How Tree Species Change: The Forest Succession Process Explained

Tree species succession is the slow, natural replacement of one dominant forest species by another — for example, a spruce stand giving way to birch and aspen after a disturbance, and then spruce gradually reclaiming the site. If a person could live several centuries, they would watch with their own eyes how tree species succession unfolds: how the appearance of our forests shifts, how soils, litter, and ground cover change over time.

How does tree species succession happen?

Tree species succession happens when a disturbance opens the canopy, light-loving pioneer species colonize the site first, and shade-tolerant species later return beneath them. Not long ago, mighty spruces and bright oak groves stood where birch and aspen now grow. Where those conifers are absent, it means a person with an axe has been there. In almost any spruce forest you can find the different stages of the struggle between spruce and broadleaved species.

Signs of species change in a forest

The clearest signs of species change are the species composition itself, charcoal in the soil, and the structure of regrowth. A stand of birch or aspen where conifers would naturally dominate signals a past disturbance. Charcoal fragments buried in the soil point to a past fire; the presence of coppice (sprout-origin) trees points to past logging. Foresters read these clues to reconstruct what stood there before and when the turnover began.

The role of humans in shaping forests

Humans drive much of the forest turnover we see, because logging, clearing, and the fires people set or allow remove the conifer canopy and hand the site to pioneer species. The widespread birch and aspen woods across the landscape are largely a legacy of human disturbance rather than the original cover. Where spruce or oak once grew and now do not, human intervention is usually the cause.

The stages of the struggle between spruce and broadleaved species

The struggle between spruce and broadleaved species moves through a predictable sequence: a clearing opens, conditions change, pioneers colonize, their crowns close, and shade returns so spruce can re-establish. Each stage can be observed within a single forest at the same time, on patches of different age.

What happens after a spruce stand is logged or burned?

How conditions change in an open clearing

In an open clearing the conditions reverse completely. The glade is now flooded with generous sunlight, evaporation from the soil intensifies, and at the same time precipitation penetrates the ground freely. The shade-loving plants of the ground cover struggle; they cannot bear the bright sun, the frosts, and the wind. They refuse to fruit, wither, and finally die. Any surviving spruce seedlings face a hard existence — fir, for example, suffers not only from frost and lack of moisture but also from solar radiation. In place of the shade-loving herbs, light-loving newcomers appear.

Pioneer trees: birch, aspen, alder

Pioneer trees — birch, aspen, and alder — are the first to seize the open clearing. The prolific birch (more detail: Tree fruiting) casts its seeds to the wind; they are carried onto the glade, and the whole area takes on a haze of birch. Its seeds could reach this spot earlier too, but back then spruce gave them no chance of life. Now it is a different matter: birch seedlings grow fast, do not fear frost, easily compete with herbaceous vegetation, and outpace it in growth. An abundance of light only benefits birch, because the species is light-loving.

Why spruce seeds die in the open

The closing of the young broadleaved canopy

The return of spruce beneath the broadleaved canopy

Spruce returns once the birch and aspen crowns close and restore the soft, humid shade it needs. The air becomes milder and damper, the sun no longer scorches, the light is dim and diffuse. Spruce had, in fact, kept casting its seeds onto the glade the whole time. People sometimes say spruce is "afraid" to settle here — that is not so. The seeds simply found no suitable conditions and did not sprout, or sprouted and died at once. But once the canopy closed and the light-loving grass disappeared, the mild atmosphere familiar to spruce prevailed beneath it. Spruce seeds stopped dying, and young spruces appeared across the glade to reclaim their rightful home — though that is still a long way off.

The full cycle of species succession: from spruce stand to spruce stand

The full cycle of species succession runs from a spruce stand, through a broadleaved phase, and back to a spruce stand — and it takes about a hundred years. The life of trees cannot be measured by human yardsticks. The process of spruce-stand recovery is very long, though it can move faster under one condition: if birch or aspen are present on the cutover, they may regenerate not from seed but from root suckers or stump sprouts. Sprout growth advances far faster than seed-origin forest, so crown closure happens earlier, and the conditions for spruce to settle arise sooner.

Once spruce has settled, the spruces themselves close their crowns and their own struggle begins. Trees of better and worse growth separate out, with both heredity and growing conditions making themselves felt. For now all the spruce sit beneath the broadleaved canopy and feel its two-sided effect — both oppression and the protection that plays the main role in spruce survival. Then comes the moment when spruce no longer needs the birch's guardianship: it has grown strong, formed its own canopy and microclimate, and now feeds and defends itself.

At that point the white-trunked neighbor only harms spruce, and spruce begins its struggle against birch. Conditions steadily improve for spruce and worsen for the light-loving birch, which thins out — plainly speaking, begins to die — and interferes less and less. Finally spruce pushes through the birch canopy, and before surrendering the birch uses its last weapon: it lashes the spruce crown with its flexible branches, what foresters call "whipping." The slightest breeze sets the sensitive birch twigs in motion, and the spruce's delicate needles cannot withstand the lashing and die. The spruce crown often becomes one-sided and misshapen — the price of victory is the loss of beauty.

Spruce grows slowly but surely and overtakes the birch, its former protector and later oppressor. In place of a two-tiered stand, a single-tiered spruce–birch stand arises, which soon becomes two-tiered again — and you can already guess who occupies the upper tier: spruce. The light-loving birch and aspen now find themselves beneath the dense canopy of shade-tolerant spruce, and the birch, passing through all the stages of suppression at accelerated speed, must yield. The circle has closed. Spruce was cut, birch settled in its place, and spruce drove the birch out — but it took no less than a hundred years.

By virtue of its biological traits, spruce is able to win back territory taken from it. Yet again and again we see only birch and aspen woods, and very rarely spruce. Why? Spruce cannot seed the whole vast space, while birch and aspen can. Foresters can answer easily and with great accuracy whether conifer forest stood here, when it gave way to broadleaves, and how. A fire leaves charcoal in the soil; logging leaves sprout-origin trees — fell a coppiced birch, count the annual rings, and you learn when the spruce stand was taken and the succession began. Spruce can be replaced by pioneer trees not only after fire or felling but also after windthrow. The persistence with which spruce returns and displaces birch and aspen earns it the status of a primary species, and the stable equilibrium is restored once more.

How is climate change affecting tree species succession and distribution?

Climate change is reshaping where tree species can grow, shifting their suitable ranges and altering the local succession story described above. The classic spruce-to-birch-to-spruce cycle assumes a stable climate, but warming temperatures and changing precipitation now move the goalposts: the conditions a species needs may migrate faster than the species itself can follow. The Intergovernmental Panel on Climate Change frames future scenarios as Representative Concentration Pathways, and forest researchers commonly model outcomes under a moderate pathway, RCP 4.5, and a high-emissions pathway, RCP 8.5 (Representative Concentration Pathway 8.5). These greenhouse-gas concentration pathways underpin nearly all projections of future tree habitat.

How does climate change move tree species ranges?

Climate change moves tree species ranges by shifting the temperature and precipitation envelope each species tolerates, generally pushing suitable habitat northward and upslope. Researchers describe this as movement at the leading edge (where a species can newly colonize) and the trailing edge (where it declines and contracts). The U.S. Forest Service Landscape Change Research Group, including Louis Iverson, Anantha Prasad, Matt Peters, and Steve Matthews, built the Climate Change Tree Atlas to map species-specific suitability zones for the eastern United States. Their methodology combines Forest Inventory and Analysis plots with climate and soil variables to project where habitat for oak, red maple, sugar maple, white pine, and other species expands or shrinks under RCP 4.5 and RCP 8.5. In Canada, Dan McKenney and the Canadian Forest Service maintain related work through Canada's Plant Hardiness Site, while the USDA Plant Hardiness Zones Map documents how hardiness zones have already shifted.

Why do cold-climate conifers face stress under warming?

Cold-climate conifers face stress under warming because species such as balsam fir, eastern hemlock, and the northern spruces are adapted to short, cool growing seasons and lose competitive ground as temperatures rise. These northern tree species sit near the trailing edge across the Northeast, New England, and the LEAP region near Lake Erie, where projections show declining suitability. Heat stress, drought, and shifting plant hardiness zones compound the pressure, and warmer winters also let pests survive — the emerald ash borer devastating black ash is one example of a climate-linked pest interaction now reshaping forest composition.

Can tree species migrate fast enough to keep up with climate change?

Most tree species cannot migrate fast enough to keep pace with climate change, creating a mismatch between migration capacity and climate velocity. Natural range migration historically proceeded at roughly tens to a few hundred meters per year through seed dispersal, but the rate at which suitable climate is now moving demands far faster colonization than seed dispersal mechanisms and reproductive strategies typically allow. Species relying on heavy, animal-dispersed seeds — many oaks — migrate especially slowly, while light-seeded pioneers like birch and trembling aspen move faster but still lag the pace of warming.

What barriers block tree migration across the landscape?

Barriers that block tree migration include landscape fragmentation, urban growth, agriculture, and roads that break the continuous habitat seeds need to leap across. Metropolitan expansion and urban forests interrupt natural corridors, and models such as SLEUTH simulate how urban growth reshapes the land available for tree distribution. Where fragmentation is severe, even species with otherwise adequate dispersal cannot colonize newly suitable ground, leaving suitable habitat empty while the trailing edge continues to contract.

What is assisted migration of tree species?

Assisted migration is the deliberate human movement of seeds or seedlings to sites projected to become climatically suitable, used to help species keep pace with change. The approach informs seed source selection and species choice in planting projects, matching planting stock to anticipated future climate rather than current conditions. The Northern Institute of Applied Climate Science and the broader Climate Change Response Framework provide guidance on when assisted migration is appropriate and how to weigh its ecological risks.

How are tree species classified by habitat and range?

Tree species are classified by habitat range so managers can group them by current distribution and projected climate response — for example, northern species expected to decline versus southern species expected to gain. In the Northeast, the Mid-Atlantic, and New England, classifications distinguish species at their trailing edge (balsam fir, black ash, eastern hemlock) from those expanding at the leading edge (oak, loblolly pine moving up from the Southern United States). The American elm illustrates how disease combined with range pressure can collapse a once-dominant species. Projections for the Southern United States to 2100 show loblolly pine and oak gaining ground as habitat shifts northward.

How do scientists predict future changes in forests?

Scientists predict future forest change by combining species distribution models with dynamic, process-based ecosystem models. The Climate Change Tree Atlas projects habitat suitability, while forest dynamic models such as LANDIS PRO simulate species demography, colonization, post-harvest regeneration, disturbance, and competition over time across landscapes; researchers including Hong S. He, Wen J. Wang, Jacob S. Fraser, William D. Dijak, and Frank R. Thompson have used these tools to model the interactive effects of harvest and climate change. The LINKAGES model simulates ecosystem processes such as nutrient cycling and growth to predict composition shifts. Peer-reviewed publications in outlets such as the Journal of Ecology, together with work from the Northern Research Station and the National Forest Service, document how these methods translate into vulnerability assessments. The Forest Ecosystem Atlas brings such projections into a usable interactive form for managers, with contributions from researchers including Todd Jones-Farrand.

What adaptation strategies help forest management under climate change?

Forest management adaptation under climate change centers on three broad strategies: resistance (defending high-value forests as they are), resilience (helping forests recover from disturbance), and transition (actively guiding forests toward future-adapted conditions). Practical decisions draw on tools and resources from the USDA Forest Service, the Climate Change Response Framework, and the Northern Institute of Applied Climate Science, and on extension guidance such as that from the University of Vermont Extension and forest scientists like Alexandra Kosiba and Patricia Leopold. A typical adaptation workflow includes:

• Conducting a forest vulnerability assessment for the stand or landscape.
• Identifying resilient and future climate-adapted species using the Climate Change Tree Atlas and the Forest Ecosystem Atlas.
• Selecting seed sources matched to projected conditions, including assisted migration where justified.
• Diversifying species and structure to spread risk across pests, drought, and disturbance.
• Monitoring outcomes and revising plans as new projections arrive.

What ecological and economic functions do tree species provide?

Tree species provide ecological, economic, and cultural functions that make succession shifts consequential well beyond the forest itself. Ecologically, species composition governs wildlife habitat, water regulation, soil protection, and the ecosystem services that depend on forest structure — the loss of a foundation species such as eastern hemlock cascades through whole communities. Economically, species choice determines wood fiber availability for the forest industry, and shifts in composition change which timber and products a region can supply. Culturally, forests carry recreational, aesthetic, and heritage value, which is why the public character of birch and aspen woods matters as much as their timber. Spatial variability means these impacts differ sharply from one microclimate and locality to the next, so management decisions must be local even when projections are regional.