The man in charge of planting and protecting the thousands of trees that line the streets of the District of Columbia has resigned, officials said last night.

Ainsley Caldwell's resignation as head of the city's Urban Forestry Administration was confirmed by Bill Rice, spokesman for the D.C. Department of Transportation.

Rice said he could not say why Caldwell left, calling it a personnel matter. Caldwell could not be reached last night.

The main task of the forestry administration, according to its Web site, is planting and maintaining street trees -- those between the curb and sidewalks, most of them oaks, elms and maples.

"I am very distressed," said Stanley Mayes, head of a tree advocacy group who portrayed Caldwell as uniquely qualified, as both an African American and a forester.

In 1999, a civic group called attention to a long decline in the condition of the city's trees. The group said 4,000 to 5,000 were being lost a year, and spotlighted a need for tree work. A later study by another group showed 106,000 trees remaining in the city and more than 20,000 empty tree spaces.

"Inevitably," said Sarah S. Boasberg, a member of the city's Urban Forestry Technical Advisory Board, "there is still a backlog" of tree work. She said it would require "a crash program and a lot of extra funds to try to catch up."

"I thought we were doing quite well," said Miles Steele III, another advisory board member. "I can't even imagine him resigning."

Caldwell came to the District in January 2003 from the Brooklyn Botanic Garden and was known as a specialist in urban ecosystem management. It was not clear last night who would replace him.

"Our hope is that the city will find an excellent candidate and develop a program that is really going to lead to protecting the urban forest of Washington, D.C.," said Jim Lyons, executive director of Casey Trees, which has been a principal D.C. tree advocate.