Structure of forest ecosystems
An ecosystem is living things plus their surroundings working together. A forest ecosystem has two parts.
- Living part: producers (green plants that make food), consumers (animals that eat plants or other animals) and decomposers (fungi and bacteria that break dead leaves into soil food).
- Non-living part: sunlight, air, water, soil and temperature.
The layers (vertical structure)
- Canopy: the tops of tall trees. It catches most sunlight.
- Understory or shrub layer: small trees and bushes in the shade.
- Ground (herb) layer: grass, ferns, seedlings, fallen leaves.
- Soil and roots: where water, minerals and decomposers live.
Energy flows one way: sun, then plants, then animals, then decomposers. Nutrients go round and round: leaves fall, decomposers rot them, roots take the nutrients up again.
Forest succession and stand development
Forest succession
Succession is the slow change of plants and animals in a place over many years. On new bare land (a lava field, a landslide or a cleared patch) it goes in steps.
- Bare ground.
- Pioneers: grass, moss and herbs that grow first and make a little soil.
- Shrubs come, with more shade and food.
- Sun-loving young trees (pioneer trees) rise above the shrubs.
- Shade-tolerant trees grow below them and finally take over. This steady forest is the climax stage.
Primary succession starts with no soil. Secondary succession starts where soil is already there, for example after a fire or after fields are left alone, and it is faster.
Stand development
A stand is a group of trees that look alike in age and kind. Its life has stages:
- Seedling: tiny plants, many of them.
- Sapling: taller than a person, crowding begins.
- Pole stage: straight thin trunks, strong competition for light.
- Mature: big crowns, thicker trunks, many seeds.
- Old: very large trees, dead wood and hollows for animals.
Self-thinning: as trees grow bigger, the weaker ones die, so fewer trees remain per hectare.
Forest structure and multiple functions
A forest has many jobs at once: it holds water, stores carbon, keeps soil, gives wood and shelters life. How well it does each job depends on its structure.
- More canopy cover means more rain is caught by leaves and slowed, so less flood and erosion.
- Deep, many roots hold soil on slopes.
- Bigger and older trees store more carbon in wood.
- More layers and dead wood give more nests, holes and hiding places, so more species.
- Gaps in the canopy let light in for young plants.
A young stand with one layer does fewer jobs than an old forest with four layers. That is why foresters plan the structure they want, not only the number of trees.
Try it: grow a forest
Go to the last 3D step and move the age slider from 0 to 100. Predict first: at what age will the floor get darkest? Then check the Light bar. Next, find the age with the most layers.
Key formulas and definitions
- Layers: canopy, shrubs, ground layer, soil
- Succession: bare ground, grass, shrubs, young trees, climax forest
- Stand stages: seedling, sapling, pole, mature, old
- More layers + bigger trees = more water held, carbon stored and homes
Worked examples
1. A landslide leaves bare rock and soil. Is the later change primary or secondary succession?
There is soil left, so it is secondary succession. Grass and herbs return first, then shrubs, then trees. It is faster than on bare rock.
2. A stand has 1000 trees per hectare at the sapling stage and 400 at the mature stage. Where did 600 trees go, and what is this called?
Weaker trees lost the race for light and died. This is self-thinning. The 400 survivors have bigger crowns and trunks.
3. Why does a forest with dead logs and hollows hold more bird species than a young neat plantation?
Logs and hollows are nests, food and hiding places. Old forests have more layers and more of these places, so more species find a home.
Common mistakes
- Thinking succession is fast. It takes many years to decades.
- Mixing up a stand (a group of trees) with one tree.
- Believing a dense young forest is as good as an old one for animals.
- Forgetting soil and decomposers when listing forest structure.