Life teems in old oaks

Last Updated: 
October 11, 2004

CORVALLIS - Protect that majestic old oak in your back yard. It is not only home to your hammock, it holds an entire ecosystem of life in its limbs.

First, consider how long the tree has stood. An Oregon white oak that is two feet in diameter may be 200 years old; a three-foot diameter tree is closer to three hundred years old. In that time, a complex community has developed amid the heavy spreading limbs.

Many people don't realize that much of that natural community, including the galls and lichens that decorate old oaks, are not life-threatening to the tree, according to Deborah Clark, biologist at Oregon State University.

For example, more than 100 kinds of lichens occur on Oregon white oaks. Some of those lichens capture nitrogen from the air and enrich the soil when they fall. Usnea, one of the long dangling lichens, supplies nest material and nutrients to foraging animals. It also contains a bright chartreuse acid that can be used to dye cloth.

The moss that grows on your tree is important, too. OSU Extension nematologist Kathy Merrifield has identified 30 kinds of mosses and related liverworts growing on large Oregon white oaks. Many of these mosses grow nowhere else but on old isolated oaks and contain their own specialized community of invertebrates. Rainwater splashed from branches and dripped through moss and lichen carries additional nutrients throughout the oak tree ecosystem.

Both mosses and lichens have the ability to dry up in the summer, then quickly rehydrate with the first flush of rain. To see this in action, add some water to a bit of dry twisted moss, and with a hand lens you can watch the leaves and capsules untwist and open before your eyes.

"It is a better show than Walt Disney," Merrifield said.

Oregon white oaks are host to a dozen different kinds of galls, some are tiny and others are the size of baseballs. The most common is the spotted gall, formed when a tiny, non-stinging wasp lays her eggs on an oak leaf. In response, the leaf develops a protective shell around the developing larva.

According to one study, only 4 percent of the larvae survive to adulthood; most are attacked by other insects in a complex food web that supports many insect-eating songbirds. Even parasitic mistletoe has its place in the oak tree's ecosystem. Its berries are especially sought after by bluebirds.

In fact, Oregon white oaks provide food and shelter for nearly 200 vertebrate species. Acorns provide a high-energy food source to many animals, especially important just prior to the harsh winter months when other foods are scarce. However, acorn production is highly variable year to year. Older, open-crowned trees generally produce more acorns than smaller trees grown in dense stands.

Some animals have developed ways to store their acorn harvest. For example, acorn woodpeckers drill holes into the oak's thick bark or into dead limbs where they stash acorns in large graneries. Acorns are an important food for western gray squirrels, valley and mountain quail, turkeys, deer and bears.

From microscopic to massive, life teems in and around the Oregon white oak. Yet these majestic old trees are not being replaced. Poor regeneration and slow growth of oaks cannot keep pace with rapid development within the Willamette Valley. Homeowners are urged to protect the remaining centuries-old oaks, and consider the life within their limbs.

For more information on the care of legacy oaks, contact your local OSU Extension Service office.

Author: Peg Herring
Source: Deborah Clark, Kathy Merrifield