Have you noticed any trees with browning foliage in your area? When looking up towards Sierra Outdoor School from the town of Sonora, the hillsides are dotted with brown. Sometimes this color is from deciduous leaves preparing to drop from trees like California buckeye and black oak, however this year there is significantly more brown on the hillsides. As you drive up to the school you see that the brown color is from ponderosa pine trees that have died over this summer. How did this happen? What can be done about it? Is this a fire risk?
After four successive years of drought, these trees have suffered and have been unable to respond normally to environmental stresses. One thing that can stress the trees are bark beetles. These insects lay eggs underneath the bark and the larvae feed on the wood. A healthy tree responds to a bark beetle attack by filling the hole with a thick, sticky, fluid called pitch. When the trees are under water stress, as they have been due to this drought, they cannot produce enough pitch to fill the holes and keep the bark beetles out. Once a few beetles get under the bark and into the wood, they send out a chemical that lets other beetles know there's food to be had. They also lay eggs in the phloem, the layer between the bark and sapwood. The area where groups of eggs are laid is called a gallery. This gallery tunneling damages the phloem which carries sap throughout the tree, further inhibiting their ability to fend off bark beetles and stopping the flow of energy to growing parts of the tree. These beetles can also bring in a fungus that, in addition to damage from gallery construction and feeding, also contributes to tree mortality.
Once the tree has died and the needles are all brown, it becomes a fire hazard because it is more flammable than green, living trees. However, once the dead foliage has dropped to the ground, the standing dead tree (or snag) does not pose an increased fire risk. In fact, snags provides great habitat for a variety of birds, mammals, insects, reptiles, and amphibians. Once the snag has fallen, though, it can become fuel for future fires.
There are things we can do to protect our forests and private property. Once you’ve identified the trees and the beetles they may be susceptible to, there are some steps you can take:
1) Thin tree stands: reduces competition for resources, keeps the healthiest trees, keeps a variety of ages of trees, hinders the chemical communication between beetles, and allows you to keep more drought tolerant species.
2) Clean up blown down trees or green slash so you don’t attract beetles to this food source.
3) Be careful not to weaken trees through injury by digging near them or removing bark.
4) Remove any trees that have beetles in them and any green material >3” in diameter or chip, bury, or burn it promptly. (Depending on the beetle species, this tactic may or may not be effective)
5) Have a professional properly apply pesticides to unaffected, susceptible, or high value trees in extended drought periods. This may help the tree(s) in the long run, but it not a guarantee.
6) Water trees during extended droughts by saturating the soil down two feet near the outer edges of branches. Careful not to over water!
Much of this information comes from a USDA pamphlet, “Bark Beetles in California Conifers”. For assistance in managing forests on private land, contact Cal Fire. For info on insect and forest management on public lands, contact the USDA Forest Service. Here is some additional information on bark beetles.
Stay tuned for the next post to see what Sierra Outdoor School has done with our ponderosa pine die-off and how we’ve managed this forest to limit the risk of forest fires. And learn how these actions affected the Oak Fire that broke out on September 8, and came within a mile of our school.