Peatlands are wetland ecosystems that accumulate dead organic matter (i.e., peat) when plant litter production outpaces peat decay, usually under conditions of frequent or continuous waterlogging. Collectively, global peatlands store vast amounts of carbon (C), equaling if not exceeding the amount of C in the Earth's vegetation; they also encompass a remarkable diversity of forms, from the frozen palsa mires of the northern subarctic to the lush swamp forests of the tropics, each with their own characteristic range of fauna and flora. In this review we explain what peatlands are, how they form, and the contribution that peatland science can make to our understanding of global change. We explore the variety in formation, shape, vegetation type, and chemistry of peatlands across the globe and stress the fundamental features that are common to all peat-forming ecosystems. We consider the impacts that past, present, and future environmental changes, including anthropogenic disturbances, have had and will have on peatland systems, particularly in terms of their important roles in C storage and the provision of ecosystem services. The most widespread uses of peatlands today are for forestry and agriculture, both of which require drainage that results in globally significant emissions of carbon dioxide (CO 2), a greenhouse gas (GHG). Climatic drying and drainage also increase the risk of peat fires, which are a further source of GHG emissions [CO 2 and methane (CH 4)] to the atmosphere, as well as causing negative human health and socioeconomic impacts. We conclude our review by explaining the roles that paleoecological, experimental, and modeling studies can play in allowing us to build a more secure understanding of how peatlands function, how they will respond to future climate-and land-management-related disturbances, and how best we can improve their resilience in a changing world.

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[1] Recent studies suggest that ebullition of biogenic gas bubbles is an important process of CH 4 transfer from northern peatlands into the atmosphere and, as such, needs to be better described by models of peat carbon dynamics. We develop and test a simple ebullition model in which a threshold gas volume in the peat has to be exceeded before ebullition occurs. The model assumes that the gas volume varies because of gas production and variations in pressure and temperature. We incubated peat cores in the laboratory for 190 days and measured their volumetric gas contents and the ebullition flux. The laboratory results support the threshold concept and, considering the simplicity of the model, the calculated ebullition compared well with measured fluxes during the final 120 days with an r 2 of 0.66. An improved, more realistic description would also include temporal and spatial variations in gas production and bubble retention terms.

[1] Water-table reconstructions from Holocene peatlands are increasingly being used as indicators of terrestrial palaeoclimate in many regions of the world. However, the links between peatland water tables, climate, and long-term peatland development are poorly understood. Here we use a combination of high-resolution proxy climate data and a model of long-term peatland development to examine the relationship between rapid hydrological fluctuations in peatlands and climatic forcing. We show that changes in watertable depth can occur independently of climate forcing. Ecohydrological feedbacks inherent in peatland development can lead to a degree of homeostasis that partially disconnects peatland water-table behaviour from external climatic influences. We conclude by suggesting that further work needs to be done before peat-based climate reconstructions can be used to test climate models. Citation: Swindles, G. T.,

Using a literature review, we argue that new models of peatland development are needed. Many existing models do not account for potentially important ecohydrological feedbacks, and/or ignore spatial structure and heterogeneity. Existing models, including those that simulate a near total loss of the northern peatland carbon store under a warming climate, may produce misleading results because they rely upon oversimplified representations of ecological and hydrological processes. In this, the first of a pair of papers, we present the conceptual framework for a model of peatland development, DigiBog, which considers peatlands as complex adaptive systems. DigiBog accounts for the interactions between the processes which govern litter production and peat decay, peat soil hydraulic properties, and peatland water‐table behaviour, in a novel and genuinely ecohydrological manner. DigiBog consists of a number of interacting submodels, each representing a different aspect of peatland ecohydrology. Here we present in detail the mathematical and computational basis, as well as the implementation and testing, of the hydrological submodel. Remaining submodels are described and analysed in the accompanying paper. Tests of the hydrological submodel against analytical solutions for simple aquifers were highly successful: the greatest deviation between DigiBog and the analytical solutions was 2·83%. We also applied the hydrological submodel to irregularly shaped aquifers with heterogeneous hydraulic properties—situations for which no analytical solutions exist—and found the model's outputs to be plausible. Copyright © 2011 John Wiley & Sons, Ltd.

Peatlands are globally important stores of carbon (C) that contain a record of how their rates of C accumulation have changed over time. Recently, near-surface peat has been used to assess the effect of current land use practices on C accumulation rates in peatlands. However, the notion that accumulation rates in recently formed peat can be compared to those from older, deeper, peat is mistaken – continued decomposition means that the majority of newly added material will not become part of the long-term C store. Palaeoecologists have known for some time that high apparent C accumulation rates in recently formed peat are an artefact and take steps to account for it. Here we show, using a model, how the artefact arises. We also demonstrate that increased C accumulation rates in near-surface peat cannot be used to infer that a peatland as a whole is accumulating more C – in fact the reverse can be true because deep peat can be modified by events hundreds of years after it was formed. Our findings highlight that care is needed when evaluating recent C addition to peatlands especially because these interpretations could be wrongly used to inform land use policy and decisions.

In the first of this pair of papers we introduced the conceptual and hydrological basis of the peatland development model—DigiBog. Here we describe the submodels which simulate (i) the production of plant litter, (ii) peat decomposition, and (iii) changes in peat hydraulic conductivity due to decomposition. To illustrate how the model works, DigiBog was applied to three example situations: Bogs 1, 2, and 3. For each, the net rainfall was held constant at 30 cm year−1 and the oxic decomposition parameter kept at 0·015 year−1. The anoxic decomposition parameter varied from 5 × 10−6 (Bog 1) to 5 × 10−4 year−1 (Bog 3). Peatland development was simulated for 5000 years. For Bogs 1 and 2, plausible large peatland domes develop. Despite having a higher anoxic decomposition rate, Bog 2 grew thicker than Bog 1. This apparently counter‐intuitive result is caused by the feedback between hydraulic conductivity and degree of peat decomposition. For both Bogs 1 and 2, DigiBog also simulates transitions from wet to dry states, demarked by sudden switches from poorly decomposed to well‐decomposed peat moving upwards in the peat profile. These regime shifts result from internal peatland dynamics and not from allogenic influences, and challenge the view that peat properties are always a reflection of climate. In Bog 3, a ‘mini‐bog’ developed and persisted near the margin of the peatland; this bog can also be explained in terms of the internal feedbacks within the model. Copyright © 2011 John Wiley & Sons, Ltd.

Peatlands represent important archives of Holocene paleoclimatic information. However, autogenic processes may disconnect peatland hydrological behavior from climate and overwrite climatic signals in peat records. We use a simulation model of peatland development driven by a range of Holocene climate reconstructions to investigate climate signal preservation in peat records. Simulated water‐table depths and peat decomposition profiles exhibit homeostatic recovery from prescribed changes in rainfall, whereas changes in temperature cause lasting alterations to peatland structure and function. Autogenic ecohydrological feedbacks provide both high‐ and low‐pass filters for climatic information, particularly rainfall. Large‐magnitude climatic changes of an intermediate temporal scale (i.e., multidecadal to centennial) are most readily preserved in our simulated peat records. Simulated decomposition signals are offset from the climatic changes that generate them due to a phenomenon known as secondary decomposition. Our study provides the mechanistic foundations for a framework to separate climatic and autogenic signals in peat records.