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Lately, I have been reading word that our latest epidemic of firestorms is so severe that "trees won't plant themselves naturally", according to the Arbor Day Foundation. Indeed, the picture of a forest sterilized by a major firestorm seems to be an interesting trope to explore in the post-apocalyptic genre or a fantasy that involves European dragons.

But if the History Channel program Life After People has to be taken into consideration, I find that concern to be short-term. In the long term, all things can come back. A moderately-burnt forest takes usually less than a decade before going back up, but our current rash of firestorms are NOT moderate, thus the concern of the forest being sterilized. So, in a likely postapocalyptic story or a Life After People, how long can this burnt forest stay barren and "sterile" before a new, young forest can grow from the ashes?

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  • $\begingroup$ A lot will depend on how "burnt" things are. Cataclysmic coronal mass ejection vs. every nuclear weapon on the planet exploded vs. asteroid impact, etc. Some things produce localized destruction - with worldwide side-effects - and some produce worldwide devastation. $\endgroup$ – manassehkatz-Reinstate Monica Jul 6 '18 at 20:47
  • $\begingroup$ The best example of this happening in the past that I can come up with off the cuff would be a large area covered in volcanic ash in a major erruption, such as Mt. St. Helens. The forests are mostly regrown now, and the most heavily deforested areas stayed barren for only a decade. Different sterilization events may cause different effects; around Chernobyl, dead trees don't decompose very quickly, but not all trees are dead. $\endgroup$ – Ghedipunk Jul 6 '18 at 20:48
  • $\begingroup$ I'm talking about the forests that have been burnt in the West these last few years. $\endgroup$ – JohnWDailey Jul 6 '18 at 20:59
  • $\begingroup$ How do you define "what it used to be"? Most forests go through phases when different tree species predominate. A regrown forest at 50 years will be completely different from a regrown forest at 250 years, and many animal species will be different, too. $\endgroup$ – user535733 Jul 6 '18 at 21:29
  • $\begingroup$ Can you be a bit more descriptive on the topography you're working with? How large an area of forest are we talking about? Brazilian rain forest levels or Yellowstone? The size of the area and surrounding geographic features will play a key part in this. Also, is this area going to be habited by creatures or humanoids during this time or after the "sterilization"? What type of forest are we dealing with, old growth, rain forest, temperate? So many more questions... $\endgroup$ – bhilgert Jul 6 '18 at 21:29
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The largest fire in North America was the Chinchanga Fire in 1950, in northern Alberta. It put enough smoke into the air that it was noticeable in Europe, darkened the sky throughout North America, and dropped temperatures in Washington.

Some parts of the fire were hot enough to change the soil chemistry so that trees couldn't regenerate.

Now looking over the region today it would be extremely difficult to find any sign of the fire. The parts that weren't super heated, recovered naturally with most tree species being well on the way to maturity within a decade, and slower growing species at least on the road to recovery.

The parts of the forest where regeneration wasn't possible, didn't make it impossible for trees to grow. It was simply that the usual methods for recovery were destroyed. The pine cones that needed fire to release seeds were incinerated instead of popped, the roots were destroyed, etc. So seeds had to be brought in by wind, animals, etc.

It's hard to find exact records on how quickly this renewal went, but from what I read in the past these spots were very scrubby with patches of trees, surrounded by ferns, weeds and grasses for over a decade. Within about 20 years this scrubby land was on the road to recovery and looked like a decade old forest.

Now boreal forests have evolved to recover quickly from fires, other types of forests will take longer to recover and colonize the dead areas. Also the most intensive flames only covered small portions of forests, a few hundred meters to a few kilometers. Obviously the larger the area of destruction will slow the recovery time.

The recolonization would start at the edges and work it's way inwards. You'll see plants with lighter seeds that travel in the wind creating green zones dozens and even hundreds of kilometers away from the edge, creating oases of life in the landscape and making the growth look extremely blotchy. If there is no radiation or other factors, grasslands will cover up a lot of the damage in a few years, even in the largest wasteland. Small, weed-like trees and bushes will follow quickly, within a decade all but the largest wastes will see scrublands. After that, it largely depends on what type of trees are growing in the area and how quickly they reach maturity to put out new seeds.

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Plant regrowth depends on the climate of the area in question.

Let us consider the extreme example, represented by the Trinity site in New Mexico. The atomic blast fused the soil into glass. We will not consider any contribution from radiation - partly because it is not asked in the OP, and also because the example of Chernobyl suggests radiation does not slow the forest down very much.

http://content.time.com/time/subscriber/article/0,33009,854500-1,00.html

Seen from the air, the crater itself seems a lake of green jade shaped like a splashy star and set in a sere disc of burnt vegetation half a mile wide. From close up the "lake" is a glistening incrustation of blue-green glass 2,400 ft. in diameter, formed when the molten soil solidified in air. The glass takes strange shapes—lopsided marbles, knobbly sheets a quarter-inch thick, broken, thin-walled bubbles, green, wormlike forms.

I wish I could find a photo of the jade lake but no luck. This is an apocalypse in which all carbon and nitrogen content of soil is vaporized and the remaining mineral soil turned to glass. Fortunately that has not happened on a wide scale in Earth from nuclear explosions. But an analogous phenomenon happens with lava flows - the forest is covered with a nutrient-poor, glassy mineral flow. We can use these as real world representations of the high heat Trinity-style apocalypse. After a lava flow of this sort, how long does it take for the forest to come back?

In a wet, warm climate, the forest will come back within decades. Lava flows in Hawaii that are 50 years old host 20 foot trees.

https://mytanfeet.com/activities/hiking-arenal-1968/ 1968 lava flow

In contrast, lava flows in colder, arid climates can take thousands of years to regrow forest. In this picture, The Costa Rica lava flow is 40 years old and I would bet turns into forest within the next 40. Craters of the Moon is 2000+ years old and desolate.

http://w3.marietta.edu/~biol/biomes/succession.htm contrast lava flows

So too your post apocalyptic glasslands. Wet tropical weather will bring colonizers and nutrients in, roots will make cracks that accumulate more dirt, and the forest will establish within a century. I would estimate it to take 2-3 times as long for a temperate forest, and millennia for drylands.

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  • $\begingroup$ Neither scenario has anything to do with what the West had been suffering the last few years. $\endgroup$ – JohnWDailey Jul 8 '18 at 1:11
  • $\begingroup$ /has anything to do with the West had been suffering / Neither do European dragons, which you reference in the OP. If you are interested in what is actually going on in the West and real life forest succession that is an interesting topic but it is not world building. I understood you to be asking about a fantasy apocalypse scenario with an ecosystem burned to extreme sterility. $\endgroup$ – Willk Jul 8 '18 at 2:07
  • $\begingroup$ Pay closer attention to the first paragraph, because that is what I was referring to. $\endgroup$ – JohnWDailey Jul 8 '18 at 2:48
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A moderately-burnt forest takes usually less than a decade before going back up, [...] So, in a likely postapocalyptic story or a Life After People, how long can this burnt forest stay barren and "sterile" before a new, young forest can grow from the ashes?

When I last seeded my lawn, I used packed ground, which was seedless. Nevertheless, weeds started growing together with my lawn.

On a scale bigger than my lawn, new born volcanic islands in the middle of the Pacific can grow seeds coming from somewhere else.

Seeds are carried by wind, water and animals. Local sterility can be achieved, but to achieve a long term sterility one would need to take rid of all the factors generating seeds elsewhere.

Give it few years, and something will grow back. From there to the original forest it will be quick, unless the climate of the place is also deeply changed.

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    $\begingroup$ That does not answer the question. $\endgroup$ – JohnWDailey Jul 6 '18 at 21:00
  • $\begingroup$ @JohnWDailey, how does "few years" doesn't answer your question? $\endgroup$ – L.Dutch - Reinstate Monica Jul 7 '18 at 3:16
  • $\begingroup$ This is both broad and vague. The question is way more specific. $\endgroup$ – JohnWDailey Jul 7 '18 at 3:31

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