PORTLAND -- The recent bid by Portland-based private investors to buy Longview Fibre Co. and its 587,000 acres of timberland is part of a rapid change in the forest-product industry that has split old-line timber companies from their timber.

Historically, giant timber companies managed vast empires that included both mills and forestland. At their peak, International Paper Co., Louisiana-Pacific Corp., Georgia-Pacific Corp. and Boise Cascade Corp. owned more than 25 million acres.

But tax and business changes over the past decade encouraged specialization, and companies increasingly split ownership of the trees from production in the mills.

Some of the new private lumber owners are known as timber-investment management organizations, or TIMOs. They own nearly all of those 25 million acres, except for International Paper's holdings, which are being auctioned off. TIMOs are expected to be among the highest bidders.

Pension funds are the biggest investors in TIMOs nationwide, as they seek a hard asset that seems less volatile than stocks and that works well as a long-term investment. Federal tax rulings in recent years have allowed non-profit endowments to invest in TIMOs with the same favorable tax treatment they have with stock-market investments.

The companies that offered $1.72 billion for Longview and plan to break it apart are part of the trend, although they are not TIMOs. They are:

The Campbell Group, one of the largest private timberland owners in the country.

Obsidian Finance Group, led by a coterie of lawyers who once cut deals for now-defunct Willamette Industries.

Obsidian is a 3-year-old equity firm that consults and leads its own strategic acquisitions -- often turnaround jobs and post-bankruptcy restructuring.

Campbell would get Longview Fibre's trees, and Obsidian would take its pulp-and-paper mill in Longview, its 15 corrugated box plants across the country, and its sawmill in Leavenworth.

The Portland area appears to be one of three regions, along with Boston and Atlanta, that draw these timber finance groups.

Aside from Forest Capital and The Campbell Group, a regional office of Hancock Timber Resource Group is in Vancouver. With $5.2 billion under management at the end of 2005, Boston-based Hancock is the world's largest TIMO.

The TIMO trend worries some who say that it could make forest ownership more distant, less transparent and perhaps less inclined toward conservation.

When some TIMOs invest, they break up large tracts of hundreds of thousands of acres into smaller parcels, sometimes one per investor, said Toby Atterbury, a Beaverton-based timberland consultant. Each investor may have different goals over the years, some of which may include real estate development, he said.

"So the timber companies have worked for hundreds of years to build these tree farms in Oregon and Washington," Atterbury said. "Now, that is being broken up. ... Is it going to get the same treatment? It's a question."