Vegetation engineers marsh morphology through multiple competing stable states

Abstract

Marshes display impressive biogeomorphic features, such as zonation, a mosaic of extensive vegetation patches of rather uniform composition, exhibiting sharp transitions in the presence of extremely small topographic gradients. Although generally associated with the accretion processes necessary for marshes to keep up with relative sea level rise, competing environmental constraints, and ecologic controls, zonation is still poorly understood in terms of the underlying biogeomorphic mechanisms. Here we find, through observations and modeling interpretation, that zonation is the result of coupled geomorphological-biological dynamics and that it stems from the ability of vegetation to actively engineer the landscape by tuning soil elevation within preferential ranges of optimal adaptation. We find multiple peaks in the frequency distribution of observed topographic elevation and identify them as the signature of biologic controls on geomorphodynamics through competing stable states modulated by the interplay of inorganic and organic deposition. Interestingly, the stable biogeomorphic equilibria correspond to suboptimal rates of biomass production, a result coherent with recent observations. The emerging biogeomorphic structures may display varying degrees of robustness to changes in the rate of sea level rise and sediment availability, with implications for the overall resilience of marsh ecosystems to climatic changes.

Fig. 1.

Zonation patterns generated by the model. (A) The time evolution of transect topography was started here from a linear initial condition, but several other initial conditions were explored with analogous results. Monospecific vegetation patches, very similar to observed zonation patterns (Inset), and terrace-like topographic structures emerge as a result of multiple stable states defined by ∂z/∂t = 0 and ∂/∂z(∂z/∂t) < 0. (B) Fitness functions of the species populating the marsh, which define the rate of organic soil production as Q_o = γ ⋅ B₀ ⋅ f_i(z), as well as the species competitive abilities (γ incorporates typical vegetation characteristics and the density of the organic soil produced; B₀ is the biomass density of a fully vegetated marsh). (C) The superscripts “(s)” and “(u)” denote stable and unstable equilibria, respectively. If the initial elevation of the site at is comprised between and , the elevation will tend toward . If the initial elevation of the site at is located below , the elevation will tend toward .

Fig. 2.

(A) Stochastic interspecific competition. Species i is selected as the successful competitor with a probability (for each site x_k and each time step), generating relatively noisy patterns. However, the multimodal frequency distribution of topographic elevation, the signature of the underlying biogeomorphic coupling, remains detectable. Each peak (color coded according to the species that is most abundant within each elevation interval) clearly is associated with the unique species that generates it. (B) In the absence of an organic soil contribution to the accretion rate, the resulting smooth topography is determined entirely by inorganic deposition. Vegetation species colonize the transect in banded patterns according to their respective fitnesses. (C) The frequency distribution of topographic elevation shows uncertain symptoms of multiple peaks when less specialized vegetation species (λ = 2) are considered, a sign that a decreased vegetation specialization produces less easily detectable multiple peaks in the topographic elevation frequency distribution.

Fig. 3.

(A and B) Observed zonation patterns. An accurate topographic survey (uncertainty smaller than 1 mm) reveals a multimodal frequency distribution of soil elevation, highly suggestive of the major role played by the biomass-elevation feedback in tuning marsh topography. Each bar is color coded according to the vegetation species that is most abundant within the pertinent elevation interval, showing that, indeed, elevation ranges are characteristic of the vegetation species (or of a typical mix of species at high elevations) that maintain them.