Towards quantifying the negative feedback regulation of peatland evaporation to drought

Abstract

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.

Hydrological controls on deep burning in a northern forested peatland

“…Furthermore, climate change and peatland afforestation pose substantial risks to peatland carbon stocks, as drying may exceed negative feedbacks (e.g. Waddington et al , ) that conserve water during drought (Kettridge and Waddington, ) and subsequently decrease peatland resilience to wildfire through a shift to hydrophysical properties that are vulnerable to smouldering. Consequently, we suggest that future research should aim to better understand landscape‐scale controls on the spatial and temporal variability in peat moisture content and bulk density as a means to identify other potential deep burning ‘hotspots’ on the landscape.…”

Role of morphological structure and layering ofSphagnumandTomenthypnummosses on moss productivity and evaporation rates

“…Thus, the Sphagnum structure provided relatively high matric pressure to fill those large pore spaces, providing the Sphagnum structure with a continuous capillary network from the water table to the capitula that favours capillary rise. At high water tables, the high water content of the Sphagnum capitula canopy provided little surface resistance to diffusion (Kettridge and Waddington 2014), thereby creating conditions favouring high evaporation and sufficient water for photosynthesis. At lower water tables, the lowering of matric pressures in the Sphagnum structure reduced the capitulaÁatmosphere pressure gradients causing evaporation and thus capillary flow to decrease.…”

“…The upward capillary water movement from underlying peat substrates is driven by soil-water pressure gradients within the moss structure caused by atmospheric demand at the canopy surface Rice et al 2001). However, when soil-water pressures become too low for free capillary rise, capillary water is evaporated and the water pressure of the moss cells drops rapidly and equilibrates with the surrounding air, thereby desiccating and ceasing all photosynthetic activity , and reducing evaporative losses to maintain saturated conditions in the peat profile below (Kettridge and Waddington 2014). Therefore, the capacity of peatland mosses to access water from underlying layers can be an important control on peatlandÁatmosphere processes.…”

“…However, the microclimate within moss growth forms and the relationship between vapour pressure of air surrounding mosses and the pressure of moss cells are not well understood, particularly for T. nitens. Some studies (Alvena¨s and Jansson 1997;Kellner 2001;Kettridge and Waddington 2014) noted the assumption that poreÁair vapour pressure is in equilibrium with soilÁwater pressure in the near-surface may not be valid where vapour is continuously transported to the atmosphere. In the case of Tomenthypnum moss, the top of the moss canopy is likely the actual evaporating surface, and thus is not in equilibrium.…”

Alpine timberline population dynamics under climate change: a comparison between Qilian juniper and Qinghai spruce tree species in the middle Qilian Mountains of northeast Tibetan Plateau

“…By contrast, tree density increases during forest development, leading to a significant increase in intraspecific competition intensity (Niu & Wan 2008;Ma et al 2012). At the same time, drought conditions could potentially increase with an increase in temperature because high temperatures could exacerbate soil moisture evaporation (Sun & Liu 2013;Kettridge & Waddington 2014). Warminginduced droughts have limited regeneration in the central Tibetan Plateau since the 1920s, even for alpine shrub up to 4900 m a.s.l.…”