How long would it take one tree to make a forest?

That depends on several factors. First and most important is the species of tree. Some species of trees must receive pollen from a different tree because pollen from the same tree will not create viable seed. If you plant only one tree of that type, it will never result in another tree, much less an entire forest.

Species, like silver maple, are fast growers and reach maturity earlier than other species. Fast maturing trees might begin producing viable seed as soon as three or four years. Other species might take as long as ten years before they produce viable seed.

Some trees produce a large quantity of seed, while others produce relative few seeds. Add to this, the seed from some trees has a high germination rate, while other species produce large quantities of seed but much of it will never germinate.

Now start looking at environmental factors. These can dramatically influence who quickly tree regeneration takes place. Among these environmental factors are:

Weather

Will the seeds get enough rain and sunlight at the right time so they germinate and survive long enough to become viable seedlings? Will really hard winters kill the seed before it ever gets a chance to sprout? Will floods wash away the seed?

Even if the seed germinates and seedlings survive, weather will impact growth rate. Different species have differing requirements for temperature and rainfall. Inclement weather, such as drought and late spring warming, can reduce annual growth by half or more.

Predation

Lots of animals and micro organisms see tree seeds as food. Mice, bear, raccoons, squirrels, weevils and fungi devour tree seeds. Depending on how heavy the predation nearly the entire seed crop for a year might be consumed by predators.

Fire

Wildland fire kills trees. It cooks the sapwood of many species which destroys the ability of the tree to move water and nutrients, even if the bark does not burn. Young trees that do not yet have thick bark are almost certainly burn over.

Some fire tolerant trees, including many species of oaks, are fire tolerant. Older trees have bark thicker enough to insulate it from fire. Young trees may be burnt to the ground (0r nearly so) but the tree has the ability to re-sprout new growth from the root crown and begin again.

Shade Tolerance

Some trees are early succession species, which means they thrive after disturbance like a fire or disease when other trees around them have died and there is lots of sunlight available. These species would be the ones that could regenerate a forest more quickly.

Other species, called late succession, thrive when grown under shade. Among these are many maple species. Some late succession species can also grow with an abundance of sunlight, but others succumb to drought too easily without damp soil to cool and keep their young roots moist.

I suspect you were looking for a definite answer to your question. Unfortunately, like most questions involving biology, the answer is, “it depends.”

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