How Do Forests Form? (Forest Succession, Pioneer Plants & Growth Stages)

Forests are some of the most fascinating environments on the planet, filled with amazing natural mysteries to explore and discover.

One of the best ways to really appreciate these environments more deeply is by learning how forests form through a process called “forest succession”.

All forests go through distinct stages of growth as they develop from non-forested lands into full canopies of old growth. This process is called forest succession, where hardy pioneer species are gradually replaced by more sensitive climax tree species.

By understanding how forest succession works in your local area, you’ll know how to read the patterns of forest formation and gain all kinds of fascinating insight about the birds, plants & animals living there.

So let’s take a closer look at the key stages of forest succession and understand how forests form all around the world!

What Is Forest Succession?

Forest succession is the core environmental process behind how an area of barren land develops gradually more complex and sensitive ecologies.

This process begins whenever an area having forest potential goes through a disturbance pattern that interrupts the life cycle of trees.

In modern times, common forms of forest disturbances include things like logging, forest fires, hurricanes & desertification.

Trees are the defining feature of a fully formed forest, but trees are also quite sensitive species that tend to not grow very well in disturbed soil conditions.

There needs to be a certain level of pre-existing plant life and soil stability in order for trees to become established on a large scale.

So nature responds by growing a succession of different plant stages over a period of years that gradually improves moisture retention, soil ph & surface organic matter.

We can name these stages of forest formation to help us observe common patterns that all forests share as they develop:

1. Forest Disturbance
2. Open Fields With Pioneer Species
3. Second Growth Forest
4. Old Growth Forest

The amazing thing is once you know how to observe forest succession, it’s one of the most effective ways to gain predictive insight about the plants, trees, birds, animals & history of any landscape.

Signs of forest succession are everywhere in nature, so this is really one of the easiest ways to study the natural world and make discoveries through direct observation.

I got into studying forest succession because of my interest in wildlife tracking, but I ended up falling in love with forests in general because they truly are so amazing.

Forests are a perfect example of nature acting like an intelligent system, and if you’ve ever stepped foot in an old growth forest, you might have noticed these ecosystems feel almost alive.

So let’s look at some examples of each stage of forest succession including some common plants & ecologies to look for…

1. Forest Disturbances

A forest disturbance is any event that returns a forest to an earlier stage of growth.

This can be as minor as a few trees blowing down in a storm to complete removal of the entire canopy and surface organic material.

The severity of a forest disturbance is determined by asking two questions:

1. How much of the canopy is removed? 
2. How much organic material is lost?

The most significant disturbances are those caused by human activity, where almost 100% of the canopy and organic material is removed from an area of land.

In this case, we see forest succession pushed all the way back to bare ground with almost no growth.

With the lost protection of ground cover, disturbed lands are susceptible to erosion and intense heat, flooding & winds.

It’s like doing a total reset on the forest back to the very earliest stage in the life of a forest.

Interestingly, this stage looks the least like a forest, but it’s actually the starting point of all forest succession!

Forest fires can also be quite devastating depending on their intensity, although much of the soil layer will remain intact and some trees can survive.

While the sudden burning of a forest may seem like a negative thing, it is a perfectly normal part of ecosystem development, and can even increase the net biodiversity.

Hurricanes are another type of forest disturbance because though they can knock down entire forests, it doesn’t actually remove any organic material from the landscape.

When you’re looking for signs of forest disturbances, ask questions like:

• How much canopy cover is here and why?
• What is the condition of the soil?
• Are there signs of downed trees?
• Are there signs of human activity like ploughing, mowing or logging?

These questions can help you observe clues about the past history of this landscape like wind, fire, & human activity. 

2. Open Fields With Pioneer Species

The magic of forest succession is that almost immediately after a disturbance, nature begins to respond by covering up any bare soil with special plants called pioneer species.

Pioneer species are unique suited to survive in disturbed landscapes because they’re extremely hardy to direct sunlight, pounding rains and long periods of hot & dry weather.

In fact, pioneers are so skilled at surviving almost in-spite of unfavourable conditions that many earn the title of being ‘weeds’ as they invade backyard gardens.

There are two general categories of pioneer species that show up after a disturbance:

1. Herbaceous Pioneers: Such as clovers, grasses, thistles, fireweed, goldenrods, etc.
2. Shrubby Pioneers: Like alders, birch, poplar, blackberries, raspberries & scotch broom.

Many pioneer plants are also noted for having special abilities to accumulate and repair certain nutrient and mineral deficiencies in the topsoil.

One great example of a soil repairing pioneer plant is fireweed:

Fireweed is often the first plant to emerge after a forest fire because its roots are uniquely adapted to harvest potassium (K) which was burned off in the heat.

The main thing to remember is that low-growing herbaceous pioneers prepare the soil for more complex and sensitive plant species.

Next Comes The Shrubbery Invasion…

After just a few years of herbaceous pioneers growing and decomposing into topsoil, the landscape has evolved to the point where shrubby pioneers can start to become established.

Shrubby plants and small trees like alders, birches & poplars continue the soil repair process, while adding shade to gradually stabilize the soil moisture and temperature.

The shrubby pioneers will continue to grow and become increasingly dominant until they eventually shade out many of the original field plants, creating something that looks like a mini-forest.

At this stage the forest is anywhere from 5-15 years old and we’re well on our way to having a true second growth forest.

So let’s move on to the next stage of forest succession!

3. Second Growth Forest

A second growth forest can be identified as the first set of canopy height trees that emerge after a disturbance.

At this point our environment finally looks like a true forest.

For the first time we now have a relatively full canopy with trees reaching their full height potential.

However, there are also many signs that this forest is still quite young & developing. For example:

• Many of the same pioneer trees that began during the shrubby invasion are still present.
• Most of the trees are quite skinny and similar in age, with an obvious lack of old growth.
• The forest still has quite a bit of light penetrating through the canopy, creating a dense understory with low visibility.
• The understory is packed with shade tolerant species that were not present during the pioneer stage.

Here we start to see a new canopy of trees beginning to emerge in the understory.

The increasing soil stability and protection from harsh elements has created conditions for increasingly sensitive trees & plants to emerge.

These are the future climax species that will one day become the old growth canopy.

As the pioneering trees reach the end of their life, new holes will open up in the canopy, gradually being filled with climax (old growth) tree species suited for the local forest region.

4. Old Growth Forest

Finally we emerge into an old growth forest as the final stage of forest succession.

• At this stage, the canopy is now fully closed with very old and thick trees spaced much further apart.
• We have multiple understory layers, with trees & plant life of all different ages & sizes co-existing in the same space.
• You’ll also find entirely new species of birds (learn how to identify birds), plants & animals that depend on the dense shelter and solitude of large old growth forests.

Because old growth forests are becoming increasingly rare in modern times, they’re essential to maintaining the biodiversity of our planet.

The old growth forest is what all environments on our planet are striving to become, but it’s important to remember that each stage of the forest succession process has it’s own unique and valuable ecology.

The particular species will change depending on where you are in the world, but the same patterns of forest succession happen everywhere.

Understanding how forests form is an essential step towards appreciating the immense beauty & intelligence of our natural world.

If you want to learn more about how forests work, check out my video tutorial called – How to Read The Secrets of A Forest.