THE PLAN TO preserve the national forests that the president outlined this week could have an enormously beneficial effect if carried out as planned. But the "if" is as large as the ambition. This is an elaborate administrative action undertaken late in the president's second term. It is not clear it can be completed before he leaves office.

If not, and perhaps even if so, an unsympathetic next president could reverse it. An unsympathetic Congress could likewise impede it. The plan thus becomes a further reminder of what is at stake in the next election.

Nor is it clear what its full extent might be even in sympathetic hands. The process the president set in motion is meant to keep in their present roadless and unlogged condition at least 40 million acres -- a little over a fifth of the national forest system -- and possibly more. But for one reason or another -- remoteness, inaccessibility, not enough trees -- many of those acres are unlikely ever to be logged anyway. The logging battleground is a much smaller area, and for political reasons some of it could end up exempt from the order. Some was exempted from a preliminary step the administration took last year -- a road-building moratorium pending further study. Alaska's powerful congressional delegation is determined to keep that state's Tongass National Forest, the largest in the system, open to logging. The administration has set it aside for special study, too.

But the president's step is a major one even if qualified. Merely having a president assert, as he did the other day, that the forests have a higher purpose than the supply of timber is a welcome departure. Policy has clearly shifted, even if not as far as some of the rhetoric surrounding the latest announcement might suggest. The government used to deal with the forests mainly by looking the other way. Conservation groups were reduced to going to court to compel the executive branch to enforce even mild law on federal land. A federal judge, in enjoining further logging on the national forests in the Northwest, declared that successive administrations had systematically ignored environmental law, and he was right. Some in Congress were prepared to respond to his and other court orders by waiving the law. The new Republican Congress was threatening a couple of years ago to pass legislation making it easier to cut the forests, when it ought to be harder.

These forests are a hugely depleted resource that ought to be conserved. The administration has taken a number of steps in that direction. The Forest Service has a strong new chief; new forest management guidelines were issued just the other day, stressing conservation. Timbering hasn't been stopped in the forests. But an industry that used to hold harmful sway there has been put on the defensive, and that is progress.