The Pennsylvania Avenue Development Corporation, in an attempt to impose a more uniform design on the thoroughfare often called America's main street, has cut down 20 mature elms and oaks on the avenue between Sixth and 10th streets NW.

PADC plans next spring to replace the trees, which had stood in front of the Justice Department and the Federal Trade Commission for about 35 years, with 85 young willow oaks--the variety designated to line the length of the avenue between the White House and the Capitol.

The mature trees measured from 4 to 35 inches across; the immature willow oaks--"The word 'sapling' throws us," PADC spokeswoman Geri Porter said--about 6 inches in diameter. The project is costing at least $50,000.

Replacement of the trees is part of PADC's plan to revitalize and beautify the city's main ceremonial route. "The whole idea is to give the vista of the avenue . . . a functional, delightful framing of trees," Porter said.

The felled mature trees will be used as firewood and mulch, according to Arnold O'Donnell, project manager for the Fort Myer Construction Co., which removed the trees last weekend for PADC.

According to PADC and the Fine Arts Commission, which approved the landscaping plan in 1977, the existing trees would have interfered with the matching willow oaks as they grew.

"Grand avenues, generally speaking, have uniform designs. That's something that hasn't existed on Pennsylvania Avenue for a long time," said Charles Atherton, secretary of the Fine Arts Commission.

"I feel like most other people about cutting trees down ," Atherton said. "There's a certain element of sadness to that. But, unfortunately, that's part of the requirements when you redevelop a thoroughfare like Pennsylvania Avenue," he said.

The planned willow oaks were chosen for their healthy constitutions in urban environments and the way their leaves filter light, PADC's Porter said.