Types of Plant Succession
Plant succession describes the gradual, directional changes in plant community composition over time. These changes typically follow a disturbance or the creation of entirely new habitat. Succession is central to understanding how plant communities develop, recover, and reorganize.
Primary vs. Secondary Succession
Primary succession occurs on newly formed or exposed substrates where no soil or vegetation previously existed. Think bare rock, fresh lava flows, or sand dunes. Because there's no soil to start with, primary succession is slow. Pioneer species must build soil from scratch through weathering and organic matter accumulation.
Secondary succession takes place where soil remains intact and some seeds, roots, or other biological legacies persist after a disturbance. Abandoned agricultural fields, burned forests, and storm-damaged areas all undergo secondary succession. It proceeds much faster than primary succession because the soil, seed bank, and sometimes root systems are already in place.
Autogenic vs. Allogenic Succession
These terms describe what's driving the successional change:
- Autogenic succession is driven by internal factors. The plants themselves modify their environment in ways that favor different species. For example, early colonizers add organic matter to soil and create shade, eventually making conditions better for shade-tolerant species and worse for themselves.
- Allogenic succession is driven by external forces like shifts in climate, changes in hydrology, or altered disturbance regimes (e.g., increased fire frequency or flooding patterns).
In practice, both autogenic and allogenic factors interact to shape succession in any given ecosystem.
Stages of Succession
Succession generally moves through a series of recognizable stages, each with distinct plant communities and environmental conditions. While the specifics vary by ecosystem and disturbance type, the broad pattern includes pioneer, intermediate, and climax stages.
Pioneer Species
Pioneer species are the first to colonize a newly exposed or disturbed area. They share a common set of traits: fast growth rates, high seed dispersal ability, and tolerance of harsh conditions like intense sunlight, nutrient-poor soil, and temperature extremes.
In primary succession, pioneers include lichens and mosses that can colonize bare rock and begin breaking it down. In secondary succession, pioneers are often fast-growing herbaceous plants. Fireweed (Chamaenerion angustifolium) is a classic example in post-fire landscapes, and beach grass (Ammophila breviligulata) stabilizes coastal dunes.
Pioneer species play a critical role: they stabilize the substrate, begin building soil, and create conditions that allow later-successional species to establish.
Intermediate Stages
As succession progresses, species diversity increases and community structure becomes more complex. Biotic interactions like competition and facilitation start to matter more than raw environmental tolerance.
Typical intermediate-stage species include shrubs (Rubus spp., Salix spp.) and fast-growing, shade-intolerant trees like birch (Betula spp.) and poplar (Populus spp.). These species outcompete pioneers for light and nutrients but will eventually be replaced by slower-growing, more shade-tolerant species.
Climax Community
The climax community is the relatively stable endpoint of succession. It's dominated by long-lived, shade-tolerant species with complex vertical structure (canopy layers, understory, ground cover).
The composition of a climax community is largely determined by regional climate and soil conditions. In eastern North American forests, sugar maple (Acer saccharum) is a common climax dominant. In coastal California, coast redwood (Sequoia sempervirens) fills that role.
Worth noting: the "climax" concept is somewhat idealized. Many ecologists now recognize that most communities exist in a state of ongoing change rather than reaching a single, permanent equilibrium.
Factors Influencing Succession
Soil Development
Soil development happens alongside succession and is tightly linked to it. As pioneer species grow and die, they add organic matter. Their roots break apart rock. Symbiotic relationships with nitrogen-fixing bacteria and mycorrhizal fungi improve nutrient availability.
This gradual soil building changes which species can thrive. Later-successional species often require deeper, more nutrient-rich soils that simply didn't exist during the pioneer stage.
Climate and Microclimate
Regional climate (temperature, precipitation) sets the broad boundaries for what the climax community can look like. A site in the Pacific Northwest will follow a very different successional trajectory than one in the Sonoran Desert.
At a finer scale, microclimate factors like slope aspect, elevation, and topographic position create local variation. A north-facing slope may stay cooler and moister than a south-facing slope just meters away, supporting different species at the same successional stage.
Climate change can also shift successional trajectories over time, potentially altering what the "climax" community looks like for a given region.
Biotic Interactions
- Competition for light, water, and nutrients drives species turnover. As taller species shade out shorter ones, community composition shifts.
- Facilitation occurs when one species improves conditions for another. Nurse plants that provide shade for seedlings or nitrogen-fixers that enrich the soil are common examples.
- Herbivory can selectively remove certain species, changing competitive dynamics. Heavy deer browsing, for instance, can prevent tree seedlings from establishing and stall forest succession.
Mechanisms of Succession
Ecologists recognize three main mechanisms that describe how species interact during succession. These were formalized by Connell and Slatyer (1977) and remain a useful framework.
Facilitation
In the facilitation model, early species make the environment more suitable for later species. Nitrogen-fixing plants like alders (Alnus spp.) enrich the soil, making it possible for nutrient-demanding species to establish. Nurse plants provide shade or wind protection for seedlings that couldn't survive in the open.
Facilitation tends to accelerate succession by actively promoting the arrival and growth of later-successional species.
Inhibition
In the inhibition model, early-arriving species make conditions worse for later species. They may monopolize light, deplete soil nutrients, or release allelopathic chemicals that suppress germination of competitors.
Under inhibition, succession slows down. Later species can only establish after the inhibiting species are damaged or die, often from disturbance, disease, or old age.
Tolerance
In the tolerance model, later-successional species are simply better at tolerating low resource levels. They can germinate and grow slowly beneath the canopy of earlier species, gradually replacing them as the early colonizers age and die.
Shade-tolerant tree species like sugar maple or hemlock are classic tolerance strategists. They don't need the early species to help them or get out of the way; they just outlast them.
Disturbance in Plant Communities
Disturbances are events that disrupt existing plant communities by removing or damaging vegetation and altering environmental conditions. Their frequency, intensity, and spatial scale all shape how plant communities respond.
Natural Disturbances
Natural disturbances include wildfires, windstorms, floods, droughts, volcanic eruptions, and pest or pathogen outbreaks. Far from being purely destructive, these disturbances are often integral to ecosystem function. They maintain diversity by preventing any single species from dominating indefinitely and create openings for regeneration.
Some ecosystems depend on specific disturbance regimes. Ponderosa pine forests and chaparral shrublands are adapted to periodic fire. Old-growth forests rely on windthrow and individual tree mortality to create canopy gaps that allow understory species to reach the light.
Human-Induced Disturbances
Human activities create disturbances that often differ from natural ones in frequency, intensity, and scale:
- Deforestation and clear-cutting remove entire forest stands at once
- Agricultural conversion replaces native communities with monocultures
- Urbanization permanently alters substrates and hydrology
- Invasive species introductions change competitive dynamics in ways native communities haven't experienced
These disturbances can lead to loss of native species, altered successional pathways, and the emergence of novel ecosystems with no historical analog.
Effects of Disturbance on Succession
Resetting Succession
Severe disturbances that remove most or all existing vegetation can push a community back to the pioneer stage. Volcanic eruptions (like Mount St. Helens in 1980), glacial retreat, and large-scale clear-cutting are all examples. After a reset, entirely new species may colonize, potentially sending the community along a different successional trajectory than before.
Altering Successional Pathways
Not all disturbances reset succession. Many redirect it instead. Frequent, low-intensity fires in grasslands are a good example: they kill woody seedlings but leave fire-adapted grasses unharmed, maintaining grassland rather than allowing succession toward forest.
When disturbance regimes change (e.g., fire suppression in historically fire-prone areas), successional pathways shift too. This can produce alternative stable states, where the community settles into a configuration that differs from the historical climax and persists even after the altered disturbance stops.
Resilience and Resistance
These two concepts describe different aspects of how communities handle disturbance. They're related but distinct.
Resilience
Resilience is the capacity of a plant community to recover after a disturbance and return to something resembling its pre-disturbance state. Communities with high species diversity and functional redundancy (multiple species filling similar ecological roles) tend to be more resilient. If one species is lost, others can fill its niche.
Resistance
Resistance is the ability of a community to withstand disturbance without changing much in the first place. Communities dominated by long-lived, stress-tolerant species with specific adaptations (thick bark for fire resistance, deep roots for drought tolerance) tend to be more resistant.
A community can be highly resistant but not very resilient, or vice versa. An old-growth forest may resist small disturbances well but recover very slowly from a catastrophic one. A grassland may be easily disturbed by fire but bounce back within a single growing season.
Succession in Different Ecosystems
Forest Succession
Forest succession typically follows a pattern where shade-intolerant pioneer trees and shrubs are gradually replaced by shade-tolerant species. In eastern North America, a cleared field might progress from herbaceous plants to birch and aspen, then to oak and hickory, and eventually to a beech-maple climax forest. The rate depends on factors like seed dispersal, gap dynamics, and herbivore pressure.
Grassland Succession
Grassland succession often involves the replacement of annual grasses and weedy forbs by longer-lived perennial grasses and native forbs. Fire and grazing are key drivers. In central North America, tallgrass prairies are maintained by periodic fire that prevents woody encroachment. After agricultural abandonment, grassland recovery can take decades, especially if the native seed bank has been depleted.
Wetland Succession
Wetland succession involves changes in hydrology, sediment accumulation, and plant community composition. One common pathway is hydrosere succession, where open water gradually fills with sediment and organic matter, progressing from submerged aquatic plants to emergent marsh vegetation and eventually to terrestrial communities. Peatlands develop through the accumulation of partially decomposed plant material (especially Sphagnum moss), a process that can take thousands of years.
Applications of Succession Theory
Ecological Restoration
Ecological restoration aims to help degraded ecosystems recover. Succession theory guides key decisions in this process:
- Which pioneer species to plant first to build soil and create conditions for later species
- Whether to mimic or manipulate natural disturbance regimes (e.g., prescribed burns)
- What realistic endpoints to target, given current climate and soil conditions
Successful restoration projects often work with natural successional processes rather than trying to skip directly to a climax community.
Invasive Species Management
Invasive species can hijack succession by outcompeting natives, altering soil chemistry, or changing fire regimes. Japanese knotweed (Reynoutria japonica), for example, forms dense stands that suppress native regeneration and effectively stall succession.
Effective management requires understanding where invasive species fit into the successional sequence. Strategies include targeted removal timed to specific successional windows and replanting with native species that can compete at each stage.
Sustainable Land Management
Succession theory helps land managers predict how ecosystems will respond to different practices:
- Rotational grazing in grasslands can mimic natural herbivory patterns, maintaining species diversity and preventing dominance by a few competitors
- Selective logging in forests can mimic natural gap formation, promoting regeneration of shade-tolerant species while maintaining late-successional habitat structure
- Prescribed fire can maintain fire-dependent ecosystems in their characteristic successional state rather than allowing them to shift toward a different community type