# Reforestation/Permanence of the Megalopolis **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2003, 11:31pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593 "2003-12-02T23:31:18Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [December 2, 2003, 11:31pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/1 "2003-12-02T23:31:18Z")
This is a hypothetical question. If humans were removed from the equation of the modern urban ecosystem (use any excuse you want, mass extinction through neutron bomb, total alien abduction of population, etc.), particularly the U.S. Eastern Seaboard Megalopolis, how long would it take for the cities to become reforested, and what form would this take? How permanent is the concrete foundation of the city and how long would it take to turn this into mostly top soil? How much of the modern city would still be obviously noticeable after one thousand years? two thousand years? one million years? Any suggestions of an answer would be helpful, thanks a bunch! ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 3, 2003, 12:29am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/2 "2003-12-03T00:29:55Z")
It’s one of those things that is hard to answer definitively. To give some idea of the minium timeframe you could take a look at Angkor Watt, which was abandoned about 1000 years ago or some of the Mayan cities abandoned at around the same time or slightly later. Using stone building materials and cobbled roads the cities the cities had been well and truly reclaimed by the forest within 800 years. Those a tropical forest, which probably works a lot faster. I also don’t think anyone knows when they vanished, but it gives some sort of idea. Modern cities would take a little longer. Solid tarmac and concrete don’t grow plants. They would need to establish in cracks and rubble. But rest assure they would do exactly that. There are several research and industrial cities in the old USSR that have been effectively abandoned. An artist did a photo exhibition on them and published a book of the results. Being soviet cities they probably weren’t well maintained for over 20 years, but even allowing for that it’s amazing how fast they have deteriorates. Playgrounds have become a mosaic of weeds growing in the cracks in the concrete. Street trees have thrown out seedlings that have established little pools of forest. Roads have a central strip of tarmac with grass completely choking the gutters and most of the sidewalk. Trees have sprung up ever 30 feet or so in cracks in the gutters and between the sidewalk and the building foundations. A lot of forest encroachment depends on suitable seed supplies. Cities with a reasonable density of parks with resident bird and squirrel populations will vanish faster. WAG to follow. I’d give a city like New York 5 years before over 50% of it had been flattened by fire. One uncontrolled fire in the high density areas would probably result in a conflagration that would take out most of the central city. Grass and shrubs would grow up in the suburbs that would carry any fires that started. Within 100 years almost no building would be standing and grass, herbs and fast growing shrubs would have colonised every available surface. It would still be noticeably a city, but a ruin. Within 200 years any site that could support trees would. That includes most of the rubble piles. I doubt that it would have obliterated the city from the air, but from the ground I imagine a lot of it would appear to be scrubby wilderness. Within 500 years I suspect that most traces aside from elevated roads would be obliterated. The buildings and might still be there, but you’d need to walk over them and dig the litter way to know that. The elevated roadways will be the last to go. With no soil and being so free draining they will have a hard time ever supporting trees. Excluding earthquakes they could last for millennia. It’s hard to say. In wet tropical regions they would soon be smothered by vines and epiphytes, but in temperate regions there isn’t much that plants could do to reclaim them until the built up enough soil to allow colonisation of larger trees. That could take along time since these roadways are almost designed to was accumulated soil away with each storm. Within 10, 000 years it’s a safe bet that almost nothing will remain to betray the city. The support columns for freeways will still stand but the concrete will have been etched by rain. The roadways themselves will probably have collapsed as a result of even minor earthquakes, snow accumulation and the weight of plant debris. There will still be lots of rubble and foundations, but it would take a search to know that a city existed there. ---
**Author:** ![\_possum\_stalker](https://avatars.discourse-cdn.com/v4/letter/_/b77776/32.png) [@\_possum\_stalker](https://boards.straightdope.com/u/_possum_stalker)\ **Post date:** [December 3, 2003, 12:38am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/3 "2003-12-03T00:38:17Z")
> [@](#): > > There is some initial variation in the rate of attaining the close cover but in the majority of seres, closed cover was formed no later than after 15-20 years of succession. Faster increase was associated with secondary rather than with primary seres such as spoil heaps, fishpond barriers, sand pit, etc. > > Cover of woody species alone, which is an important indication of how fast a sere can be incorporated into the surrounding landscape exhibits relatively high variation. The establishment of shrubs and trees is easier under moderate environmental conditions and retarded in extreme habitats (dry or wet, nutrient-poor or acid). No difference in the pattern of shrub and tree estalishment was found between primary and secondary seres. [-from this site](http://www.ibot.cas.cz/research/443sken.html) ---
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [December 3, 2003, 1:18am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/4 "2003-12-03T01:18:30Z")
Thanks to both of you! But, one question, wouldn’t the complete absence of topsoil, on say the island of Manhattan, make any vegetative succession almost impossible? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 3, 2003, 1:21am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/5 "2003-12-03T01:21:01Z")
Intersting lin 'possum stalker, but I’m not sure how much we can infer from “studying succession in human-made habitats, starting on bare ground”. It does show that once we get bare ground it may only take 15 years to get a good solid shrubland. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 3, 2003, 1:31am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/6 "2003-12-03T01:31:25Z")
I can only assume that manhattan still has much topsoil as it ever had. No one would bother to cart it away except where they excavated, so it’s all still there under the streets. Even during excavation I expect much of the soil was just moved around and not shipped off the island. Aside from that plants don’t need topsoil, they just need soil. With no topsoil at all plant growth normally only slows 50% at most, usually not even that. For trees the difference would only be during establishment. Soil is formed from all sorts of things. A combination of concrete dust and decaying plant matter would make quite an acceptable soil if a little alkaline. Plants can establish in the tiniest cracks and form their go on to produce more soil. The weathering of the bedrock itself takes care of the rest. It amazing how fast plants returned, even to lava flows. The colonisers just need enough dust to hold some water. Once they are established the pave the way for the later stages. ---
**Author:** ![Laughing\_Lagomorph](https://avatars.discourse-cdn.com/v4/letter/l/6bbea6/32.png) [@Laughing\_Lagomorph](https://boards.straightdope.com/u/Laughing_Lagomorph)\ **Post date:** [December 3, 2003, 1:32am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/7 "2003-12-03T01:32:03Z")
> [@](#): > > \*Originally posted by myles \* > \*\*Thanks to both of you! > > But, one question, wouldn’t the complete absence of topsoil, on say the island of Manhattan, make any vegetative succession almost impossible? \*\* I don’t know about that. First of all there obviously is topsoil on Manhattan…in the parks, in empty lots, tree planters etc. And plant succesions do take place in places with no topsoil, new volcanic islands being probably the most notable example. ---
**Author:** ![Ranchoth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranchoth/32/1066_2.png) [@Ranchoth](https://boards.straightdope.com/u/Ranchoth)\ **Post date:** [December 3, 2003, 8:24am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/8 "2003-12-03T08:24:20Z")
[geeky sci-fi novel reference] Well, in any case, the [Jungfraujoch observatory](http://www.upv.es/geos/Pict1.jpg) should go pretty untouched, even after the Tripods order the capped to abandon the cities. [/geeky sci-fi novel reference] ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [December 3, 2003, 1:26pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/9 "2003-12-03T13:26:55Z")
> [@](#): > > \*Originally posted by Blake \* > \*\* > > WAG to follow. > > I’d give a city like New York 5 years before over 50% of it had been flattened by fire. One uncontrolled fire in the high density areas would probably result in a conflagration that would take out most of the central city. Grass and shrubs would grow up in the suburbs that would carry any fires that started. Within 100 years almost no building would be standing and grass, herbs and fast growing shrubs would have colonised every available surface. It would still be noticeably a city, but a ruin. Within 200 years any site that could support trees would. That includes most of the rubble piles. I doubt that it would have obliterated the city from the air, but from the ground I imagine a lot of it would appear to be scrubby wilderness. Within 500 years I suspect that most traces aside from elevated roads would be obliterated. The buildings and might still be there, but you’d need to walk over them and dig the litter way to know that. > > \*\* Point of order–the Empire State Building is _very_ “overbuilt” by moderns standards, and would likely survive in recognizable form at _least_ 200 years longer than the rest. Ditto the Chrysler Building, albeit the top would come off. And, of course, Grant’s Tomb will still be there. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 3, 2003, 9:30pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/10 "2003-12-03T21:30:31Z")
But are they overbuilt in such way that they can withstand an uncontrolled fire? ---
**Author:** ![Spoke](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@Spoke](https://boards.straightdope.com/u/Spoke)\ **Post date:** [December 3, 2003, 10:01pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/11 "2003-12-03T22:01:16Z")
> [@](#): > > \*Originally posted by Blake \* > \*\*But are they overbuilt in such way that they can withstand an uncontrolled fire? \*\* Like so many old castles and abbeys in Europe, the structure would be gutted, but remain standing. IMO. Our skyscrapers could be around for a long time. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 3, 2003, 10:54pm UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/12 "2003-12-03T22:54:43Z")
> [@](#): > > Like so many old castles and abbeys in Europe, the structure would be gutted, but remain standing. I’m skeptical. Firstly I know of no castles that were gutted 200+ years ago that remain standing. The standing castles like Arundel and Windsor have been constantly inhabited and repaired after fires. In contrast a castle like Tintagel stopped being maintained about 600 years ago and is now a complete ruin. Records show it was a ruin by 1500, less than 200 years after maintainence stopped. What now remains are the foundations and low sections of wall, maybe 10 feet high. I suspect that’s what would remain of a skyscraper. Castles are made of stone, not reinforced concrete. Concrete is great so long as it doesn’t crack and let water into the reinforcing. If it cracks the reinforcing corodes rapidly. Without reinforcing a skyscraper comes down fast. I’m no engineer. but I imagine that a hot internal fire would be garaunteed to crack the concrete because the reo would expland much faster than the stone. ---
**Author:** ![Ranchoth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranchoth/32/1066_2.png) [@Ranchoth](https://boards.straightdope.com/u/Ranchoth)\ **Post date:** [December 4, 2003, 5:05am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/13 "2003-12-04T05:05:20Z")
Yeah, but buildings like the ESB have a steel superstructure—and a much more “bulky” one that the WTC had, for that matter. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [December 4, 2003, 11:02am UTC](https://boards.straightdope.com/t/reforestation-permanence-of-the-megalopolis/216593/14 "2003-12-04T11:02:59Z")
If water gets near the steel, it will rust; this wont affect its structural integrity much, but I believe it will cause expansion and open up more cracks in the concrete or brick outer skin; the more water gets in the faster the steel framework will fail. Stone castles on the other hand are ruinous because their stone has been stolen; here in York you can see reused medieval and even Roman stoneworkall through the central city.