Northern peatlands provide important global and regional ecosystem services (carbon storage, water storage, and biodiversity). However, these ecosystems face increases in the severity, areal extent and frequency of climate-mediated (e.g. wildfire and drought) and land-use change (e.g. drainage, flooding and mining) disturbances that are placing the future security of these critical ecosystem services in doubt. Here, we provide the first detailed synthesis of autogenic hydrological feedbacks that operate within northern peatlands to regulate their response to changes in seasonal water deficit and varying disturbances. We review, synthesize and critique the current process-based understanding and qualitatively assess the relative strengths of these feedbacks for different peatland types within different climate regions. We suggest that understanding the role of hydrological feedbacks in regulating changes in precipitation and temperature are essential for understanding the resistance, resilience and vulnerability of northern peatlands to a changing climate. Finally, we propose that these hydrological feedbacks also represent the foundation of developing an ecohydrological understanding of coupled hydrological, biogeochemical and ecological feedbacks.

Northern and tropical peatlands represent a globally significant carbon reserve accumulated over thousands of years of waterlogged conditions. It is unclear whether moderate drying predicted for northern peatlands will stimulate burning and carbon losses as has occurred in their smaller tropical counterparts where the carbon legacy has been destabilized due to severe drainage and deep peat fires. Capitalizing on a unique long-term experiment, we quantify the post-wildfire recovery of a northern peatland subjected to decadal drainage. We show that the moderate drop in water table position predicted for most northern regions triggers a shift in vegetation composition previously observed within only severely disturbed tropical peatlands. The combined impact of moderate drainage followed by wildfire converted the low productivity, moss-dominated peatland to a non-carbon accumulating shrub-grass ecosystem. This new ecosystem is likely to experience a low intensity, high frequency wildfire regime, which will further deplete the legacy of stored peat carbon.

We compared median runoff (R) and precipitation (P) relationships over 25 years from 20 mesoscale (50 to 5,000 km2) catchments on the Boreal Plains, Alberta, Canada, to understand controls on water sink and source dynamics in water‐limited, low‐relief northern environments. Long‐term catchment R and runoff efficiency (RP−1) were low and varied spatially by over an order of magnitude (3 to 119 mm/year, 1 to 27%). Intercatchment differences were not associated with small variations in climate. The partitioning of P into evapotranspiration (ET) and R instead reflected the interplay between underlying glacial deposit texture, overlying soil‐vegetation land cover, and regional slope. Correlation and principal component analyses results show that peatland‐swamp wetlands were the major source areas of water. The lowest estimates of median annual catchment ET (321 to 395 mm) and greatest R (60 to 119 mm, 13 to 27% of P) were observed in low‐relief, peatland‐swamp dominated catchments, within both fine‐textured clay‐plain and coarse‐textured glacial deposits. In contrast, open‐water wetlands and deciduous‐mixedwood forest land covers acted as water sinks, and less catchment R was observed with increases in proportional coverage of these land covers. In catchments dominated by hummocky moraines, long‐term runoff was restricted to 10 mm/year, or 2% of P. This reflects the poor surface‐drainage networks and slightly greater regional slope of the fine‐textured glacial deposit, coupled with the large soil‐water and depression storage and higher actual ET of associated shallow open‐water marsh wetland and deciduous‐forest land covers. This intercatchment study enhances current conceptual frameworks for predicting water yield in the Boreal Plains based on the sink and source functions of glacial landforms and soil‐vegetation land covers. It offers the capability within this hydro‐geoclimatic region to design reclaimed catchments with desired hydrological functionality and associated tolerances to climate or land‐use changes and inform land management decisions based on effective catchment‐scale conceptual understanding.

Abstract:The frequency and intensity of drought is projected to increase within the boreal region under future climatic conditions. Peatlands are widely considered to regulate water loss under drought conditions, increasing surface resistance (r s ) and reducing evaporative losses. This maintains peat moisture content, increasing the resilience of these globally important carbon stores. However, the magnitude and form of this important negative feedback response remains uncertain. To address this, we monitored the response of r s to drought within four peat cores under controlled meteorological conditions. When the water-table was dropped to a depth of 0.30 m and the humidity reduced to ≤40%, a step shift in r s from~50 s m -1 up to 1000 s m -1 was observed within burned and unburned peat, which virtually shuts down evaporation, limiting water loss. We show that measured near-surface tension cannot be used to directly calculate this transition in peat surface resistance. However, empirical relationships that account for strong vertical variations in tension through the near-surface and/or disequilibrium between pore air and near-surface pore water pressure provide the potential to incorporate this negative feedback response into peatland ecohydrological models. Further observations are necessary to examine this response under dynamic atmospheric conditions. We suggest that the link between surface temperature and evaporation provides potential to further examine this feedback in either burned peatlands or peatlands with a low vascular vegetation cover.

The surface pattern of vegetation influences the composition and humification of peat laid down during the development of a bog, producing a subsurface hydrological structure that is expected to affect both the rate and pattern of water flow. Subsurface peat structures are routinely derived from the inspection of peat cores. However, logistical limits on the number of cores that can be collected means that the horizontal extent of these structures must be inferred. We consider whether subsurface patterns in peat physical properties can be mapped in detail over large areas with ground‐penetrating radar (GPR) and complex conductivity by comparing geophysical measurements with peat core data along a 36 m transect through different microhabitats at Caribou Bog, Maine. The geophysical methods show promise. Peat horizons produced radar reflections because of changes in the volumetric moisture content. Although these reflections could not be directly correlated with the peat core data, they were related to the depth‐averaged peat properties which varied markedly between the microhabitats. Well‐decomposed peat below a hollow was characterized by a discontinuous sequence of chaotic wavy reflections, while distinct layering of the peat below an area of hummocks coincided with a pattern of parallel planar reflections. The complex conductivity survey showed spatial variation in the real and imaginary conductivities which resulted from changes in the pore water conductivity; peat structures may also have influenced the spatial pattern in the complex conductivity. The GPR and complex conductivity surveys enabled the developmental history of the different microhabitats along the studied transect to be inferred.