The role of hydrological transience in peatland pattern formation

Abstract

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.

Microform‐scale variations in peatland permeability and their ecohydrological implications

“…They proposed a hypothetical mechanism for ecological memory ( sensu Hendry & McGlade ) in peatlands whereby former surface vegetation patterns can leave a three‐dimensional imprint in the hydraulic structure of peat even after they are buried by litter from more recent plant assemblages. In a bog that possesses strong ecological memory of the type proposed by Morris, Baird & Belyea (), differences in peat properties (e.g. K ) that characterize particular microform types would be evident not only near the surface in upper peat but also in deeper peat.…”

“…A group of theoretical studies (Swanson & Grigal ; Couwenberg & Joosten ; Eppinga et al . ; Morris, Baird & Belyea ) used cellular landscape models to investigate the linkages between the hydrological and ecological functioning of peatlands, in particular how these linkages can lead to the development of hummock–hollow patterns. These models assume that the hydraulic properties of peat under different microforms are also different – they assume there is lateral variability – but also that most water flow occurs in the uppermost layers of a bog.…”

“…These models assume that the hydraulic properties of peat under different microforms are also different – they assume there is lateral variability – but also that most water flow occurs in the uppermost layers of a bog. Morris, Baird & Belyea () showed that these models, despite considering horizontal variability, likely lack some important ecohydrological feedbacks and may produce realistic patterns for the wrong reasons. Specifically, Morris, Baird & Belyea () raised the question of whether microforms such as hummocks and hollows can be considered as features only of shallow peat, or whether their structural and hydrological importance at both the scale of the microform and the whole bog extends into deeper peat.…”

“…Morris, Baird & Belyea () showed that these models, despite considering horizontal variability, likely lack some important ecohydrological feedbacks and may produce realistic patterns for the wrong reasons. Specifically, Morris, Baird & Belyea () raised the question of whether microforms such as hummocks and hollows can be considered as features only of shallow peat, or whether their structural and hydrological importance at both the scale of the microform and the whole bog extends into deeper peat. They proposed a hypothetical mechanism for ecological memory ( sensu Hendry & McGlade ) in peatlands whereby former surface vegetation patterns can leave a three‐dimensional imprint in the hydraulic structure of peat even after they are buried by litter from more recent plant assemblages.…”

Microtopographical and hydrophysical controls on subsurface flow and solute transport: A continuous solute release experiment in a subarctic bog

“…During high water table conditions, such as those present during this spill, solute migration occurred preferentially through hollows in the landscape. Although preferential hydrological connectivity through hollows has been theorized (Oosterwoud et al, ) and flow models have assumed this process (Couwenberg & Joosten, ; Morris, Baird, & Belyea, ), this study provides the first empirical evidence of this important process on the transport of both water and solutes.…”