What is a plant community?
A plant community is a group of plants of different species that live together in the same place and affect each other. Examples are a forest, a grassland, a pond edge or a desert scrub. Plants in a community compete for light, water and soil. They also help each other, for example by making shade, holding soil or giving shelter to animals.
Species composition and dominant species
Species composition means which species are in the community and how many individuals each one has. Scientists count plants in small squares (quadrats) to find out.
- The species that is most common, or covers the most ground, is the dominant species. It gives the community its name, such as an "oak forest" or a "sal forest".
- Species that are few are rare but can still be important.
- A community with many species is more diverse and usually stronger against disease and drought.
Example: in a plot of 40 trees, if 25 are species A, 10 are B and 5 are C, then A is dominant: 25 รท 40 = 62.5% of the trees.
Layers of a plant community
A forest is not one flat mass of green. It is arranged in layers (also called stratification):
- Tree layer (canopy): tall trees with spreading crowns that get the most sun.
- Shrub layer: bushes and young trees in partial shade.
- Herb layer: grasses, ferns and flowering herbs on the ground.
- Ground layer: mosses, fungi and leaf litter, where decomposers turn dead matter into soil.
Layers let many plants share one place without all fighting for the same light. Tropical rainforests have the most layers, and some have an extra emergent layer of the tallest trees above the canopy. Layers also give homes to different animals: birds in the canopy, insects in the shrubs, beetles in the litter.
Why layers form: light
Light is the main reason. Leaves of the upper layer catch most of the sunlight. Only a part passes down, then the next layer takes some more. If the canopy lets about 20% through and the shrubs let only a quarter of that pass, the herbs get about 5%. So sun-loving plants grow on top, and shade-loving plants with large thin leaves and low light needs live below. (Real numbers vary by forest.)
Seasonal changes
A plant community looks different through the year.
- Spring: some forest-floor flowers bloom early, while tree leaves are still small and a lot of light reaches the ground.
- Summer or monsoon: the canopy is full, shade is deep and the herb layer gets little light.
- Autumn: leaves of deciduous trees change colour and fall. Leaf litter builds up, and rots to feed the soil.
- Winter: deciduous trees are bare. In dry tropical places, trees may drop their leaves in the dry season instead. Evergreen trees such as pines keep their leaves.
These changes also affect animals: insects, pollinators and migrating birds follow them.
Succession: communities change over years
When bare ground appears (after a landslide, a fire or a field is abandoned), a community slowly builds up. This is ecological succession.
- Bare ground or rock: first mosses and lichens may start.
- Grasses and herbs (pioneer plants) arrive and make a thin soil.
- Shrubs follow and shade the grasses.
- Young trees of fast-growing species grow up.
- Mature forest (the climax community) is stable and changes little unless disturbed.
It can take 100 years or more to reach a forest. Each stage changes the soil and shade so that the next stage can grow. See the lesson on ecological succession for more detail.
Try it: map a patch
Mark a 1 m by 1 m square on the ground in a park (a string square works). List each kind of plant and count how many. Which species is dominant? Draw a side view with layers. Visit again in another season and compare. In the 3D free play, check what the light does in each season.
Key formulas and definitions
- Key terms: community, species composition, dominant species, layers (stratification), succession, climax
- Layers from top: tree โ shrub โ herb โ ground (moss and litter)
- % of a species = (number of that species รท total number) ร 100
- Light reaching a lower layer = light above ร the share each upper layer lets through
- Succession: bare ground โ grasses โ shrubs โ young trees โ mature forest
Worked examples
1. In a plot there are 25 trees of species A, 10 of B and 5 of C. Which is dominant, and what share does it have?
Species A is dominant. Total = 25 + 10 + 5 = 40. Share of A = 25 รท 40 ร 100 = 62.5%.
2. The canopy lets 20% of sunlight pass. The shrubs let one quarter of that pass. How much reaches the herbs?
20% ร 1/4 = 5% of the original sunlight reaches the herb layer.
3. Why do some spring wildflowers bloom before the trees are fully in leaf?
Early in spring the canopy is open, so a lot of light reaches the ground. The flowers use this time to grow, flower and store food before the tree leaves block the light.
4. Put these in order for succession on bare ground: shrubs, grasses, mature forest, young trees.
Grasses โ shrubs โ young trees โ mature forest.
Common mistakes
- Thinking the dominant species is the tallest. It is the most common or the one that covers the most area.
- Thinking a community is just a list of plants. It also has layers and changes through seasons.
- Believing succession takes a few weeks. It takes years to centuries.
- Forgetting that decomposers on the floor are part of the community too.