Conceptual frameworks in peatland ecohydrology: looking beyond the two‐layered (acrotelm–catotelm) model

Abstract

Northern peatlands are important shallow freshwater aquifers and globally significant terrestrial carbon stores. Peatlands are complex, ecohydrological systems, commonly conceptualized as consisting of two layers, the acrotelm (upper layer) and the catotelm (lower layer). This diplotelmic model, originally posited as a hypothesis, is yet to be tested in a comprehensive manner. Despite this, the diplotelmic model is highly prevalent in the peatland literature, suggesting a general acceptance of the concept. We examine the diplotelmic model with respect to what we believe are three important research criteria: complexity, generality and flexibility. The diplotelmic model assumes that all ecological, hydrological and biogeochemical processes and structures can be explained by a single discrete boundary-depth in relation to a drought water table. This assumption makes the diplotelmic scheme inherently inflexible, in turn hindering its representation of a range of ecohydrological phenomena. We explore various alternative conceptual approaches that might offer greater flexibility, including the representation of horizontal spatial heterogeneity and transfers. We propose that the concept of hot spots, prevalent in terrestrial biogeochemistry literature, might be extended to peatland ecohydrology, providing a more flexible conceptual framework. Hot spots are areas of a peatland which exhibit fast processing rates in a number of mechanistically linked hydrological, ecological and biogeochemical processes. The complementary concept of cold spots may also be useful in peatland ecohydrology, particularly with regards to understanding the vulnerability of peatlands to disturbance. The flexibility of our suggested scheme may allow the future incorporation of ecohydrological phenomena yet to be identified as important in peatlands.

Biochemical determinants of litter quality in 15 species of Sphagnum

“…Bogs are composed of meters-thick layers of peat accumulated during thousands of years; the low hydraulic conductivity of peat facilitates the creation of waterlogged, anoxic habitats. Anoxia is particularly linked to the lower peat layers (the permanently water-logged catotelm), while the acrotelm is intermittently oxic (Morris et al 2011;Rydin and Jeglum 2013). Decay resistance of dead plants differ for different growth forms (Dorrepaal 2005), but also between different species of Sphagnum (Turetsky et al 2008;Hájek 2009;Bengtsson et al 2016a).…”

Differential response of carbon cycling to long-term nutrient input and altered hydrological conditions in a continental Canadian peatland

“…CH 4 is released to the atmosphere by three different processes: (i) diffusion through the acrotelm, which is a relatively slow process, (ii) ebullition, i.e., fast evasion of CH 4 bubbles, and (iii) fast diffusion or pressurized throughflow convection through the aerenchymatous tissues of vascular plants (Morris et al, 2011;Schütz et al, 1991;Whiting and Chanton, 1996;van den Berg et al, 2016;Hornibrook et al, 2009). Due to the slow diffusion of CH 4 in peat, up to 100 % of diffusive CH 4 is oxidized in the acrotelm before it reaches the atmosphere, while the other processes effectively bypass oxidation and thus contribute a major fraction to observed fluxes (Whalen et al, 1990;Whalen, 2005).…”

Re-assessing the vertical distribution of testate amoeba communities in surface peats: Implications for palaeohydrological studies

“…hummock, hollow and lawn microforms), the interaction between water tables and processes of peat accumulation (Belyea and Clymo 2001;Quinty and Rochefort 2003;Rydin and Jeglum 2006). However, the concept of two distinctive layers is arguably too simplistic, as the acrotelm contains anaerobic sections and aerobic channels that can extend into the catotelm via vascular plant roots (Hayward and Clymo 1983;Morris et al 2011). Notwithstanding this, the boundary between the acrotelm and catotelm has been considered to occur at the minimum water table in summer (Clymo 1984).…”