Autogenic Succession

Autogenic succession (from the Greek autos, meaning "self," and genesis, meaning "origin") is the directional process of ecological succession in which sequential species replacement and community structural development are driven endogenously by the biological activities of the resident organisms themselves. Formalized in 1935 by British plant ecologist Sir Arthur George Tansley to distinguish internally generated biotic community trajectories from externally forced allogenic succession, autogenic succession builds upon Frederic E. Clements's (1916) concept of biotic reaction—the cumulative alteration of soil chemistry, humus depth, moisture retention, mineral weathering, and sub-canopy irradiance by colonizing plants, microbes, and soil fauna.

Across both primary succession on newly exposed mineral substrates (such as deglaciated forelands, cooled basaltic lava flows, and coastal sand dunes) and secondary succession following canopy disturbance (such as windthrow, wildfire, or agricultural abandonment), autogenic modifications progressively transform the local abiotic template. In doing so, pioneer species often render the microenvironment less suitable for their own juvenile recruitment while facilitating or inhibiting the establishment of later-successional guilds across the biosphere.

Pedogenesis, Litter Accumulation, and Symbiotic Nitrogen Fixation

On newly exposed parent rock or glacial till devoid of organic carbon and bioavailable nitrogen, autogenic succession initiates through coupled biological weathering and pedogenesis (soil formation):

• Lithobiontic Weathering and Organic Acid Exudation: Crustose lichens (symbioses between ascomycete fungi and green algae or cyanobacteria) and pioneer bryophytes (such as Racomitrium and Polytrichum mosses) colonize bare silicate or carbonate rock. By secreting oxalic, citric, and carbonic acids (H₂CO₃) and chelating divalent cations (Ca²⁺, Mg²⁺, Fe²⁺), these non-vascular pioneers accelerate mineral dissolution and trap aeolian dust to form a nascent organo-mineral horizon (protosoil).
• Rhizosphere Symbiotic Nitrogen Fixation: Because primary mineral substrates lack fixed reactive nitrogen (NH₄⁺ and NO₃^-) despite abundant rock-derived phosphorus (PO₄^3-), early vascular colonists are frequently actinorhizal or rhizobial nitrogen-fixers that host diazotrophic bacteria (such as Frankia or Rhizobium) inside root nodules. Classic examples include mountain avens (Dryas drummondii) and Sitka alder (Alnus viridis subsp. sinuata) on deglaciated moraines, or sea buckthorn (Hippophae rhamnoides) on coastal dunes. Symbiotic fixation reduces atmospheric N₂ to ammonia (NH₃), adding 50 to 150 kg N ha^-1 yr^-1 to developing soil horizons while leaf litter deposition lowers bulk density, raises cation exchange capacity, and acidifies soil pH from >7.8 to <5.0 through nitrification (NH₄⁺ + 2O₂ → NO₃^- + H₂O + 2H⁺) and organic acid leaching.
• Mycorrhizal Network Shifts: As soil organic matter and C:N ratios increase during autogenic progression, the belowground fungal community shifts from ruderal arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) toward ectomycorrhizal (ECM) and ericoid mycorrhizal basidiomycetes and ascomycetes capable of secreting extracellular peroxidases and acid phosphatases that mine organic nitrogen and occluded PO₄^3- directly from recalcitrant humus.

Canopy Light Attenuation and Sub-Canopy Microclimate Buffering

Above ground, the vertical accrual of leaf area index (LAI) and woody stem biomass by foundation species restructures the radiative and hydrological microclimate:

• Beer-Lambert Irradiance Extinction: According to the Monsi-Saeki (1953) application of the Beer-Lambert law to plant canopies, photosynthetically active radiation (PAR, 400–700 nm) at cumulative leaf area index L decays exponentially as I(L) = I₀ exp(−k L), where k is the canopy foliar extinction coefficient (typically 0.4 to 0.8). While pioneer herbs and shade-intolerant early trees (such as birches Betula, aspens Populus tremuloides, and loblolly pine Pinus taeda) require high light compensation points (15–30% full sunlight), their closing canopy reduces forest-floor PAR to 1–3% of open-sky irradiance and lowers the red-to-far-red photon ratio (R:FR from ~1.15 in direct sunlight to ~0.20 beneath foliage). Shade-intolerant pioneer seedlings fail to maintain a positive carbon balance beneath their own parents, whereas shade-tolerant climax species (such as sugar maple Acer saccharum, American beech Fagus grandifolia, and eastern hemlock Tsuga canadensis) regenerate in the shaded understory.
• Vapor Pressure Deficit and Soil Moisture Regulation: The closed canopy and litter layer insulate the soil surface against convective wind drying and diurnal thermal extremes, lowering daytime vapor pressure deficit (VPD) and enhancing hydraulic conductivity for desiccation-sensitive saplings and nurse plant beneficiaries.

In a landmark 1977 synthesis in The American Naturalist, Joseph H. Connell and Ralph O. Slatyer replaced Clements's strictly directional view of facilitation with three testable mechanistic models describing how early-arriving species modify the environment for subsequent colonists:

Glacier Bay Foreland Chronosequence (Alaska)

Studied by William S. Cooper (1923), Donald B. Lawrence (1958), and F. Stuart Chapin III et al. (1994), the 250-year glacial recession chronosequence at Glacier Bay, southeastern Alaska, illustrates coupled autogenic facilitation and inhibition across four stages:

1. Pioneer Stage (0–20 years post-deglaciation): Newly exposed calcareous glacial till (pH 8.0–8.4, soil N < 5 g m^-2) is colonized by black organic soil crusts, Epilobium latifolium, dwarf willows (Salix), and mat-forming nitrogen-fixing Dryas drummondii.
2. Alder Thicket Stage (20–70 years): Dense thickets of actinorhizal Sitka alder (Alnus viridis subsp. sinuata) fix atmospheric N₂ via Frankia root nodules, raising total soil nitrogen sevenfold (to >300 g m^-2) while leaf litter decomposition and nitric acid leaching reduce surface soil pH from 8.0 to 4.6 within five decades.
3. Sitka Spruce Forest Stage (70–170 years): Facilitated by high soil nitrogen, Sitka spruce (Picea sitchensis)—while simultaneously experiencing partial competition for light and root space from alder—eventually grows above the 8-meter alder canopy, shading out the shade-intolerant Alnus and shifting the C:N ratio of forest-floor litter upward.
4. Spruce–Hemlock Climax Complex (>170 years) and Hydrosere Paludification: Shade-tolerant western hemlock (Tsuga heterophylla) invades the spruce understory. On poorly drained benches, acidic conifer litter and colonizing peat mosses (Sphagnum spp.) acting as autogenic ecosystem engineers exchange H⁺ ions for mineral cations, waterlog the soil profile, induce iron-pan (spodosol ortstein) formation, and convert upland forest into an ombrotrophic Sphagnum bog (autogenic paludification).

Hydrosere Terrestrialization of Kettle Lakes

In freshwater kettle lakes and shallow basins, hydrosere autogenic succession converts open water into terrestrial peatland or swamp forest through progressive autochthonous organic sedimentation (gyttja and peat accrual). Submerged macrophytes (Potamogeton, Ceratophyllum) trap suspended detritus and elevate the benthic sediment surface into the depth zone of floating-leaved plants (Nymphaea, Nuphar), which are successively replaced by emergent reedswamp graminoids (Phragmites australis, Typha latifolia, Carex sedges), minerotrophic fen carr (Alnus, Salix), and raised ombrotrophic Sphagnum bog domes perched above the regional water table.

1. Tansley, A. G. (1935). The use and abuse of vegetational concepts and terms. Ecology, 16(3), 284–307. https://doi.org/10.2307/1930070
2. Connell, J. H., & Slatyer, R. O. (1977). Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist, 111(982), 1119–1144. https://doi.org/10.1086/283241
3. Chapin, F. S., Walker, L. R., Fastie, C. L., & Sharman, L. C. (1994). Mechanisms of primary succession following deglaciation at Glacier Bay, Alaska. Ecological Monographs, 64(2), 149–175. https://doi.org/10.2307/2937039
4. Walker, L. R., & del Moral, R. (2003). Primary Succession and Ecosystem Rehabilitation. Cambridge University Press. https://doi.org/10.1017/CBO9780511615078