Allogenic Succession

Allogenic succession (from the Greek allos, meaning "other," and genesis, meaning "origin") is a mode of ecological succession in which sequential shifts in species composition, guild dominance, and community architecture are driven by external abiotic forcing—such as geomorphological sediment accretion, glacio-isostatic crustal rebound, climatic fluctuations, hydroperiod modification, volcanic tephra deposition, or watershed nutrient loading—rather than by the internal biological activities of the resident organisms. Introduced in 1935 by British plant ecologist Sir Arthur George Tansley as the mechanistic counterpart to autogenic succession, allogenic succession recognizes that ecological communities across the biosphere are embedded within non-stationary geological, hydrological, and atmospheric boundary conditions.

Whereas autogenic succession progresses toward an internally buffered community state as plants and soil microbes modify their own substrate, allogenic succession tracks a moving abiotic target: external environmental parameters (elevation relative to mean sea level, soil salinity, flooding frequency, redox potential, or sediment burial rate) change independently of—or at a velocity that overrides—biotic feedback. In virtually all natural ecosystems, observed community trajectories reflect a quantitative vector sum of allogenic physical forcing and autogenic biological modification.

Fluvial, Estuarine, and Coastal Geomorphic Forcing

Across sedimentary landforms and dynamic riparian zones, physical fluid transport continuously reconfigures substrate elevation, grain-size sorting, and soil aeration:

• Fluvial Point-Bar Accretion and Floodplain Avulsion: Meandering rivers erode cutbanks and deposit bedload sand and suspended silt along convex point bars during seasonal flood pulses. As allogenic sediment deposition raises a point bar incrementally above the baseflow river channel, flood duration drops, water-table depth increases, and soil redox potential shifts from anoxic (dominated by Fe²⁺ and H₂S) to oxic. Along boreal and temperate floodplains (such as the Tanana River in Alaska or the Rhône in France), this physical elevation gradient drives a directional replacement from flood-tolerant pioneer sandbar willows (Salix interior) and cottonwoods (Populus balsamifera) to alder (Alnus incana), white spruce (Picea glauca), and upland hardwood forests.
• Glacio-Isostatic Land Uplift: Following the melting of Pleistocene continental ice sheets (such as the Fennoscandian and Laurentide ice sheets), viscoelastic mantle rebound continues to elevate coastal bedrock and marine clays along the Gulf of Bothnia (at ~8–9 mm yr^-1) and Hudson Bay (at ~10–12 mm yr^-1). As former subtidal seafloors emerge into intertidal mudflats, supratidal coastal meadows, and uplifted inland terraces, marine macroalgae and halophytic marsh graminoids (Puccinellia phryganodes, Carex subspathacea) are sequentially replaced by willow-birch scrub and boreal Picea–Pinus taiga purely as a consequence of crustal uplift and meteoric rainwater leaching of marine NaCl.
• Aeolian Dust, Loess, and Tephra Deposition: Episodic volcanic ashfalls (tephra) and chronic aeolian mineral dust inputs supply exogenous base cations (Ca²⁺, Mg²⁺, K⁺) and rock-derived orthophosphate (PO₄^3-) that reset soil pH and rejuvenate phosphorus-depleted late-successional ecosystems, such as Hawaiian montane Metrosideros polymorpha rainforests receiving Asian tropospheric dust.

Hydroclimatic and Geochemical Boundary Shifts

On multi-decadal to millennial timescales, allogenic succession is forced by regional shifts in precipitation-to-evapotranspiration ratios (P/PET), temperature, and watershed solute fluxes:

• Lake-Level Fluctuations and Hydroperiod Shifts: Changes in basin hydrology raise or lower wetland water tables independently of peat accumulation, driving retrogressive succession or cyclic community replacement in prairie potholes, Everglades sawgrass (Cladium jamaicense) marshes, and boreal peatlands.
• Watershed Eutrophication and Acid Deposition: Anthropogenic or geogenic inputs of nitrate (NO₃^-), ammonium (NH₄⁺), phosphate (PO₄^3-), or atmospheric sulfur dioxide (SO₂) alter aquatic and terrestrial stoichiometry, shifting oligotrophic isoetid lakes (Lobelia dortmanna, Isoetes lacustris) toward elodeid macrophyte or phytoplankton dominance and eventually triggering transitions to alternative stable states.

Tidal Sediment Accretion and Halophyte Zonation

Coastal salt marshes and mangrove ecotones provide the canonical model system illustrating how allogenic geophysical forcing and autogenic biological engineering interact continuously:

1. Allogenic Tidal and Eustatic Forcing: Astronomical tides inundate intertidal mudflats twice daily, delivering suspended mineral silt, clay, and marine salts while imposing hydrostatic submergence stress and root-zone hypoxia (low dissolved O₂). Simultaneously, eustatic sea-level rise and deltaic sediment compaction alter the vertical position of the marsh platform relative to the tidal frame.
2. Biogeomorphic Coupling by Foundation Cordgrass: Once allogenic mudflat accretion elevates the sediment surface above the lower physiological threshold of smooth cordgrass (Spartina alterniflora, a classic foundation species and autogenic ecosystem engineer), colonizing Spartina stems exert hydrodynamic drag that attenuates tidal current velocity and wave orbital shear stress. This stem-induced baffling accelerates suspended sediment settling (up to 5–20 mm yr^-1), while belowground rhizome mats bind the accreted silt and oxygenate the rhizosphere via aerenchyma gas transport.
3. Transition to High-Marsh Zonation: As coupled allogenic mineral delivery and autogenic organic peat accumulation raise the marsh platform above mean high water (MHW), tidal inundation frequency declines. In the infrequently flooded high marsh, porewater evaporation concentrates salinity in shallow pans while well-drained levees experience reduced sulfide (H₂S) toxicity, driving the competitive replacement of Spartina alterniflora by saltmeadow cordgrass (Spartina patens), spikegrass (Distichlis spicata), black needlerush (Juncus roemerianus), and marsh elder (Iva frutescens).

When allogenic sea-level rise or watershed damming (which starves estuaries of fluvial suspended sediment) outpaces the maximum vertical accretion capacity of the marsh vegetation, allogenic forcing overrides autogenic homeostasis, driving retrogressive marsh drowning and conversion back to unvegetated subtidal mudflat.

Holocene Pollen Stratigraphy and Milankovitch Forcing

High-resolution palynological (fossil pollen) and macrofossil records preserved in lake sediments and ombrotrophic peats demonstrate that over time spans exceeding 10³–10⁴ years, continental vegetation dynamics are governed predominantly by allogenic climatic forcing rather than static Clementsian climax equilibria. Following the Last Glacial Maximum (~21 ka), orbital Milankovitch insolation shifts and Laurentide ice-sheet retreat drove individualistic species migrations across North America and Europe (Margaret B. Davis, 1981; Thompson Webb III, 1986), repeatedly reassembling novel forest communities as regional temperature and hydroclimate shifted.

Cyclic Allogenic Replacement in Wave-Regenerated and Permafrost Forests

In high-elevation subalpine balsam fir (Abies balsamea) forests of the northeastern United States (Douglas G. Sprugel, 1976) and permafrost black spruce (Picea mariana) taiga of interior Alaska, directional winds, rime-ice loading, and seasonal active-layer thaw depth act as chronic allogenic drivers that generate migrating "regeneration waves" and thermokarst thaw-lake cycles, perpetually resetting local community age structure across the landscape.

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2. Sprugel, D. G. (1976). Dynamic structure of wave-regenerated Abies balsamea forests in the North-Eastern United States. Journal of Ecology, 64(3), 889–911. https://doi.org/10.2307/2258815
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