8
$\begingroup$

On a continent similar to modern-day Europe with a temperate climate, 90% of the continent's flat and hilly land was once covered by native forest. Human activity has reduced that to 10%.

A relatively sudden event (such as plague, famine, or a massive exodus) removes most humans from the area, causing human activity to almost cease and allowing the forests to gradually regrow.

How long does it take for the forests to return to 90%?

$\endgroup$
  • $\begingroup$ There are many factors that effect the rate of reforestation. An urban center is going to reforest at a very different rate than a meadow at the edge of a wood. $\endgroup$ – sphennings Feb 14 '18 at 23:42
  • 1
    $\begingroup$ 90 percent of what? $\endgroup$ – user47438 Feb 14 '18 at 23:54
  • $\begingroup$ On a suitable land, you will see fresh brush in a few years, which will turn into a real forest in a few decades. But not all of your land will be equally suitable. $\endgroup$ – Alexander Feb 15 '18 at 0:05
  • 2
    $\begingroup$ After the 1986 Chernobyl disaster, the town of Pripyat and in general a 2600 sq. km exclusion zone were evacuated; they are still uninhabited, and will remain so for quite a long time. There are many documentaries and pictures showing how nature has reclaimed the land. $\endgroup$ – AlexP Feb 15 '18 at 1:54
11
$\begingroup$

Tree growth

There are two ways to determine if a forest is regrown. First, the trees can reach approximately their maximum height. Second, the first generation of trees can die, leaving lots of fallen decayed wood and a second generation of trees, that might be of a different species (this is called forest succession) takes over.

I don't know many details about European trees, but I can tell you a lot about forests of Eastern North America.

Lake Anna state park in Virginia was clear cut in the 1920s. It was logging land from then into the 1970s, so it was planted in pine and cut every ~25 years. In 1971, the lake (Lake Anna, believe it or not) in the park was used as cooling for a nuclear power plant, so the land was turned into a state park.

Originally, the land (alluvial bottomland on the North Anna river) was covered with fire resistant oak-hickory forest. That was all removed by the 1920, and pine was planted. Since the forest has become a protected park, the early pine has given way in succession to tulip-tree, red maple, and sweetgum, mixed with some remaining shortleaf pine. Pines don't live that long, and they don't grow as fast as the hardwoods; so despite a ~20 year head start the tulip-tree and sweetgum in particular are already as tall as the tallest pines, perhaps 25 meters maximum after ~50 years of growth (the maples are probably only 15 meters, maximum).

So in the one sense, the forest is fully regrown in 50 years. On the other hand, the first generation of hardwoods have not died yet. Tulip-tree and sweetgum are both shade intolerant, so they will not produce a second generation in the same forests that currently exist. Instead, the forest will be replaced by beech and maples.

However, there is still another twist. The state forest service removes fallen wood and prevents wildfires. But Virginia sees regular summer droughts, where high temps make for fire conditions. There would normally be understory fires every 5 years or so, under pure natural conditions. With these fires, beech and maples will have difficulty establishing. Instead, white oak and shagbark hickory will grow; these are not as shade tolerant, but are fire resistant as young trees.

What would happen in Virginia, from a bare field

In cleared fields, there isn't much danger of fire, since there isn't much material to burn. A variety of shrubby trees will start growing. After about 50 years, tulip-tree and sweetgum will have 'won' the race. They will form a closed canopy about 25 meters in height. In the understory, shade tolerant red maple will dominate, along with shrubbier trees like holly, dogwood and persimmon. After this time, the amount of tree litter will build up to the point that understory fires every five years will prevent trees from being established that are not fire resistant.

The only young trees that will survive are oaks and hickories, particularly white oak. After 100 years, the first white oaks will reach the 25 meter canopy. Some of the Tulip-trees (the tallest hardwood in Eastern US) will have reached 50 meters. The short-lived red maples will start falling at this point, and due to fires they will not be replaced.

After 200 years, the tulip-tree and sweetgum will start dying; the only remaining specimens will be the biggest ones. After 400 years, all the tulip-tree and sweetgum will be gone, the first generation of hickory will have fallen and even the first generation of oaks will start falling as well. At this point you will finally have an old growth forest, which will change little over time.

Conclusion

50 years for tall trees and a closed canopy. 100 years for succession to final forest type. 400 years for an old-growth forest.

Of course, this is for Virginia. You could do an analysis for any region with its specific forest type. But the numbers here are reasonable for any temperate forest.

$\endgroup$
  • 1
    $\begingroup$ In British Columbia (and presumably Washington), "old growth" forest in the coastal rainforest is labelled as such if the trees (primarily Douglas fir, Sitka spruce and redwood) are 250+ years old. In the drier, more fire-prone interior, it's 120 years for lodgepole line and broadleaf forests, 140 years for spruce and interior Douglas fir. $\endgroup$ – Keith Morrison Feb 15 '18 at 18:56
3
$\begingroup$

Well, we have some ideas about past events, when glaciation redeced from European soil. A major factor in quickly (within a few decades) reforesting the continent were birds.
Them flying over empty land and shitting out treeseeds they have been eating a few kilometers back contributed quickly to the process.

Another thing how quickly this works is the fresh volcanic islands in front of Japan, that were colonized by brushery and shrubbery during a few years, mostly owing to bird shit as well.

This is an incomplete analysis, but if you have birds in your world, and they have a working digestive system, they'll do the job within a century. This is assuming quickly growing tree species (40 years harvest) which are being used in the French industry. Naturally growing tree species have more like 80 years growth time until sufficiently usable (after being shat out).

$\endgroup$
2
$\begingroup$

There's no one simple answer, since it depends on both local conditions (like the amount of rainfall). and exactly what you mean by "regrown". For an example, the forests around where I live (east side of the Sierra Nevada, fairly dry mountains) were pretty much clear-cut in the Comstock mining era (roughly 1860-1900). The forests have mostly regrown, in the sense that the mountains are pretty well covered with trees (mostly conifers). However, those trees are much smaller than the few remaining stands of old-growth trees - roughly 18 in/45 cm trunk diameter, vs 4-6 ft (120-180 cm) for old-growth trees. So regrowing to something resembling the original forest will take roughly 500 years.

$\endgroup$
1
$\begingroup$

The recovery of the almost completeley destroyed Lolo national forest following the fires of 1910 took about 100 years and is still considered a juvenile forest with another 100 years before it hits full growth. Keep in mind the CCC mobilized tens of thousands of great depression relief-hiree workers to replant it probably accelerating its recovery. You're looking at about 200 to 300 years on a fast scale. After 100 years a lot of swiftly growing pioneer species will have encroached, by 200 your looking at a full forest, 300 is primordial old growth. Its all reliant on climate, terrain, and flora species however so such timelines are usually an approximation at best.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.