Forest Succession: How Ecosystems Rebuild Themselves

Primary versus secondary succession

Ecologists distinguish two fundamentally different starting points for succession, separated by one critical resource: soil. Primary succession begins on lifeless substrate where no soil exists — newly cooled lava, land exposed by a retreating glacier, fresh volcanic ash, or a sandbar. With no organic matter, no seed bank, and no microbial community, the first colonists must survive on bare mineral rock, and building the deep, fertile soils a forest requires can take centuries to millennia. The classic textbook example is the retreating glaciers of Glacier Bay, Alaska.

Secondary succession follows a disturbance that removes much of the existing community but leaves the soil — and usually a seed bank, roots, and surviving organisms — intact. Abandoned farmland, logged plots, and burned forests all undergo secondary succession, and because the soil and biological legacy already exist, recovery is far faster, often producing a recognisable forest in decades rather than centuries. Primary succession must build soil from scratch; secondary succession inherits it — a single difference that accounts for the order-of-magnitude gap in how long the two processes take to reach a mature community.

Pioneer species and the seral stages

The first organisms to colonise open ground are the pioneer species. In primary succession these are typically lichens and mosses — organisms that can cling to bare rock, fix atmospheric nitrogen (often via cyanobacterial partners), and survive extremes of temperature and desiccation. As lichens secrete acids and trap dust, they begin the slow manufacture of soil. Pioneer species share a characteristic r-selected life-history strategy: rapid growth, early reproduction, prolific output of small, wind-dispersed seeds or spores, high tolerance of harsh, sunny, exposed conditions, and short lifespans — excellent colonisers but poor competitors.

The intermediate communities that follow are called seral stages (each a sere). A typical secondary sequence on abandoned farmland runs: annual weeds and grasses (years 1–3), perennial herbs and shrubs (years 3–15), fast-growing shade-intolerant pioneer trees such as birch, aspen and pine (years 15–80), and finally shade-tolerant hardwoods such as oak, maple and beech forming the eventual canopy.

The climax community

The endpoint that succession tends toward is the climax community — a relatively stable, self-perpetuating assemblage in equilibrium with the regional climate and soils. In a temperate climate this is typically a closed-canopy forest dominated by shade-tolerant, slow-growing, long-lived trees. These late-successional species are K-selected: they invest in fewer, larger seeds, grow slowly, tolerate shade as seedlings, and persist for centuries.

Early 20th-century ecologist Frederic Clements imagined the climax as a single, deterministic endpoint dictated by climate — a "superorganism" developing toward maturity. Henry Gleason countered that communities are individualistic and contingent on chance and dispersal. Modern ecology sits between them: succession is directional and partly predictable, but the precise endpoint depends on local conditions, history, and ongoing disturbance — many ecologists now prefer to speak of a shifting climax mosaic rather than a single fixed community.

Net primary productivity (NPP) over succession:
NPP rises rapidly in early stages, peaks in mid-succession, then declines as the forest matures.

Biomass accumulation:  B(t) ≈ B_max · (1 − e^(−k·t))
where  B_max = carrying-capacity biomass of the climax stand
       k     = growth-rate constant set by climate and soil

Facilitation, inhibition, tolerance

In 1977 Joseph Connell and Ralph Slatyer proposed three distinct mechanisms by which one successional stage gives way to the next; real successions usually combine all three. In the facilitation model, early species modify the environment in ways that make it more suitable for later species — nitrogen-fixing pioneers enrich the soil, leaf litter builds humus, and shade reduces evaporation — a mechanism that dominates classic primary succession. In the inhibition model, whoever arrives first holds the site and actively resists invasion through shading, allelopathic chemicals, or pre-empting space and nutrients, and succession proceeds only when incumbents die or are removed by disturbance. In the tolerance model, later species are simply those able to tolerate the lower resource levels — especially light — that develop as the community matures, so early and late species can establish together but the shade-tolerant ones eventually dominate.

The Intermediate Disturbance Hypothesis

If succession always ran to a single climax dominated by a few superior competitors, mature forests would be relatively species-poor. Yet many of the most diverse ecosystems on Earth are far from undisturbed. The Intermediate Disturbance Hypothesis (IDH), also from Connell, resolves the paradox: at intermediate levels of disturbance frequency and intensity, the system never settles into competitive exclusion by dominant climax species and never collapses to only the hardiest pioneers, so both early- and late-successional species coexist across a patchwork of recently disturbed and recovering ground, and total diversity peaks.

Species diversity as a function of disturbance:

Low disturbance   -> competitive exclusion -> few dominant climax species -> low diversity
High disturbance  -> only fast pioneers survive              -> low diversity
Intermediate      -> pioneers AND late species coexist        -> MAXIMUM diversity

Diversity D(disturbance) is hump-shaped, peaking at intermediate frequency/intensity.

Coral reefs and many forests fit this hump-shaped pattern, though the IDH is now understood as one important mechanism among several rather than a universal law.

Fire ecology and disturbance-dependent forests

For some ecosystems, disturbance is not an interruption of succession but an integral part of it. Many forests are fire-adapted and even fire-dependent, having evolved over millions of years with recurring burns as a normal feature of the landscape. The adaptations are striking: serotiny in lodgepole pine, jack pine and many eucalypts holds seeds in resin-sealed cones that open only when heated by fire; thick insulating bark lets ponderosa pine and giant sequoia survive low-intensity surface fires that kill their competitors; epicormic and lignotuber resprouting lets many eucalypts and oaks regenerate rapidly from protected buds after the canopy burns; and smoke chemicals called karrikins trigger fire-stimulated germination in dormant chaparral and fynbos seeds.

A century of aggressive fire suppression in western North America illustrates the danger of removing a disturbance an ecosystem evolved to need: without periodic low-intensity burns, fuel accumulates for decades, and when fire inevitably comes it burns hotter and higher, becoming a catastrophic crown fire that even fire-adapted species cannot survive. Modern forest management increasingly uses prescribed burning to restore the natural disturbance regime, keep fuel loads low, and maintain the mosaic of seral stages that sustains biodiversity — fire, used well, is a tool of succession rather than its enemy.

Frequently asked questions

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