For the $13 billion Oregon timber industry -- which employs an estimated 30,000 people -- forestland free of alder, Scotch broom and blackberry is considered critical.

When forested land is clear-cut, the sun strikes ground that was shaded for decades. Dormant seeds wake up; grasses and other swift opportunists compete with the Douglas fir, the midnight-green monarch that is a symbol of the state and even graces license plates here. So clear-cut hillsides are aerially doused multiple times with chemical agents designed to destroy all other plants.

Herbicide sprayed on cut forestland is supposed to stay on company land. But a growing number of Oregonians suspect chemicals carried on the capricious currents of these mountains do not obey human intentions. There are signs, they say: the absence of fish jumping, a smell on the afternoon breeze, a grapevine suddenly dead, a litter of aborted rabbits.

Three years ago, King had an epiphany after reading the Eugene Weekly, which regularly prints notices about the areas where timber companies are spraying. She found one for Fish Creek. I live on Fish Creek, she thought.

Not long after that, King and her partner, Justin, and their sons, Rowan and Tobbe, then 8 and 3, heard helicopters cresting the ridge that faces their house. "The first spray we ever witnessed, we could watch from my kids' bedroom window," King remembers. "They saw the spray."

Since then, she says, she can expect to see a helicopter spraying somewhere in the steep slopes around her home twice a year, in the fall and spring. The job can last several hours.

"As soon as I hear the helicopters in the morning, I can call the kids inside," King said. "But, I mean, you can't just vacate the land within five minutes of hearing a helicopter. What about our animals? The goats, those are the big thing, and the chickens? Their barn is open."

It's unclear how many residents have been affected by the spraying, though a rough estimate based on U.S. Census data shows about 100,000 residents live near privately owned forests in Oregon. For a $25 fee, the Oregon Department of Forestry sends King notifications when timber companies plan to spray. But the time windows for spraying are so wide -- several months, even a year -- it's difficult to judge when to stay inside. So far, she's received 75 spray notices, for parcels nearby and upstream, with some forms listing as many as a dozen chemicals.

For the timber industry, herbicide has become an essential tool. Without it, much forestland would not be profitable, said Terry Witt, who for 25 years was executive director of Oregonians for Food and Shelter, a trade group for the timber, agriculture and chemicals industries.

Weeds and brush represent competition for water and sunlight. "It really prohibits or hampers the growth potential of that seedling," said Witt, now a consultant. Witt said spraying chemicals from the air -- especially with the aid of global positioning systems -- is "a very economical way to apply a uniform rate of pesticides according to the label."