark Beetles and Firewood, and Bark Beetles in the Southwest.
If you need professional help, it's best to consult a certified professional. To find a certified forester consult the yellow pages, call your local University of Arizona County Extension office, or log on to http://www.safnet.org/certifiedforester/index.cfm, or log onto www.isa-arbor.com to find a certified arborist. Also, fire departments want to help assist land owners with forest thinning.
Tom DeGomez is a Forest Health Specialist, University of Arizona, and the former Coordinator of the Master Gardener Program. For more information about the Master Gardener Program, call Hattie Braun, Coordinator of the Master Gardener Program, at 774-1868 ext.17 or visit our Web Site: highelevationgardening.arizona.edu.)
Posted by maxmaddy at November 2, 2008 7:28 AM