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
Animals can be a culprit. We had a small but dense forest between our house and our only other neighbors. But we had a family of deer that lived there. Mama popped out triplets 3 years in a row. After 5 years, the underbrush is completely gone and now we can see everything.
Hi, I’m a field scientist that has worked extensively on projects involving this issue! So this question is right up my alley.
White-tailed deer are the most impactful herbivore on the east coast of the US. Their populations have exploded without predators controlling them. They love eating tasty plants like oak and maple seedlings and ignore plants that don’t taste so great like sedges and beech seedlings. This causes a dramatic change in the understory of the forest since plants cannot grow out of reach of the deer’s mouth. Tree saplings don’t grow into adult trees, shrubs can’t grow taller than a few inches from the ground. The forest becomes much simpler, filled with tall, established trees and plants that grow no taller than a few inches off the ground.
When the plants disappear so do the wildlife that rely on them. Insects don’t have somewhere to hide or pollinate, small mammals lose their homes and sources of food, and birds don’t have somewhere to nest and raise their babies!
Overpopulation of deer is a serious issue. It has recently gained more attention and traction as something that needs to be fixed. There are many way to control the populations and one of the most common is permitting hunting. This is often not taken well by the public, but that’s the nature of the beast when you’re working with the public on a topic that isn’t necessarily well known or understood.
I know that pine forests shed pine needles, which makes the soil to acidic for other plants to grow. You can often see the grass stop exactly at the edge of a stand of pine trees for this reason.
Also, not all rain forests are tropical. I’m from New Zealand, and we have lots of temperate rainforests. Mature podocarp forests often don’t have very dense undergrowth. The seedlings have a scraggly half-dead phase, which they can maintain for years, and if a mature tree falls, letting in light, all the seedlings will shoot up to compete for the light source.
Local deer numbers can play a big part.
In my area of the UK there are woodlands that were previously thick with undergrowth, but since deer have moved in you can see right the way through at ground level.
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.
This video does a great job of explaining why some forests have thick undergrowth and how forest fires help create healthier more diverse forests
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
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
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.
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
Latest Answers