The idea of succession
Succession is the gradual change in the plants and animals of a place over time. After a clearing, fire, storm, farm abandonment or logging, a site does not usually jump straight back to its original forest. Instead, a series of communities replaces each other, each changing conditions in ways that favor the next. The word describes a pattern, not a fixed script, and real sites vary a great deal.
Ecologists distinguish primary from secondary succession. Primary succession begins on bare ground with no soil, such as fresh volcanic rock or ground exposed by a retreating glacier, and takes a very long time. Secondary succession begins where soil, seeds and roots remain, such as an abandoned field or a burned stand. Secondary succession is far more common in forest management and is the focus here.
Stage one: the open site
Right after a disturbance, the site is open and sunny. Soil may be exposed, and competition is low. Fast-growing plants arrive from seeds stored in the soil, blown in from nearby, or carried by animals, or resprout from roots and stumps that survived. Grasses, herbs, brambles and fast-growing shrubs often form the first cover, together with weedy species that thrive on bare ground. In burned areas, species like fireweed are well known for colonizing early.
This stage can be brief or long. Where a grassy sward is thick, or where deer browse hard, tree seedlings may struggle to get started.
Stage two: pioneer trees
Next come trees that are adapted to full sun and rapid growth. In many northern and temperate regions, these include birches, aspens, poplars, cherries, alders and certain pines. They typically produce abundant light seeds that travel far, grow quickly when young, and need plenty of light. Their lifespan is often relatively short compared with later species.
As they grow, a young canopy forms and the ground becomes shaded. Stems compete intensely, and some die in a process called self-thinning. Stands at this age tend to be dense and similar in size. Foresters sometimes use stand thinning to give the best trees more room.
Stage three: the transition
Under the pioneer canopy, conditions change. Shade increases, the soil cools and stays moister, litter builds up, and the sun-loving seedlings of the pioneers can no longer establish. But seedlings of shade-tolerant species, such as maples, beech, hemlock, spruce or fir depending on the region, can survive in low light. These species may have arrived early, or they may arrive later from seed trees nearby.
Some trees, such as oaks and many pines, fall between the two groups. They are moderately light-demanding and can persist for a long time if disturbance such as fire or thinning keeps the canopy fairly open.
As the pioneers age and begin to die, the shade-tolerant understory takes over the canopy. This is the point where the stand changes in character, from a sun-loving community to one that can replace itself in shade.
Stage four: the maturing stand
A maturing stand has taller, larger trees and an increasingly complex structure. Gaps open as individual trees die, and the processes described in the article on canopy gaps begin to shape the stand. Dead wood accumulates, and layers of vegetation develop from the ground to the canopy. The soil gains organic matter, and fungal and animal communities become richer.
This stage often supports more kinds of wildlife, particularly species that need large trees, cavities or fallen logs. Forest birds that nest in tree holes, for instance, often depend on older stands.
Stage five: old forest
If the stand escapes major disturbance for a long enough time, it may develop the features associated with old-growth forests, including very large trees, multiple age classes and large amounts of standing and fallen dead wood. How long this takes depends on the forest type: a few centuries is a common order of magnitude in many temperate regions, though it varies, and some forests reach this condition sooner or later.
A caution on the word “climax”: older textbooks described succession as marching toward a stable final forest. Modern ecologists generally see forests as dynamic, with continuing disturbance, climate shifts and chance events. A mature stand is not static, and it may not match what stood there before.
Why the path varies
Real succession rarely follows the neat sequence. Many things alter it:
- The kind of disturbance. A fire, a storm, a clearcut and an abandoned pasture leave different soil, seed supplies and surviving plants.
- Seed sources nearby. A site far from seed trees may stay open for years.
- Browsing. Heavy deer or livestock pressure can hold back some trees and favor others. Livestock overgrazing is one version of this.
- Invasive plants. Non-native shrubs and vines may dominate and divert the pathway, as covered in the article on invasive species.
- Weather and chance. A drought or a heavy seed year can change what establishes.
- Soil and climate. Wet, dry, poor and rich sites follow different routes.
In boreal forests, repeated fires often keep stands in earlier stages, with aspen, birch and pine giving way to spruce and fir only if long fire-free intervals occur.
Succession in practice
Understanding succession helps with land management. Natural regeneration relies on these processes, and planting can speed or redirect them. Where the aim is a particular forest type, managers often work with the stage a site is in rather than against it, for instance planting shade-tolerant species beneath a pioneer canopy. In the tropics, similar regrowth after clearing is described under secondary forests.
Restoration projects also draw on the sequence. Allowing a site to regrow with little intervention is sometimes the cheapest option where seed sources are close, while heavily degraded sites may need planting and other help. The best choice depends on the site and goals.