Why do some forests have undergrowth so thick you can’t get through it, and others are just tree trunk after tree trunk with no undergrowth at all?
In: Biology
Well it would really depend on the climate, age of the forest, and species of plants and animals native to the region.
Also probably access to resources for said living things.
It’s called succession. Plants compete for sunlight. Some only grow to a certain height so grasses, vines, and hardy shrubs will start out growing strong and fast but end up getting shaded out once trees or taller plants begin to grow. The crowns of trees (leaves or needles on the branches above) block the sunlight from reaching the forest floor which stunts the growth of the understory.
Trees do this to each other as well. That’s why the lower branches get naturally pruned. You can sometimes see where the branches once were. The larger the tree/crown, the more difficult it is for forest floor plants to grow.
If a forest has a dense, mature tree population the ground is typically clear of undergrowth. If the trees are further apart allowing sunlight to reach the forest floor then the sun will help germinate the seeds in the seedbed resulting in other species flourishing until they are shaded out.
Edit: I am completely aware that I did not cover every situation resulting in less dense undergrowth. Other factors are as follows: logging practices, wildlife foraging, elevation, landscape, natural and artificial disturbances (wind/fires/wildlife routes/trails where compaction of soil occurs)/soil type/ plant zones/forest type/shade tolerance or shade intolerance/etc Thank you all for commenting.
Undergrowth struggles to grow in the acidic soil under pine trees. Perhaps that’s what you’re thinking of.
https://www.youtube.com/watch?v=BG-05r0oTuc
Lyrics: Neil Peart
There is unrest in the forest
There is trouble with the trees
For the maples want more sunlight
And the oaks ignore their pleas
The trouble with the maples
And they’re quite convinced they’re right
They say the oaks are just too lofty
And they grab up all the light
But the oaks can’t help their feelings
If they like the way they’re made
And they wonder why the maples
Can’t be happy in their shade?
There is trouble in the forest
And the creatures all have fled
As the maples scream, “oppression”
And the oaks just shake their heads
So the maples formed a union
And demanded equal rights
“The oaks are just too greedy
We will make them give us light”
Now there’s no more oak oppression
For they passed a noble law
And the trees are all kept equal
By hatchet
Axe
And saw
You can divide forest in 3 types
Primary(Old Forest), Secondary(Middle age forest) and Tertiary(Young forest)
Primary= Virgin and old forest with big trees that blocks sunlight and very little undergrowth.
Secondary = Forest that have been intervene by humans or natural disasters such as fires, the sun can reach the ground and there is a good amount of undergrowth.
Tertiary= New forest with a lot of sunlight reaching the ground and lots of undergrowth.
Of course there are exceptions like tropical forest.
Edit: Typos and I add other information
In my area(the north eastern US) the forests have little undergrowth because people have removed the apex predators and deer are allowed to breed mostly uncontrolled and they eat most of the young plants
This video does a great job of explaining why some forests have thick undergrowth and how forest fires help create healthier more diverse forests
Several people have mentioned pine forests and soil types but they are missing the mark. A lot of plants exude chemicals from their roots that repel other plants. This action is called allelopathy and is fairly common in the plant world. Pines interestingly have these chemicals in their needles and not the roots. Often when you see big areas of a single tree this is why.
Not eligible, but I highly recommend “the hidden life of trees” by peter Wholloben. It goes into great detail about interdependent forest systems, in easy to grasp language.
The amount of underbrush is determined by the amount of natural light available at lower levels and by grazing patterns of wild life. And by forest fire patterns.
In places in the pacific coastal rainforest (west coast of bc), there is a big difference between old growth forests and “reforested” areas post -clearcutting.
In old growth forests, trees are regularly being knocked and branches broken off in the wind. This creates a lot of patches of light that filter down to ground level. So there is a lot of underbrush, but the density of underbrush varies quite a bit. Most of the time, there’s lots of ferns and spindly bushes. But anytime there is a decent patch of light, salal, salmonberry, maple and many other bushes take off- if a plant was well established in a shady environment and a tree toppled over creating a patch of light, all those established plants and trees grow madly as soon as the sun hits them.
In an artificially reforested (tree planted) all of the preexisting Forest is eliminated. Seedlings are planted the same age and spacing and variety so they grow at approximately the same rate. There are fewer wind falls and so the area at ground level is really consistently dark. Once the trees are taller than the normal bushes, the bushes are strangled by lack of light, resulting in almost no undergrowth.
Some organizations that rehabilitate forests intentionally go through and fall trees, kill and leave standing dead wood, and top trees to bring more diversity to the undergrowth. Essentially more light means more plant diversity means more animal diversity
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