IF life in the big city is hard on humans, just think how rough it is on trees, especially the half million or so along New York's traffic-choked streets. Under constant assault from exhaust, accidents, disease, vandalism, dog waste, parasites and countless bicycle-lock chains, urban trees look as if they could use some T.L.C.

Bloomin' of the Green

Here are the 10 most common species among the city's 490,000 street trees. Norway Maple 22.8% London Plane 18.4% Pin Oak 7.6% Honey Locust 7.1% Callery Pear 6.5% Little Leaf Linden 5.5% Silver Maple 4.7% Red Maple 3.8% Green Ash 3.7% Sugar Maple 3.2%

But the city has an important message for tree huggers who are just itching to get out there and do some spring pruning: Learn before you lop.

That old elm, maple or ginkgo at curbside belongs to the city. To legally bob a single bough, hopeful gardeners need something called Citizen Pruner Certification. Without it, pruners court fines up to $1,000.

For far less than that — $70, to be exact — any New Yorker can take a course from Trees New York, which teaches people not only how to prune trees, but also how to identify, care for and protect these lungs of the city. Everyone completing the 12-hour course will receive an official Parks Department certificate and, as the brochure of this nonprofit group points out, "become an informed steward of NYC's urban forest."

The classes have been around for years, but demand has risen with some recent, well-publicized fines, said officials at Trees New York. "We had a startling increase in enrollment in Staten Island after a woman there received a $1,000 fine for pruning a tree," said Susan Gooberman, education director of the group. "It was made very clear to people that they can't just take a saw and do whatever they feel like, even if the tree is right in front of their house."

Joe Bernardo, director of urban forestry for Trees New York, said most pruning students just want to cut back a single tree. But once certified, he said, "Many graduates want to go out and prune every tree in their neighborhood."

Since the city set up the certificate program in the 1970's, more than 5,200 people have become citizen pruners. Enrollees study a variety of topics. They learn that the roots of street trees cannot thrive under the streets, where the asphalt blocks all oxygen. As a result, roots tend to grow under the sidewalk, where they can breathe through the cracks. Mr. Bernardo also tells his students the rule of thumb for estimating the age of a tree: measure its diameter in inches at chest height, about four and half feet, and multiply by four.

Cutting Class

Trees New York, a nonprofit group, offers the 12-hour course that New Yorkers must take to be authorized to prune city trees. Classes are given at the locations below. Advance registration is required. More information is available at 212-227-1887 or www.treesny.com. Downtown Manhattan National Arts Club, 2nd floor parlor 15 Gramercy Park South Classes begin April 11 Uptown Manhattan The Arsenal, 3rd floor 830 Fifth Avenue at 54th St. Classes begin April 27 Bronx New York Botanical Garden 200th Street and Southern Blvd. Watson Building, Room 301 Classes begin April 17 Brooklyn Brooklyn Botanic Garden 1000 Washington Avenue Classes begin May 3 Queens Queens Botanical Garden 43-50 Main Street, Flushing Classes begin May 9 Staten Island Staten Island Botanical Garden Snug Harbor Cultural Center 1000 Richmond Terrace Classes begin May 3

Courses are offered in all five boroughs and begin as early as April 11. Enrollees begin by learning to identify the main species in the city, which has more than 2.5 million trees, counting the half million that line the streets. In addition to pruning, students learn about tree biology, pest and disease identification and tree-pit gardening. They also find out how to get a tree planted on their block, and spend four hours at a hands-on tree-shearing lesson.

"The main thing is, they are learning to provide first aid for trees," Mr. Bernardo said. "They are out there looking for dead and damaged branches, which are the biggest problems for trees and the public."

Dead branches invite disease and harmful insects, including the voracious Asian longhorned beetle. Big branches, can do serious damage if they fall on people or property.

The pruning season usually starts in April, when greenery returns. "If a branch doesn't turn green, it's dead and it should come down," Mr. Bernardo said. Likewise, if two branches are touching, the weaker should be removed, he said.

These tasks have traditionally been done by trained city employees, with every agency responsible for trees on its own properties. The Parks Department cares for the largest number: the trees in most city parks, and every single street tree along New York's 33,287 blocks. But budget cuts in the 1970's drastically reduced the army of municipal pruners, who once numbered 325 in the Parks Department alone.

At first, Mr. Bernardo said, Local 1506 of the Climbers and Pruners Union, which represents the city workers who prune trees, were alarmed at the prospect of citizen pruners running around the city, possibly taking away jobs. So a compromise was reached: civilians could use only hand-held pruners, and could neither climb trees nor use ladders or lifts to reach higher branches. The arrangement left large trees in the care of city workers, while civilians tended often-neglected younger trees.

"The hope was that by taking care of smaller trees, these would grow into bigger trees," Mr. Bernardo said, "thus providing future work for union members." In the 1980's, Parks Commissioner Henry J. Stern amended the rules, allowing citizen pruners to carry 10-foot pole saws.

Once certified, graduates can prune any street tree and any tree in any city park except Central and Prospect Parks, which are administered by private conservancies. Citizen pruners need to get permission from the park manager before they begin lopping in a particular park, however. Trees New York also offers a professional job-training program and a youth program, which gives summer jobs to young people taking care of trees.

Don Dodelson, 70, is a retired dentist from Long Island City who developed an interest in trees. "I wanted to become more of an activist in community affairs and the world," he said. "I took the course last fall and I passed my test and got my little green card and they turned me loose."

Dr. Dodelson says he has "a big job to do," pruning some 40 young street trees in Long Island City next to Gantry State Park.

"They were baby trees crying for help, and the city was not about to take care of them," he said. "But I saw they needed pruning and watering and to have their guy wires clipped. I recognized all that from the course."

"I'm looking forward to watching the fruits of my pruning," Dr. Dodelson added.

That remark is music to the ears of Mr. Stern, who usually speaks at citizen-pruner graduation ceremonies. "We have a lot of trees to prune and we couldn't do it all without them," he said. "They are a very helpful band of citizens. They remind me of Robin Hood and the merry men and women of Sherwood Forest."