What Is Forest Succession and How Does It Work?

Forest succession is the gradual, largely predictable process by which a forest’s plant and animal communities replace one another over time after a disturbance or on newly exposed ground. A meadow abandoned by a farmer does not stay a meadow forever; grasses give way to shrubs, shrubs yield to fast-growing sun-loving trees, and those trees are eventually overtopped by shade-tolerant species that can regenerate under their own canopy. The whole sequence can unfold over decades to centuries, and the forces driving it range from shifting light conditions to changing soil chemistry to competition among species with very different survival strategies.

Two Paths Into a Forest

Ecologists distinguish two broad categories of succession based on what exists at the starting line. Primary succession begins on surfaces that have never supported a biological community, or where all previous life and soil have been stripped away: fresh volcanic lava, retreating glaciers, newly exposed rock. Pioneer organisms on these bare substrates are often microbes capable of weathering rock and fixing atmospheric nitrogen, building the first thin film of soil that later plants can exploit.¹ Lichens, mosses, and hardy grasses follow, and only after enough organic material accumulates do shrubs and eventually trees gain a foothold. Primary succession is slow, often requiring centuries before anything resembling a mature forest appears.

Secondary succession is far more common and far faster. It begins on land where an established community has been disrupted but soil and seed banks remain intact: after a fire, a windstorm, logging, or the abandonment of a farm. A study tracking the first five years of forest regrowth in recently abandoned agricultural fields across six tropical landscapes found that succession typically began with woody remnants left in the landscape and progressed slowly but directionally toward structurally complex, biodiverse forests.² The speed of that progression depended heavily on what happened before: fields that had experienced short-duration, low-intensity, non-mechanized farming retained more biological legacies and recovered faster than heavily worked land.²

A global comparison of succession across biomes found that both types are more likely to reach their target vegetation in cold climates than warm ones, and primary succession has better odds of success in humid climates than arid ones.³ This makes intuitive sense: heat and drought impose extra stress on colonizing plants, and thin soils in dry regions rebuild slowly.

How Light Drives the Changeover

If you had to pick a single engine of forest succession, light competition would be a strong candidate. Early in succession, sunlight floods the forest floor. Pioneer trees thrive on it: they grow fast, produce leaves cheaply, and invest little in dense wood. Later-successional species take the opposite approach. They grow slowly, build dense wood, and produce leaves that are efficient at capturing the dim light filtering through a closed canopy.⁴ This trade-off is fundamental: no species can do both equally well.

As the forest canopy fills in, less and less light reaches the understory. Research in tropical secondary forests shows that in later stages, stronger vertical light gradients limit the regeneration of light-demanding pioneer species while increasing the proportion of shade-tolerant late-successional species beneath the canopy.⁵ A similar pattern holds in boreal forests: older stands have lower light availability and different light quality in the understory compared to younger stands in the same successional sequence.⁶

The practical result is a relay race. Fast-growing pioneers establish quickly after disturbance, shoot up into full sun, and dominate for a few decades. But their seedlings cannot survive in their own shade. Meanwhile, shade-tolerant species germinate and grow slowly beneath the pioneers, eventually replacing them as the canopy matures. In tropical rainforests, this shift in competitive strategy, from fast growth in high light to efficient light use in low light, is the primary driver of species turnover during succession.⁷

What Happens Underground

Succession is not just a story about trees. The soil microbial community reshuffles alongside the plants, and the underground changes feed back into what grows above. Research in subtropical forests dominated by Phoebe bournei found that the dominant groups of soil fungi changed more frequently across successional stages than the dominant groups of bacteria, suggesting fungi are more sensitive to the shifts in litter chemistry, root structure, and canopy cover that accompany succession.⁸ As the forest matures, microbial interaction networks become more complex: species form denser webs of positive and negative relationships, and the assembly of the community becomes more influenced by random ecological events rather than strict environmental filtering.

These microbial shifts matter because soil organisms decompose litter, cycle nutrients, and form partnerships with tree roots that help them access phosphorus and water. An early-successional soil community and a late-successional one deliver different nutrient profiles to the plants above, which in turn affects which species thrive. This below-ground turnover is one reason why simply planting late-successional tree species on bare ground often fails: the soil community may not yet be ready to support them.

Carbon Storage Through the Life of a Forest

One of the most practically important dimensions of succession is how much carbon a forest stores at different ages. Very young forests, particularly in the first decade after disturbance, can actually release more carbon than they absorb. A biome-scale analysis of forests worldwide found that the average net carbon balance in the youngest temperate forests (zero to ten years old) was negative, releasing roughly two tonnes of carbon per hectare per year to the atmosphere.⁹ Intermediate-aged forests in the 30-to-120-year range were the most productive in terms of net carbon uptake, while forests older than 120 years generally sequestered carbon at lower rates. Carbon pool sizes, however, continued to increase with age across all biomes, including soil carbon.⁹

This distinction between rate and stock is easy to miss but critical for climate policy. A middle-aged forest is gaining carbon fastest, like a savings account with the highest annual deposit rate. But an old-growth forest holds the most total carbon, like a savings account with the largest balance. Cutting an old forest and replanting does not recover its carbon stock for decades, and the young replacement goes through an initial period of net emissions before it starts pulling its weight. In Masson pine plantations, for example, the total ecosystem carbon stock increased significantly with stand age, even as the per-year rate of biomass accumulation slowed.¹⁰

How Wildlife Follows the Trees

Animals do not wait passively for succession to play out; different species track the structural changes in the forest and assemble into communities that reflect the current stage. In mixed-species bird flocks in tropical forests, small-bodied species with large clutch sizes, traits associated with fast life histories, dominated early-successional stages. As the forest matured, these were gradually replaced by larger, longer-lived species with smaller clutch sizes.¹¹ The social structure of flocks shifted too: early-stage flocks consisted mainly of associations among similarly fast-lived species, while late-stage flocks were built around associations among slow-lived species.

Bird richness does not always climb in a straight line from young to old forest. In boreal aspen forests, old-growth stands had the highest bird species richness, but young forests came in second, with mature middle-aged forests actually ranking lowest.¹² The explanation lies in structural complexity: both young forests with dense shrubby cover and old forests with snags, canopy gaps, and fallen logs offer varied nesting and foraging opportunities, while uniform middle-aged stands do not. In Atlantic rainforest, the pattern was slightly different: bird diversity was similar between early and intermediate stages but significantly higher in advanced successional forests, where some species and ecological guilds were found only in that final stage.¹³

The lesson for conservation is that no single successional stage supports all species. A landscape that contains only old-growth or only recently disturbed forest will miss species that depend on the other stage. Maintaining a mosaic of ages across a landscape, something that natural disturbance regimes once provided, is key to supporting the full range of forest wildlife.

What Old-Growth Actually Looks Like

Old-growth forests are often treated as the endpoint of succession, and their defining trait is structural complexity. Compared with early-successional stands, old-growth forests consistently contain a wider variety of individual structures: trees spanning a broad range of sizes, standing dead trees (snags), large fallen logs on the forest floor, and multiple canopy layers.¹⁴ This diversity of structures creates microclimates that buffer against daily and seasonal environmental swings, while storing large amounts of energy, water, and nutrients.

This complexity supports organisms that simply cannot live elsewhere. Cavity-nesting birds need snags. Fungi and invertebrates need large-diameter dead wood in various stages of decay. Lichens and bryophytes need the stable humidity found deep under a multi-layered canopy. Old-growth forests are not “over-mature” or stagnant; they continue cycling carbon, generating habitat, and supporting biodiversity in ways that younger forests cannot replicate.

When Succession Gets Stuck

Succession does not always march neatly from pioneer to old-growth. Under certain conditions, the process stalls, producing what ecologists call arrested succession. The classic culprits are dense understory layers of ferns, grasses, or shrubs that blanket the ground and prevent tree seedlings from establishing. Many of these plants are clonal, spreading vegetatively and persisting for surprisingly long periods, which makes them especially effective at suppressing the next wave of trees.¹⁵

In central European forests, the strongest predictor of arrested succession was large herbaceous biomass on the forest floor, followed by browsing pressure from deer and other herbivores, and then large-scale disturbance history. Combinations of these factors mattered more than any single one, and abiotic conditions played a role: mesic (moderately moist) sites were less likely to get stuck, while drier landscapes were more vulnerable.¹⁶

Invasive species add another layer of risk. In the Brazilian Caatinga, a tropical dry forest biome, heavily disturbed areas invaded by exotic grasses showed classic signs of arrested succession: impoverished plant assemblages dominated by a handful of herbs, shrubs, and subshrubs, with a dramatic loss of native tree species.¹⁷ The number of rare native species declined as invader cover increased. Once an invasive grass locks in dense cover and its own fire regime, the feedback loops can be extremely difficult to break.

Disturbance, Diversity, and the Role of the Mosaic

A completely undisturbed landscape would eventually converge on a single late-successional forest type, which sounds stable but can actually reduce biodiversity. Periodic disturbance, whether from fire, wind, insects, or flooding, resets patches of the landscape to earlier successional stages, creating a mosaic of different ages and structures. Research in tropical production forests of Nepal found that intermediate levels of periodic disturbance sustained tree diversity by maintaining a balance between species that are good at colonizing disturbed ground and species that are good at competing in closed-canopy conditions.¹⁸

Biological legacies left behind by disturbance, such as surviving large trees, seed banks, dead wood, and remnant patches of vegetation, accelerate recovery and shape what comes next. A study of a large landscape in Oregon’s western Cascades found that patches of remnant old-growth trees after disturbance enhanced the recovery of total ecosystem carbon, increased canopy structural complexity, and helped late-successional species recolonize more quickly.¹⁹ A broader meta-analysis confirmed the pattern globally: retaining biological legacies substantially enhances forest resilience and accelerates recovery, with seed banks showing the largest effect and structural legacies like snags and coarse woody debris contributing moderately but consistently.²⁰

Climate Change Is Rewriting the Playbook

Forest succession has always been shaped by climate, but rapid warming is pushing some landscapes into territory where historical successional trajectories no longer hold. In the southwestern United States, post-fire modeling found that forest recovery after wildfire was generally associated with cooler, wetter sites close to forested refugia, while warmer, drier locations distant from seed sources were more likely to shift permanently to scrub or grassland. Under projected future climate scenarios, models predicted decreases in post-fire forest recovery and increases in non-forest vegetation.²¹ In other words, fires that historically would have initiated a successional sequence back toward forest may instead permanently convert the land to something else.

Arctic and subarctic regions face analogous disruptions. After tundra fires in Canada’s Northwest Territories, moderately and severely burned sites recovered their vegetation structure toward pre-fire levels fairly quickly, but community composition at severely burned sites did not return to what it was. Shrub and lichen cover decreased, while ruderal species and grasses increased.²² If fires become more frequent or more severe as the climate warms, these compositional shifts could become permanent, representing a novel ecosystem rather than a temporary successional stage.

Using Succession to Restore Forests

Restoration practitioners increasingly work with successional processes rather than against them. One of the most successful techniques is nurse-plant facilitation, which mirrors the natural role of pioneer species. A large-scale experiment across the Sierra Nevada in Spain planted over 18,000 seedlings of 11 woody species either in the open or beneath existing shrubs. Seedlings planted under nurse shrubs survived and grew significantly better across every environmental situation tested.²³ The shrubs moderated temperature extremes, reduced water stress, and shielded seedlings from herbivory, essentially replicating the facilitative role that pioneer species play in natural succession.

Follow-up modeling work confirmed that nurse facilitation is fundamental for launching secondary successional trajectories in fire-prone Mediterranean landscapes. Where succession remained arrested in the pioneer state, targeted planting of specific shrub-tree species pairs could restart the process.²⁴ In Scandinavia, a different approach used fast-growing nurse tree species to build canopy structure rapidly, which suppressed competing herbaceous vegetation and improved stem form in target late-successional trees, though the vegetation-control effect took more than a decade to become fully effective.²⁵

Indigenous Burning and the Forests That Existed Before

Long before ecologists formalized succession theory, Indigenous peoples across several continents managed forests through deliberate burning and cultivation practices that profoundly shaped successional dynamics. In British Columbia, fire-history reconstruction in a dry Douglas fir forest within T’exelc (Williams Lake First Nation) territory revealed that areas of highest Indigenous use, including winter villages, fishing camps, and travel corridors, coincided with plots that experienced frequent, low-severity fire. That high fire frequency ceased in the 1870s after the smallpox epidemic, forced relocation, and the prohibition of Indigenous burning. By the time of the study, about two-thirds of tree cohorts across the landscape had established and persisted in the absence of fire, creating a dense, homogeneous forest more prone to burning at uncharacteristically high severity.²⁶

In California’s Klamath Mountains, a millennium-long biomass record showed that Native burning practices, combined with natural lightning fire, promoted long-term stability of forest structure and composition for at least a thousand years. Climate alone could not account for the observed forest conditions; frequent intentional fire was a co-driver.²⁷ In the Brazilian Amazon, Indigenous agricultural and forest management systems also showed deep integration with successional processes: practices like choosing previously recovered forest areas for cultivation, limiting the number of cultivation cycles, protecting individual trees during farming, and attracting seed dispersers all facilitated natural regeneration on degraded land.²⁸

These examples reveal something that textbook succession diagrams often obscure: many of the forests that European settlers encountered were not untouched wilderness moving through a natural sequence. They were actively managed landscapes in which human fire and cultivation had been shaping successional dynamics for centuries or millennia. Restoring some of those practices is now being explored as a tool for reducing wildfire risk and rebuilding forest heterogeneity in landscapes that have grown dangerously uniform since colonization.

Tracking Succession From the Sky

Monitoring succession across large landscapes used to require decades of repeated ground surveys. Remote sensing is changing that. A recent study in Alaska and northwest Canada used drone-mounted laser scanning and color imagery to classify individual trees into functional types representing different successional stages, including evergreen conifers typical of late succession and deciduous broadleaf trees typical of earlier stages. By building a network model of how forest patches interact and transition, the researchers could characterize current successional stages and predict future trajectories across 48 forested sites.²⁹ Satellite-based tools are scaling these approaches further, making it possible to map the age and recovery status of forests at continental scales, which is essential for carbon accounting and conservation planning in a rapidly changing climate.