F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and Function

Plants, Succession and Biodiversity

F3.5 F1.1 A1.2 A1.6 A1.10About 70 minutes

Learning goals

I can explain how plants change their environment during succession and why one group of plants replaces another.
I can describe how plants such as jack pine, trembling aspen and fireweed help ecosystems recover after a fire.
I can explain how plants maintain biodiversity by providing habitat, food, shelter, soil and a suitable microclimate.
I can interpret data from plots at different stages of succession.

Words to know

ecological succession
The gradual, fairly predictable change in the species of a community over time.

primary / secondary succession
Primary succession starts where there is no soil; secondary succession starts after a disturbance that leaves soil behind.

pioneer species
The first species to colonize disturbed or bare ground; usually fast-growing, sun-loving and good at spreading.

shade-tolerant species
Plants able to grow in the low light under a forest canopy, such as sugar maple, beech, balsam fir and hemlock.

disturbance
An event, such as fire, flood, windstorm, insect outbreak or logging, that removes much of a community.

serotinous cone
A cone sealed with resin that opens to release its seeds only after it is heated, for example by fire.

facilitation
When early species change conditions (soil, shade, moisture, nitrogen) in ways that help later species establish.

invasive species
A non-native species that spreads quickly and harms native ecosystems.

A burned forest isn't a dead forest

A year after a wildfire in the boreal forest, the ground is black, but it is already turning green and pink: fireweed flowers, young aspen shoots and millions of tiny jack pine seedlings. Within decades there will be a young forest again, full of birds, insects and mammals. Plants drive this recovery. In this lesson you'll see how plants rebuild ecosystems after disturbance and why they are the foundation of biodiversity.

Remember from Grades 7 and 9

You learned that primary succession starts on bare rock with pioneers such as lichens and mosses, while secondary succession starts where soil remains (after a fire, flood or abandoned farm) and is faster. You also learned that each stage changes conditions (soil, moisture, shade) so that other species can move in. Note: lichens are partnerships of a fungus and an alga or cyanobacterium, not plants. This lesson focuses on the role of plants.

Why do plant communities change?

The key is light and soil.

Early (pioneer) plants are sun-loving, fast-growing and good at spreading: many tiny wind-blown seeds, or roots that survive underground. They need full sun and cope with poor soil.
As they grow and die, they build soil: roots break up the ground, and fallen leaves add organic matter and nutrients. Some, such as clover and alder, add nitrogen through root partnerships. This facilitation makes it easier for other plants to establish.
They also create shade. Their own seedlings can't grow well in shade, but shade-tolerant species can. Over time, shade-tolerant trees grow up under the pioneers and eventually replace them.

An old field in southern Ontario shows this well. When a farm field is abandoned: annual weeds such as ragweed appear in the first year or two; then grasses and perennials like goldenrods and asters; then shrubs such as dogwood, sumac and hawthorn; then sun-loving trees such as trembling aspen, white birch and white pine; and finally, after many decades, a forest dominated by shade-tolerant species such as sugar maple and American beech. The timing varies a lot from place to place.

Secondary succession on an abandoned field in southern Ontario (years are rough guides). Each stage adds height, shade and soil, favouring the next group of plants.

Fire and the boreal forest

Canada's boreal forest is shaped by fire. Many of its plants don't just survive fire; they depend on it.

Jack pine has serotinous cones, sealed shut with resin. The heat of a fire melts the resin, the cones open, and seeds fall onto the bare, sunny, mineral-rich ash, exactly where jack pine seedlings grow best. Black spruce has partly serotinous cones too.
Trembling aspen often survives fire underground: its roots send up thousands of new shoots (suckers) within a year. Because the roots are already big, the shoots grow very fast.
Fireweed produces huge numbers of tiny seeds with silky hairs that drift on the wind and quickly colonize burned ground.
Blueberries, raspberries and many other shrubs resprout from roots and rhizomes and often fruit heavily a few years after a fire.

The new plants quickly hold the soil against erosion, provide food (berries, young leaves, seeds) and cover for animals, and shade the ground, starting the climb back to forest. Over decades, shade-tolerant balsam fir and white spruce may grow up under the aspen and birch.

Three plant strategies after a boreal fire: seeds released by heat (jack pine), regrowth from surviving roots (trembling aspen) and wind-dispersed seeds (fireweed).

How plants maintain biodiversity

Habitat structure: a forest with a canopy, understory, shrub layer and herb layer offers many more places to live and feed than a field. Dead standing trees (snags) are homes for woodpeckers and owls; fallen logs shelter salamanders and fungi and can become "nurse logs" for tree seedlings.
Food: plants are the producers at the base of almost every food web, from caterpillars on oak leaves to moose browsing on willow.
Microclimate: shade and leaf litter keep the forest floor cooler and moister, which many species need.
Soil: plant roots and litter build and hold soil, which supports fungi, bacteria and invertebrates.
Recovery: after a disturbance, fast-growing plants give surviving animals food and shelter and keep the soil from washing away.

That's why losing plant diversity has a ripple effect. Invasive plants such as European buckthorn, garlic mustard and dog-strangling vine in Ontario can crowd out native plants, change soil chemistry and stall succession, which reduces the insects, birds and other animals that depend on native plants. For more on why biodiversity keeps ecosystems stable, see the Diversity of Living Things unit.

Worked example

Let's interpret plots at different stages

Ecologists counted plant species in 10 m × 10 m plots in boreal forest stands of different ages since the last fire (sample data). Results: 1 year: 12 species; 5 years: 28; 15 years: 34; 40 years: 30; 100 years: 22. Describe and explain the pattern.

Describe: the number of plant species rises quickly after the fire, peaks at about 15 years (34 species), then gradually declines (22 species at 100 years).
Early increase: open, sunny, nutrient-rich ground lets many pioneer herbs, shrubs and tree seedlings establish, joining species that resprout from roots.
Later decline: as trees grow and the canopy closes, shade increases. Sun-loving species can't survive under it, and a few shade-tolerant trees come to dominate.
Conclusion: both early and late stages matter. A landscape with patches of different ages (a mosaic created by occasional fires) holds more total biodiversity than one where every stand is the same age.
Limitation: one plot per age, and different places, so other factors (soil, moisture) may affect counts. More plots per age would make the pattern more reliable.

Species richness peaks at an intermediate stage (about 15 years) and declines as the canopy closes; a mosaic of stand ages supports the most biodiversity.

Sample data from the example: plant species per plot rises after a fire, peaks at an intermediate stage, then falls as shade-tolerant trees dominate.

Worked example

Let's explain fire suppression effects

For decades, many forest fires near communities were put out as quickly as possible. In some jack pine forests, fewer young jack pines are now found. Explain using what you know about jack pine.

Jack pine cones are serotinous: they need heat to open.
Jack pine seedlings need full sun and bare mineral soil, which fire provides by removing the canopy and the thick layer of needles and moss.
Without fire, cones stay sealed, few seeds are released, and those that are released land on shaded, thick litter where seedlings do poorly.
Over time, shade-tolerant species replace the jack pine stands. This is one reason forest managers in some areas now use prescribed burns.

Without fire, jack pine can't release its seeds onto the open, sunny ground it needs, so it is gradually replaced by other species.

Just for fun

Fireweed, the Yukon's floral emblem, earned its name honestly: it is one of the first plants to cover burned ground with tall spikes of pink flowers, often the very next summer after a fire.

Hands-on activity

Succession walk with a quadrat

You need: 4 sticks or pencils, 4 m of string, a measuring tape, a notebook, and a phone camera. Optional: a plant identification app or field guide.

Make a 1 m × 1 m quadrat by pushing the four sticks into the ground at the corners of a square and running string around them.
Choose three sites at different stages, for example: a mowed lawn; an unmowed field edge or vacant lot; and a woodland or the edge of a forest in a park.
At each site, place the quadrat in a typical spot. Count the number of different kinds of plants (you don't need names: "plant A, plant B"), estimate the tallest plant height, and describe the light (full sun, part shade, shade) and the ground (bare soil, leaf litter).
Make a table and a bar graph of the number of plant types at each site. Which site is earliest in succession? Which is latest? Explain using light and soil.

Safety: go with a grown-up, stay on public trails or land you have permission to use, and learn to recognize poison ivy ("leaves of three, let it be") and avoid it. Check for ticks afterwards and wash hands.

What to look for: a mowed lawn is kept at an early stage (few species, all short); a field edge often has the most kinds of plants; a woodland floor has fewer, shade-tolerant plants and a thick litter layer. This matches the pattern in the example.

Your turn

Explain why pioneer species are usually replaced by other species over time.

Describe three ways plants help an ecosystem recover after a fire.

What is a serotinous cone, and why is it an advantage?

How does trembling aspen recover so quickly after a fire?

Give the order of plant stages in old-field succession in southern Ontario.

Explain how dead trees help maintain biodiversity.

How can an invasive plant reduce biodiversity?

More practice: warm-up

What is the difference between primary and secondary succession?

Name two shade-tolerant trees found in Ontario forests.

Name a plant that colonizes burned ground with wind-blown seeds.

What is facilitation?

Name one invasive plant in Ontario.

More practice: on your own

Why is secondary succession usually faster than primary succession?

Explain how alder shrubs might speed up succession on poor, sandy soil.

Why would keeping a lawn mowed stop succession?

A student finds 9 plant types in a lawn quadrat, 21 in a field edge and 8 in a mature maple forest. Explain the pattern.

Explain why a landscape with patches of forest of different ages may support more species than one large forest of the same age.

Challenge

Climate change is expected to make some boreal fires more frequent. Predict one possible effect on jack pine and one on biodiversity, explaining your reasoning.

Design a study to test whether removing European buckthorn increases native plant diversity in an Ontario woodlot.

Evaluate this statement: "The last stage of succession is the best stage, so we should protect only old-growth forests."

Check your understanding

Quiz

Why do shade-tolerant trees eventually replace pioneer trees?

Jack pine cones release their seeds mainly after:

Trembling aspen regrows quickly after a fire because:

Which is the correct order of old-field succession in southern Ontario?

Plants support biodiversity by:

In plots of different ages after a fire, plant species richness is often highest:

For parents and teachers

Big idea: F3.5 asks students to explain the process of ecological succession, including the role of plants in maintaining biodiversity and the survival of organisms after a disturbance to an ecosystem. Grade 9 introduced succession; this lesson focuses on the plant mechanisms (light, soil, facilitation) and on Canadian examples such as boreal fire.

How to help: The quadrat walk is a great family outing in any season except deep winter. Go together, stay on public land, avoid poison ivy and check for ticks afterwards. Ask your teen to explain why the forest floor has fewer plants than the field edge.

Common difficulties:

Thinking succession always ends in one fixed "climax". Disturbances keep resetting parts of the landscape.
Thinking fire only destroys. Many boreal plants depend on it.
Calling lichens plants. They are partnerships of fungi with algae or cyanobacteria.