Discharge Competence and Pattern Formation in Peatlands: A Meta-Ecosystem Model of the Everglades Ridge-Slough Landscape

Abstract

Regular landscape patterning arises from spatially-dependent feedbacks, and can undergo catastrophic loss in response to changing landscape drivers. The central Everglades (Florida, USA) historically exhibited regular, linear, flow-parallel orientation of high-elevation sawgrass ridges and low-elevation sloughs that has degraded due to hydrologic modification. In this study, we use a meta-ecosystem approach to model a mechanism for the establishment, persistence, and loss of this landscape. The discharge competence (or self-organizing canal) hypothesis assumes non-linear relationships between peat accretion and water depth, and describes flow-dependent feedbacks of microtopography on water depth. Closed-form model solutions demonstrate that 1) this mechanism can produce spontaneous divergence of local elevation; 2) divergent and homogenous states can exhibit global bi-stability; and 3) feedbacks that produce divergence act anisotropically. Thus, discharge competence and non-linear peat accretion dynamics may explain the establishment, persistence, and loss of landscape pattern, even in the absence of other spatial feedbacks. Our model provides specific, testable predictions that may allow discrimination between the self-organizing canal hypotheses and competing explanations. The potential for global bi-stability suggested by our model suggests that hydrologic restoration may not re-initiate spontaneous pattern establishment, particularly where distinct soil elevation modes have been lost. As a result, we recommend that management efforts should prioritize maintenance of historic hydroperiods in areas of conserved pattern over restoration of hydrologic regimes in degraded regions. This study illustrates the value of simple meta-ecosystem models for investigation of spatial processes.

Coupled local facilitation and global hydrologic inhibition drive landscape geometry in a patterned peatland

“…Kaplan et al (2012) demonstrated this feedback with a hydrodynamic model of surface water flow across randomly generated patterned landscapes of varying anisotropy, finding that hydroperiod decreased exponentially with increasing patch anisotropy. Using a patchscale analytical model, Heffernan et al (2013) demonstrated strong feedbacks between the soil elevation at any given location and an adjacent location perpendicular to flow, but no such feedback parallel to flow, lending support to the central mechanism of the SOC hypothesis. However, that analytical model was limited to two patches, where flow in one cell was directly controlled by flow in the only other cell.…”

“…Although multiple hypotheses exist for explaining the ridge-slough pattern, most of them attribute the development of these landscapes to one dominant process. The self-organizing-canal hypothesis (Cohen et al, 2011;Heffernan et al, 2013), on the other hand, ascribes pattern formation and maintenance to reciprocal feedbacks between landscape pattern and hydrology. Moreover, evidence of a strong feedback between pattern and hydroperiod (Kaplan et al, 2012) lends support for the SOC.…”

“…An alternate explanation for ridge-slough patterning that includes a global negative feedback is the self-organizingcanal (SOC) hypothesis (Cohen et al, 2011;Heffernan et al, 2013). In this conceptualization, spatially anisotropic patterning emerges from global constraints on patch (both ridges and sloughs) expansion created by feedbacks between landscape geometry, water flow, and hydroperiod, which controls peat accretion (and therefore affects landscape geometry).…”

“…In this conceptualization, spatially anisotropic patterning emerges from global constraints on patch (both ridges and sloughs) expansion created by feedbacks between landscape geometry, water flow, and hydroperiod, which controls peat accretion (and therefore affects landscape geometry). In short, the SOC hypothesis proposes that elongated patches develop as the landscape incrementally adjusts its spatial geometry (ridge density, size, and shape) to optimize the discharge competence (i.e., ability to convey water; Heffernan et al, 2013;Kaplan et al, 2012;Cohen et al, 2011), and that this pattern may evolve without sediment or nutrient redistribution. Decrease in discharge competence primarily results from high ridge density but can be further intensified by reduced patch elongation (which lowers the probability of longitudinally connected sloughs).…”

“…In this study we implemented the local and global feedbacks described in the SOC hypothesis (Cohen et al, 2011;Heffernan et al, 2013) in a stochastic cellular automata framework to model temporal evolution of the ridgeslough pattern. Transition probabilities between ridge and slough states were driven by hydroperiod (a global negative feedback) and local facilitation.…”