Utilities that operate the nation's high-voltage electric transmission lines have done extensive tree trimming that should reduce the chances of blackouts, federal officials concluded in a report released yesterday.

But the Federal Energy Regulatory Commission report said that utilities vary widely in how they manage vegetation surrounding their transmission lines. The 21-page report, prepared for Congress, includes a number of recommendations for making the electricity grid more reliable -- including mandatory standards for trimming trees.

The report was prompted by the blackout of Aug. 14, 2003, the biggest in American history, which affected the Northeast, Midwest and Canada. A joint U.S.-Canadian task force concluded that FirstEnergy Corp.'s failure to adequately prune trees near its lines was partly to blame.

In April, the commission ordered those who own, control or operate the highest-voltage portions of the nation's electrical grid to provide information on their vegetation management practices. The commission sought information about the results of the companies' most recent inspections of vegetation around their lines, remedial actions the companies identified and whether action had been taken before the peak summer electricity season.

The report concluded that "transmission owners and operators have performed extensive vegetation management along the nation's high-voltage transmission network."

It also said, "While this does not guarantee that there will not be adverse impact to grid reliability caused by vegetation interfering with transmission lines, it is a positive indication of reduced risk to reliability."

The commission could not say whether transmission line operators had done more tree-trimming this year than in previous years because this was the first time it had requested such information. FirstEnergy Corp., based in Akron, Ohio, has said that it has stepped up its vegetation control program since the blackout.

Spokesmen for utilities that own transmission lines in the Washington area said they had been aggressively trimming trees near their lines before the blackout and have not changed their approach.

Pepco Holdings Inc., which serves the District and suburban Maryland, runs a "vigorous program" to trim trees and other vegetation, said spokesman Robert A. Dobkin. He said Pepco occasionally takes landowners to court if they refuse to allow the company to trim trees threatening transmission lines.

In early 2003, Dominion Resources Inc.'s Dominion Virginia Power started paying more attention to trees deemed to pose a risk, said spokesman Dan Genest. "Since the blackout we have not really changed our program because we feel it's a good program," Genest said.

Some of the tree-trimming has caused consternation among residents and nature lovers, particularly along the bucolic Washington & Old Dominion Trail in Northern Virginia. Dominion's power lines run above a bike path there, and the company's contractors last year took chain saws to thousands of towering trees lining the trail.

Many utilities reported to the commission that their efforts to keep vegetation away from transmission lines were impeded by federal and state regulations. Some said that state governments had hindered their efforts to improve electrical reliability.

PacifiCorp, based in Portland, Ore., complained that the Utah Department of Transportation had planted trees directly under some of its transmission lines and would not allow them to be pruned, the report said. The New York State Department of Environmental Conservation requires transmission line owners to obtain permits to access to some trees and the permit process can take up to two years, the report said.