Unless you see it for yourself, you can have a tough time making the connection between a tree and a 2-by-4.

That's why a recent trip to Northern California and Oregon was an eye-opener for someone like me who makes great use of the finished product but has never seen the forests that produce it.

In the 550-mile stretch on Interstate 5 from San Francisco to Eugene, Ore., I saw more felled timber and lumber and lumber products than I imagined existed.

It was as if someone had built a Home Depot as big as all outdoors, completely devoted to lumber products, with a continuing demonstration of how it is all made.

Trucks haul the timber, which is lashed between giant two-pronged steel, fork-like contraptions, to the mills. If you are on the road long enough, you can witness a procession of wood products--a load of timber followed by a truckload of hemlock 2-by-4s, followed by a load of exterior plywood and then one of oriented strandboard, a wood-composite product.

Just about every settlement along the highway that cuts through the Trinity and Scott mountains, the Cascades and the Coastal Range has at least one lumber mill, with thousands of logs piled 100 feet high.

The logs are kept under a constant spray of water to keep them from drying out before they can be milled into sticks for house construction.

On the slopes of some of the mountains in Douglas County, Ore., if you look carefully, you'll see one tree, and then another, falling as it is cut. As a result, the hillsides are a patchwork of stands of young trees and virtual moonscapes of stumps.

Because a lot of the area is national forest, most shouts of "timber"--I didn't hear any, and I don't know if they really say it, like "fore" on a golf course--would echo on private land.

Eighty percent of all lumber harvested in Oregon and Washington comes from land owned by large producers such as Willamette Industries Inc.--which has 600,000 acres--and 150,000 smaller landowners.

It adds up to $4 billion worth of logs annually in an industry that employs 110,000 people in the two states.

From Oregon alone in 1997--the last year for which data are available--3.4 billion board feet of lumber were harvested from private land, while 0.67 billion board feet came from federal land, the lowest level since the Great Depression, according to a study at Oregon State University in Corvallis.

The average annual statewide harvest from 1987 to 1996 was 6.1 billion board feet. In the last 10 years, logging on federal land has decreased considerably as a result of the federal Northwest Forest Plan, passed in 1989.

That action increased the price of U.S. lumber, forcing the building industry to turn to Canadian provinces for wood products. As a result, softwood lumber prices dropped.

On the other hand, Asia consumes a huge percentage of the hardwoods produced in the Northwest, boosting prices that U.S. consumers have to pay. Last year's Asian economic crisis, however, reduced foreign demand for hardwoods and, in turn, pushed down prices.

Removing federal land, which accounts for 51 percent of Oregon's forest, from timber harvesting has resulted in greater use of alternatives to lumber in home construction, and to building-industry efforts to reduce unused leftovers by using engineered lumber produced from waste in factories.

According to statistics from the National Association of Home Builders, it takes 13,127 board feet of lumber to frame a 2,085-square-foot house.

The price of 1,000 board feet of framing lumber reached $490 on July 16, according to Random Lengths Publications Inc., a Eugene firm that tracks prices. A year ago, the same lumber cost $342.

That means that framing lumber for the typical house cost $6,432 this year and $4,489 last year, with the difference adding almost $2,000 to the base price of a house. Those increased costs would indeed be passed on, because these are good times for the home-building industry, and prices usually are increased during periods of peak demand.

Prices for studs, or 2-by-4s, reached $435 per 1,000 board feet July 16, according to Random Lengths. Though this is hardly an accurate way to measure, I buy kiln-dried 8-foot studs at a retail price of $2.35 each, so based on the Random Lengths price, I'm getting a bargain.

But after looking at those clear-cut slopes, I am feeling sort of guilty. Either I need to switch to light-gauge steel or hang the drywall quickly so I won't have to look at the studs.

What is being harvested are not "old-growth" trees but new-growth. The harvest cycle for fir, hemlock and spruce runs from 40 to 60 years, when the rate of growth begins to slow. Lumber companies clear-cut, then replant, millions of seedlings a year, in the hope that the growth-harvest cycle will endure.

"Regardless of what is finally decided on federal lands, Oregon will not run out of timber," said John Beuter, an Oregon State University professor and a forestry expert.

Case in point: Willamette Industries now harvests 1.3 percent of the timber on its lands and expects that the harvest will increase to 2 percent after the turn of the century.

Old-growth trees in the Northwest often grow to heights of 200 feet and were four to six feet in diameter. New-growth trees look more like utility poles, with diameters of no more than two feet.

There are strict laws against harvesting old-growth trees--unless nature has felled them herself--owing to the effect that their removal has on the environment. Clear-cutting of new growth also is under attack from environmentalists because it affects drainage in an area of the country that typically receives 100 inches of rain annually.

Reforestation is mandatory in Oregon. The state's Forest Practices Act requires that private lands be replanted within two planting seasons of harvest, and within six years of harvest the site must be certified as free to grow, meaning that the trees have topped the brush and can grow successfully.

There are severe penalties for failure to follow the policy. State officials say that more than 90 percent of harvested forest acres are replanted to stocking levels that meet or exceed legal requirements.

And even little trees on a hillside are a vast improvement over a moonscape of stumps.