The role of hydrological transience in peatland pattern formation

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Abstract

The sloping flanks of peatlands are commonly patterned with non-random, contour-parallel stripes of distinct micro-habitats such as hummocks, lawns and hollows. Patterning seems to be governed by feedbacks among peatland hydrological processes, plant micro-succession, plant litter production and peat decomposition. An improved understanding of peatland patterning may provide important insights into broader aspects of the long-term development of peatlands and their likely response to future climate change. We recreated a cellular simulation model from the literature, as well as three subtle variants of the model, to explore the controls on peatland patterning. Our models each consist of three submodels, which simulate: peatland water tables in a gridded landscape, micro-habitat dynamics in response to water-table depths, and changes in peat hydraulic properties. We found that the strength and nature of simulated patterning was highly dependent on the degree to which water tables had reached a steady state in response to hydrological inputs. Contrary to previous studies, we found that under a true steady state the models predict largely unpatterned landscapes that cycle rapidly between contrasting dry and wet states, dominated by hummocks and hollows, respectively. Realistic patterning only developed when simulated water tables were still transient. Literal interpretation of the degree of hydrological transience required for patterning suggests that the model should be discarded; however, the transient water tables appear to have inadvertently replicated an ecological memory effect that may be important to peatland patterning. Recently buried peat layers may remain hydrologically active despite no longer reflecting current vegetation patterns, thereby highlighting the potential importance of three-dimensional structural complexity in peatlands to understanding the two-dimensional surface-patterning phenomenon. The models were highly sensitive to the assumed values of peat hydraulic properties, which we take to indicate that the models are missing an important negative feedback between peat decomposition and changes in peat hydraulic properties. Understanding peatland patterning likely requires the unification of cellular landscape models such as ours with cohort-based models of long-term peatland development.

Figures

  • Figure 1. Aerial photograph showing contour-parallel, striped patterning on a peatland complex in the James Bay lowlands, Ontario, Canada. The directions of slope and regional water flow are from the top right to the bottom left of the picture. Horizontal distance between tops of successive ridges is approximately 5 to 10 m. Image belongs to Brian Branfireun, reproduced here with kind permission.
  • Table 1. Glossary of algebraic terms, including default values where appropriate, for each of the four model versions.
  • Table 2. Summary of the four models and the objectives addressed by each.
  • Figure 2. Time series of daily-average absolute water-table change per grid square during five-year test periods with Models 1, 2 and 3. See main text for full details. Note the logarithmic scales on both axes.
  • Figure 3. Graphical representation of the linear probability function used by the ecological submodel to assign hummock and hollow states to each model cell, based on water-table depth.
  • Figure 4. 365-day time series of daily observed rainfall, and its conversion to net rainfall, U, after scaling by a factor of 0.245. See main text for full details.
  • Figure 5. Final micro-habitat maps after 100 developmental steps, showing effects of increasing ∆te on patterning in typical simulations with Model 1 (top panel), Model 2 (middle panel) and Model 3 (bottom panel). The value of ∆te assumed for each simulation is indicated immediately above the map. Light pixels represent hummocks, dark pixels represent hollows. Low values of y represent upslope locations (y = 0 is the upslope boundary); high values of y represent downslope locations (y = 200 is the downslope boundary); as such, groundwater flow is generally down the page. All maps have the same horizontal (x, y) scale as that shown for the upper-leftmost map.
  • Figure 6. Influence of hydrological equilibration time, ∆te, over temporal development of relative variance of hummocks per model row, R (top row); SL1 turnover rate, Q (middle row); and proportion of model landscape occupied by hummocks, S (bottom row); in Models 1 (left column), 2 (middle column), 3 and 4 (right column).

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CITATION STYLE

APA

Morris, P. J., Baird, A. J., & Belyea, L. R. (2013). The role of hydrological transience in peatland pattern formation. Earth Surface Dynamics, 1(1), 29–43. https://doi.org/10.5194/esurf-1-29-2013

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