Density, in many ways, is what makes New York City great. But New York is getting ever denser; between 1990 and 2000, it grew by 685,000 people -- the entire population of Austin, or twice the population of St. Louis.

Crowded indoor living needs to be balanced with open space. For every new apartment tower built, brownstone rehabbed or single-family home subdivided, we need a small parcel of compensating parkland for our outdoor well-being -- for recreation, health, social connections, views, appreciation of nature and trees, environmental improvement. Elsewhere, residents have yards; New Yorkers have parks. With more New Yorkers, there is a need for more parks.

In an ideal world, government would bear the full responsibility of providing and maintaining parks. But in today's real world of unfunded federal mandates and local-level anti-tax pressure, a patchwork quilt of public-private partnerships has grown up. If developers are making money building housing for new residents, it seems reasonable to ask them to provide some new parkland, too.

Many communities around the country require developers to donate land or pay in-lieu fees to help governments acquire parkland. Known as "developer exactions" or "developer impact fees," the concept is generally thought of as suburban – setting aside parcels for playing fields or stream-valley trails among the new tract houses. But it's also the law in Atlanta, Portland, Oregon and Los Angeles and in very densely populated Chicago, Miami and Long Beach, California.

Miami charges developers $157 for every 1,000 square feet of downtown residential construction and $104 for every 1,000 square feet of downtown commercial construction. The money must be spent in the general vicinity of the new building, it must be spent within six years, and it can only be spent for capital purposes (buying land or constructing a park facility), not ongoing maintenance.

Chicago's Open Space Impact Fee is similar, though somewhat steeper -- about $313 for a small apartment and up to $1,253 for a large one. Affordable (below-market) housing units are assessed a flat fee of $100. The city has seven years to spend the money or it must return it to the developer. In case of larger planned developments, the city prefers to receive not money but a comparable gift of land.

Long Beach, California, which charges up to $2,680 per dwelling unit, hopes to produce precisely 38.72 square feet of new parkland for every new resident – about the size of a double-bed.

Maybe it's time for New York to create a developer exaction program, too. New York has an informal system of bargaining with developers for street-level plazas in return for allowing more building height, but this is not an exaction program - and it doesn't do much for the outer boroughs where the population growth is taking place.

If New York had had an exaction law from 1990 to 2000, housing the 685,000 new residents could have yielded something like $360 million for park acquisition - perhaps more than 300 acres of relatively desirable land around the city. This is not a panacea, but it's not insignificant, either. That's one park the size of Riverside Park, or 35 of them the size of Bryant Park.

Do these programs really result in more parkland? To determine this, the Trust for Public Land's Center for City Park Excellence, where I am the director, undertook a study of the experience of 12 cities. Results were mixed. In six of the communities for which records were kept, a total of 1,329 acres of new parkland was acquired through developer fees. However, this was only about 60 percent of what we calculated should have been acquired. Still, 60 percent is better than zero percent, where New York is right now.

Developer exactions can be controversial. Most developers feel that they are already doing enough, after all, by providing housing. They also argue that paying for parkland will raise the cost of apartments even higher than they already are. That may be true, but doing so might also help keep taxes a bit lower. (Also, with parkland,the apartments will be worth more.) And, ultimately, city living has to be more than a clenched-teeth trip through urban canyons between an apartment and a job. New York has already experienced parkless tenement life, back in the 19th century, and no one wants to return to that.

By every measurement, New York City has one of the most fabulous park systems in the nation. Much of it was created by our forebears beginning in 1859. While we continue to upgrade and steward this priceless legacy, it is important that we also leave more parkland to our children. Instituting a developer exaction program would be one way to do so.

Peter Harnik is the director of the Center for City Park Excellence at the Trust for Public Land.