Modelling the influence of mechanical-ecohydrological feedback on the nonlinear dynamics of peatlands

Peatlands are complex systems that exhibit nonlinear dynamics due to internal and external feedback mechanisms. However, the feedback of vegetation on peat volume changes that potentially affect peatland dynamics is not well understood. Here, we analyse the consequences of coupling between plant functional types with peat stiffness on a nonequilibrium model of a peatland by developing MPeat model. In this formulation, the peat systems prefer to exist in two possible states defined by two limit cycles, one corresponding to a wet and the other to a dry attractor. These states can also coexist under the same net rainfall indicating bistability in which a crucial drying threshold leads to a tipping point and associated regime shift from soft-wet to stiff-dry states with related changes in rates of carbon storage. While the shift from wet to dry states constitutes a tipping point, to shift from the dry to wet states requires more sustained increases in net rainfall, indicating that dry state is the more stable attractor as the peatland grows. As the model peatland evolves, the response of surface motion, carbon accumulation, and water table depth to the same external forcing becomes increasingly higher amplitude indicating that a degree of caution may be required when interpreting the paleorecord. Investigation of the behaviour of these states in response to seasonal variations in water budget suggests that the wet state will display high amplitude and later peak timing when compared to the dry state, a phenomenon that is observed in measures of surface motion. Our study highlights the possible importance of mechanical-ecohydrological feedback and, in particular, the role of the coupling between the proportion of plant functional types, peat Young's modulus, plant weight, and water table position in influencing peatland regime shifts, critical thresholds or tipping points, and both short- and long-term peatland dynamical behaviour.