This month, as the Douglas firs and blue spruces are evicted to the city sidewalks, a group of amateur historians will begin trimming an entirely different sort of family tree -- the genealogical kind.

In workshops like "Immigration and Naturalization" and "Military Records," taught at the Philadelphia branch of the National Archives, they'll attempt to document their ancestors, and maybe also discover a few skeletons lurking in the family closets. The Archives' past offerings were so immensely popular -- sellout crowds and repeat students -- that it has expanded its selection and begun offering Saturday classes.

Workshop participants learn how to navigate the Archives' bountiful holdings, some 58,000 cubic feet of photographs, maps, documents and architectural blueprints, all dating from 1789 to 1991. Even if you've only got a name and a date to go on, the Archives can all but guarantee you insight into a family member's past. While it's impossible to put a price on such personal-history treasures, the workshops' $5 registration fee makes it one of the best educational values in the city.

The Archives' Family History Program was created to satisfy local demand, which first began building when the Roots miniseries spurred a nationwide genealogy boom in the 1970s. "Out of that, the National Archives saw a need to have programs to introduce people to the types of records that we have here," says Rebecca Warlow, coordinator of the workshop series. Every year, she says, "over 12,000 people come to [Philadelphia's] Archives to do family history research. The advent of the Internet has really led to a second big wave."

Often stories blossom out of seemingly antiseptic government records, like the National Census. While teaching one of the census workshops, Warlow stumbled upon an entry for one of her own ancestors, an immigrant from Germany. "There was a little line in the census that said he was an instrument maker." She later learned that the instruments he made were a unique brand of banjo -- so unique that a specimen now resides in the Smithsonian Institute.

This winter, the Archives will conduct census workshops, covering different time periods from 1790 to 1930. Every decade's installment holds specific data on household members -- names, ages, occupations -- as well as perspective into the national Zeitgeist at the time (the 1930 Census asked every citizen if they owned a radio). Although there is a congressionally mandated 72-year waiting period before individuals' records are made public, census records are the best place to start looking for anyone who is deceased.

Connie Thompson, a 66-year-old schoolteacher, enrolled in the census workshops because she wanted to track down her mysterious great-grandmother, Anna Ward. While rummaging through her aunt's "junk," Thompson found a dusty obituary and photograph of Ward. What intrigued Thompson was that, in the photograph, Ward appeared "black as coal." Until this discovery, the variation in skin tone across Thompson's family was always a mystery. Some relatives, like her grandmother (Ward's daughter), were "as white as snow," she says. Armed with the photograph, Thompson questioned her relatives about Ward, curious to learn if she'd married a white man. Nobody was pleased with her inquiries; one elder aunt responded with a menacing admonishment: "Let sleeping dogs lie."

Thompson did the opposite; she scoured the national censuses for clues. At first, the search was difficult because "before 1860, there were no blacks listed by name," says Thompson. The 1880 Census, however, proved to be very fruitful. Not only was Anna Ward there, but so was a man named Levin Ward. The two were listed as living together, but classified as "NR" -- no relation. Thompson continued to dig through censuses and death certificates, ultimately learning that the Wards had 12 children together, but never married.

Allan Spulecki, a landscape architect helping to redesign Independence Mall, also enrolled in the Archives' workshops last year. As the genealogical point man for his family, Spulecki, 39, had researched off and on since college, but it was the Archives' immigration and naturalization records that provided his biggest breakthrough. In ship logs, Spulecki discovered that his great-grandfather had emigrated from Poland in 1905, two years before his wife and children. What stunned Spulecki was that, in the log, there was another man of the same age, with the same unique last name -- presumably, a long-lost brother. "When I scan death notices, I see a very similarly spelled name, " he says. "There may be other branches of the family that we never knew about."

Due to overwhelming demand, the Archives have added a workshop this winter called "Preserving Your Family Records," a primer on how to save discolored photos and preserve brittle newspaper clippings. Throughout January and February, there will also be weekday classes devoted to customs, court, military and census records, as well as two new Saturday classes for the work-week crowd. The goal, says Warlow, is to "bring alive people who you might only know by a name or a gravestone." Not a bad New Year's resolution.