How does this benefit urban forests, though?

The biggest way this benefits forests, urban, suburban, rural, wilderness, etc. is by raising awareness in an urban population who generally does not see too many trees, outside of Central Park. While this isn't as beneficial as planting some trees, it will hopefully make both New Yorkers and tourists consider why there are so few trees in urban areas.

My concern is that reserving green space in urban areas just pushes the population outward, increasing urban sprawl. This means more land taken up for development and more polution-causing traffic as commuters drive in from the tree-lined suburbs.

Actually, there is a good body of evidence to show that green space in cities encourages people to move inward, not outward. Here's one sudy on the topic: www.pennsylvaniahorticulturalsociety.org/
phlgreen/seeinggreen.htm

There's no doubt that green space increases the value and the desireability of nearby property as the cited article notes. This has nothing to do with limiting sprawl, though. As a resident of the DC suburbs, I personally see how vast amounts of green space and high property values force families to live miles away from where they work.

What are "vast" amounts? What are "miles"? I live in a medium sized city with a lot of green space (as well as two large lakes). I work 2 miles from home, my husband works 5.5 miles from home. We are a family of 4 earning under $75,000 annually.

Trees and green space also allow dense (sustainable/efficient) cities to
1) reduce the damage they do to local and regional watersheds by managing stormwater rather than washing pollutants into rivers and lakes,
2) improve the mental and physical health of urban residents and even the learning of children (http://www.herl.uiuc.edu/),
3) preserve ecosystems by creating corridors for animals who migrate through areas that have become urbanized,
4) reduce heat island effects by shading asphalt and other heat-absorbing materials.

Lucky for me in Minneapolis, they also increase property values. These benefits can be gained through pocket parks, green roofs, narrow greenway trails (http://en.wikipedia.org/wiki/Midtown_Greenway) and boulevard trees.

Another resource: http://www.treelink.org/

Vast is a lot. I don't know specifically, but quick research reveals DC encompases 43,737 acres, 4,447 of which is water. In addition to over 800 acres of city-maintained parks there are at least 98 parks in DC maintained by the National Park Service. Of these, Rock Creek Park, the National Mall and surrounding memorials, Anacostia Park, and Fort Dupont alone account for at least 5396 acres. (Wikipedia, NPS.gov)
As for miles, the average DC commute is 29 minutes. (census.gov) Commutes of 50 miles or more are not unusual as people seek more affordable real estate near Baltimore of Fredericksburg. According to the Washington Post, 80% of DC homes are not affordable to the average city resident.
I don't want to get into this too much here. I think we're getting off topic.

But you brought it up, Brad!

There is definitely a problem with housing costs outpacing incomes for service industry employees and other middle-income wage earners. But I think green space has very very little to do with the issue. A frozen federal minimum wage, lack of requirements for affordable housing in new developments and the - now popping - real estate bubble are the bigger culprits.

From Fact Sheet #3, Univ.of Washington, College of Forest Resources: "A 25 foot tree reduces annual heating and cooling costs of a typical residence by 8 to 12 percent." "A healthy tree, say a 32 ft tall ash tree, can produce 260 lb of oxygen annually." "Several studies have analyzed the effects of trees on actual sales prices of residential properties....In one area a 6% increase in value was found to be associated with the presence of trees."