How long would it take for a forest ecosystem to overtake New York City?

Question

Deer bound across broken pavement, eating from bushes growing were there were once cars beneath trees where there were once light posts. Busses have been replaced by bears, rumbling slowly down ancient, abandoned avenues and wolves build dens in the uncollapsed portions of what were once subways.

The inexorable process of nature reclaiming a city is a staple of post-apocalyptic sci-fi, but how long would it actually take? Naturally, it's going to depend on the environment, so for this question, I'm asking about New York City, or at least a city very similar to New York.

For a large, built-up, concrete covered city like New York, how long would it take for native plants and animals to re-colonize what is now a city? This doesn't have to mean that all traces of humans are gone, only that plants and animals have sufficiently rebounded to create a mature forest ecosystem in what was previously downtown.

Answer 1

You're in luck. This question has been investigated quite thoroughly by Alan Weisman in his book The World Without Us. The short story is that within 5 years, it'll start looking like a forest, within 20 years, the skyscrapers will start coming down, and within 200 years, the place will have been fully colonized by trees.

Weisman's premise is that humanity suddenly disappears, and whatever it is that causes this doesn't do any collateral damage (like fallout or dead bodies everywhere, just empty streets).

He starts the thought experiment with New York, specifically Manhattan, and the first new is bad. Manhattan basically requires 24/7 attention to keep 13 million gallons of groundwater out of its subway tunnels. The minute humans disappear, the power goes off, and the foundations of all those skyscrapers start eroding:

Even if it weren’t raining, with subway pumps stilled, that would take no more than a couple of days, they estimate. At that point, water would start sluicing away soil under the pavement. Before long, streets start to crater. With no one unclogging sewers, some new watercourses form on the surface. Others appear suddenly as waterlogged subway ceilings collapse. Within 20 years, the water-soaked steel columns that support the street above the East Side’s 4, 5, and 6 trains corrode and buckle. As Lexington Avenue caves in, it becomes a river.

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Before that, the freeze/thaw cycle would already have destroyed most pavements and asphalt, so plants would have no trouble colonizing the streets. Weisman estimates that withing five years, Ailanthus trees are growing all over town, ruining the streets and sewers with their roots.

After that first violence, it will take a long time for the final signs of humanity to be eroded. Many small objects will last for thousands centuries. We'll have to wait for a glacier to grind it all to a pulp:

Even if the Statue of Liberty ends up at the bottom of the harbor, Appelbaum says, its form will remain intact indefinitely, albeit somewhat chemically altered and possibly encased in barnacles. That might be the safest place for it, because at some point thousands of years hence, any stone walls still standing—maybe chunks of St. Paul’s Chapel across from the World Trade Center, built in 1766 from Manhattan’s own hard schist—must finally fall. Three times in the past 100,000 years, glaciers have scraped New York clean. Unless humankind’s Faustian affair with carbon fuels ends up tipping the atmosphere past the point of no return, and runaway global warming transfigures Earth into Venus, at some unknown date glaciers will do so again. The mature beech-oak-ash-ailanthus forest will be mowed down. The four giant mounds of entombed garbage at the Fresh Kills landfill on Staten Island will be flattened, their vast accumulation of stubborn PVC plastic and of one of the most durable human creations of all—glass—ground to powder.

So in the end, it all depends on your definition of what a forest is. The process would start immediately. It would look post-apocalyptic in a few years, and more like a forest than a city within decades.

Answer 2

We have a modern city to look at for guidance: Pripyat, Ukraine.

Image by 2wid on flikr. More images.

We're about 30 years out from the sudden abandonment of the city. That's a nice beginning of a forest, but it's just a beginning.

In Vermont, where I live, the forests have regrown significantly over the last century since the sheep ranching craze died out. Still, most of the forests don't have the maturity of the older growth regions. A young forest features young trees that are relatively dense, while older growth areas feature mature (100+ year old) trees with clear space around them and a mix of tree ages. Vermont's forests are about 150 years old, and now becoming a thriving ecosystem again. But that are wasn't covered with asphalt - the ground itself was left fairly intact. You can find some interesting before-after photos here on UVM's site, check out the presentations ForestsVT and Forests2. The forest has changed significantly, and there was a ready source of seed and wildlife nearby. I would guess that for a bigger, better built city like NYC, you'd need more time, closer to 200 years.

Vermont Statehouse, 1870, clearcut surroundings.

Vermont Statehouse, 2012, both from Wikipedia.

Another avenue to research that I think supports a 200 year timeline is the reforestation of Mt. Saint Helens'. 30 years on, with human help and it's still far from what it was, though many of the animals have returned.

Answer 3

30 - 50 years.

I'm basing this off of actual abandoned modern cities. The two examples that come to mind right away are Tchernobyl(1986) and Fukushima(2012). Both cities evacuated in response to a nuclear incident. << A very efficient deterrent to human presence.

Both cities are smaller than New York and it is also an island which might or might not hinder the return of wildlife, however if you look up present pictures of both cities you'll see that they are both already well underway of being entirely overrun by nature. (Especially Tchernobyl)

I didn't post any link because honestly google image ('tchernobyl today' i.e.) is your best bet.

EDIT: From there, depends on how old you want your forest to be. (And how ruined you want the buildings to look - but anywhere it freezes in the winter, that doesn't take a long time. Frost and Thaw in spring are evil to anything artificial.

Answer 4

So essentially you're asking how long it takes to get from a clean slate to a full forest. I hate when people answer like this, but, it just depends. This article speculates that a forest could develop in wet regions of Hawaii in less than 150 years. Calculating exactly how long it would take for New York City to get to an unspecified point of primary succession would be extremely difficult.

My final answer would be somewhere between 75-125 years, though NYC is surrounded by plenty of foliage that may expedite this process!