Water and heat transport in boreal soils: Implications for soil response to climate change

Abstract

Soil water content strongly affects permafrost dynamics by changing the soil thermal properties. However, the movement of liquid water, which plays an important role in the heat transport of temperate soils, has been under-represented in boreal studies. Two different heat transport models with and without convective heat transport were compared to measurements of soil temperatures in four boreal sites with different stand ages and drainage classes. Overall, soil temperatures during the growing season tended to be over-estimated by 2–4 °C when movement of liquid water and water vapor was not represented in the model. The role of heat transport in water has broad implications for site responses to warming and suggests reduced vulnerability of permafrost to thaw at drier sites. This result is consistent with field observations of faster thaw in response to warming in wet sites compared to drier sites over the past 30 years in Canadian boreal forests. These results highlight that representation of water flow in heat transport models is important to simulate future soil thermal or permafrost dynamics under a changing climate.

Introduction

A large fraction (approximately 33–37%) of terrestrial carbon (C) is stored in boreal soils (Ping et al., 2008, Tarnocai et al., 2009b). Over the last four decades, air temperatures in the boreal region have increased during winter and spring by 0.2 to 0.3 °C per decade, accompanied by increased precipitation in autumn and winter (Jones and Moberg, 2003, Smith and Reynolds, 2005). In some regions, higher air temperatures are likely triggering increases in net primary production (Bond-Lamberty et al., 2004) as well as changes in soil hydrology (Striegl et al., 2007, Walvoord and Striegl, 2007).

Changes in boreal hydrology may result in a positive feedback to global warming, either via increased emissions of CH₄ in previously frozen and now flooded areas or via increased production of CO₂ from decomposition or combustion in drained areas. Hydrological changes might also shift the distribution of peatlands, wetlands, and lakes (Rapalee et al., 1998, Vitt et al., 2000). While in some areas thawing of permafrost can promote the development of wetlands and lakes (thermokarst), in other areas permafrost degradation can lead to drainage (Jorgenson and Osterkamp, 2005). In central Siberia, 11% of lakes > 40 ha have shrunk or disappeared between 1973 and 1997/1998 (~ 93,000 ha of regional lake surface) (Smith et al., 2005). Degradation of permafrost over the last two decades has been reported from northern Alaska (Jorgenson et al., 2001), yet the environmental factors responsible for the loss of permafrost are debated with evidence for both the direct effects of warmer temperatures and/or the effects of increasing precipitation (Agafonov et al., 2004).

Heat transport through soils is a critical factor in determining how permafrost changes in response to climate. In most of the existing biophysical models used in boreal settings, pure heat conduction, which is mainly influenced by the particle contact area (Jury and Horton, 2004), is assumed to be the main mechanism of heat transport in the soils. However, heat conduction is not the only mechanism for heat transport in soils and ecosystems. Cahill and Parlange (1998) indicated that liquid water and thermally-induced vapor movement (convection) contributed to ~ 50% of soil heat flux in temperate soils. In boreal soils where moisture is highly variable across space and through seasons, the role of water movement could be an important factor in seasonal soil energy dynamics and in the long term response of boreal systems to changes in climate. Additionally, the convection of latent heat by water vapor movement in response to temperature gradients will release energy through evaporation–condensation processes, resulting in rapid heat transfer with only minor changes in soil moisture content (Jury and Horton, 2004, Kane et al., 2001).

Two heat transport models used to describe soil thermal dynamics were compared in this study. In Model 1, heat was transported by conduction and convection via the movement of liquid water and water vapor; in Model 2 heat was transported exclusively by conduction, a formulation similar to the commonly used soil physical models for boreal soils, for example, FORFLUX (Zeller and Nikolov, 2000), CLASS (Verseghy, 1991), LPJ-GUESS (Wolf et al., 2008), and Wetland-DNDC (Zhang et al., 2002).