I’ve touched on the topic of succession in a few of my posts. Here I will delve a little deeper into the mechanics of an aging plant community. When I began to learn more about forests and trees, I wondered: how it is that an “old” forest has not only bigger and taller trees, but different kinds of trees? Why does a hemlock like to grow in old forests, and a birch like to grow in young ones?
A key concept in understanding how and why forests change over time (aka forest succession) is disturbance. A young forest requires the opportunity that a disturbance provides. Whether an entire forest is devastated by a disease, or a tiny patch of forest opens up after a tree falls down in a storm, succession can begin after a disturbance occurs.
Stage 1: Let There Be Light
If you were to stand in a very old forest, you might notice that it’s rather dark. If you were to stand in a recently disturbed area where trees have been cut down, knocked down, or a farmer has abandoned a field, there is no shortage of sunlight. It’s an obvious and critical difference. As Claire has explained in her post, plants are adapted to different levels of light, so the light will determine to some extent what species will grow there. To simplify, we can break trees into two groups:
1. Shade Intolerant: these trees love light. They will not grow well in shady conditions, such as under other trees. When they find themselves in full sun, they grow very fast.
2. Shade tolerant: these trees need light too of course, but they are much more patient. They can grow in the shade of other trees, and they don’t tend to grow very fast. Shade tolerant plants have adaptations that allow them to expend less energy, as they have less incoming solar energy for photosynthesis.
These two groups of trees exist because of nature’s love of diversity. Their differences allow them to fill different niches, utilize different resources, and provide stability to the forest. E.g., if there were no shade tolerant trees, there would be no understory trees to provide food and shelter to wildlife, and to take advantage of that space. On the other hand, shade-intolerant plants are very competitive in high light situations and can overtop the shade tolerant plants to gain more resources.
Stage 2: The Competition is On
A few years after a disturbance and the cherries, birches, and other shade-intolerant species appear to be dominating the forest. Take a closer look though, and creeping up in the understory are the more shade-tolerant species: oaks, hemlocks, sugar maple. They don’t look like much now, but these species are using the “tortoise” strategy: slow and steady wins the race.
Eventually, the shade-intolerant trees up in the canopy will begin to die. They tend to have shorter lifespans, and also will succumb to storms and diseases as any tree will. Each time one dies, openings in the canopy let light fall down to the understory. The understory trees that have been chugging along get more and more access to resources as the upper canopy trees die back.
Over time, the shade-tolerant trees replace the shade-intolerant species in the canopy.
Stage 3: Stability
The now old forest could continue on with a similar species composition for a long time. Occasionally a large tree will fall and a few sun-loving plants will have a chance to fill in the gap. Eventually though, a major disturbance like a fire, hurricane, glacier, or tree harvest will renew the cycle.
Another factor that determines the species composition of a forest over time is the availability of seeds. The right seeds must be present in the right conditions in order for the trees to grow. Trees have adaptations that make it more likely that their seeds will be in the right place at the right time. A shade-intolerant tree’s seeds need to find their way to a newly disturbed area, which might not be nearby! So many of these trees have wind or bird- dispersed seeds, like birches (wind) and cherries (bird). On the flip side, a shade-tolerant seedling could grow near or even directly under its parent, so those seeds tend to be heavier so that they end up close by (e.g. acorns, beechnuts, avocados). Of course there are many exceptions to this rule: coconuts are big and heavy but can be water-dispersed, and of course even oaks and avocados benefit from spreading their seeds across the landscape to some extent.
All of this ties into another ecological concept: biogeography, or, why are species where they are? Check in for a future post on this topic, or, you know, google it. Thanks for reading!