Complex Dynamics in Models of Peat Accumulation lead to Dramatic Regime Shifts under a Steady Climate

Peatlands are highly concentrated and potentially fragile stores of organic carbon (C), and the accumulation of deep peat deposits during the Holocene represents a globally-significant terrestrial C sink. Several existing models predict the 1-D (vertical) accumulation of peat based on representations of peatland ecohydrological feedbacks. However, many such models make oversimplifying assumptions. In particular, hydraulic conductivity, the rate of formation of fresh peat, and peatland lateral (drainage) extent are often assumed to be constants. The available evidence suggests that complex feedbacks between these quantities and other neglected elements of peatland systems may play an important role in determining long-term rates of peat accumulation and trends in peatland surface-wetness. We observed the response of a 1-D model of peat accumulation to assumed long-term net rainfall rates and peat decay rates over 5,000 simulated years. We increased model complexity incrementally so as to represent: rate of formation of fresh peat as a humpback function of surface wetness; hydraulic conductivity as a logarithmic function of peat decay; and peatland lateral extent as various functions of time. Each incremental increase in model complexity caused qualitative and quantitative alterations to model behaviour, suggesting that the accurate understanding of peat accumulation may require the consideration of complex, interacting feedbacks not previously considered in modelling studies. Importantly, maximum long-term rates of peat accumulation occurred under intermediate peat decay rates and intermediate rainfall rates. This is a novel finding which challenges the assumptions of traditional, static models commonly used to estimate rates of carbon accumulation from dated peat profiles. Also, the assumed rate and mode of peatland lateral expansion are identifiable in model peat profiles, suggesting that they may also have to be taken into account when analysing peat records. Model peat profiles after 5,000 simulated years are suggestive of abrupt changes in hydrological regime, despite a steady climatic influence (in the form of constant net rainfall rate). We extended our 1-D model into an extra (horizontal) spatial dimension and modelled the development of a 2-D peat profile for a growing bog. Rates of peat accumulation were quantitatively and qualitatively different between the 1-D and 2-D models. Differences partly reflect the numerical integration methods used, but the inclusion of horizontal space clearly had a significant effect upon model behaviour. More so than for the 1-D model, peat profiles for the 2-D model after 5,000 years very clearly show abrupt shifts in water-table depth and peat accumulation regime. The shifts represent autogenic changes in hydrological regime which occur under a constant climatic influence, and are likely natural stages in peatland spatio-temporal development. The possibility of abrupt transitions such as these occurring under a constant climate fits with existing reports, and has both theoretical and practical implications for the use of peat records as indicators of palaeoclimatic change.