Boreal permafrost thaw amplified by fire disturbance and precipitation increases

James P Fisher,Georgios Xenakis,Julian B Murton,Iain P Hartley,Cristian Estop-Aragonés,Dan J Charman,Gareth K Phoenix,Yu Zhang,Mathew Williams

doi:10.1088/1748-9326/abbeb8

Abstract

Permafrost soils store huge amounts of organic carbon, which could be released if climate change promotes thaw. Currently, modelling studies predict that thaw in boreal regions is mainly sensitive to warming, rather than changes in precipitation or vegetation cover. We evaluate this conclusion for North American boreal forests using a detailed process-based model parameterised and validated on field measurements. We show that soil thermal regimes for dominant forest types are controlled strongly by soil moisture and thus the balance between evapotranspiration and precipitation. Under dense canopy cover, high evapotranspiration means a 30% increase in precipitation causes less thaw than a 1 °C increase in temperature. However, disturbance to vegetation promotes greater thaw through reduced evapotranspiration, which results in wetter, more thermally conductive soils. In such disturbed forests, increases in precipitation rival warming as a direct driver of thaw, with a 30% increase in precipitation at current temperatures causing more thaw than 2 °C of warming. We find striking non-linear interactive effects on thaw between rising precipitation and loss of leaf area, which are of concern given projections of greater precipitation and disturbance in boreal forests. Inclusion of robust vegetation-hydrological feedbacks in global models is therefore critical for accurately predicting permafrost dynamics; thaw cannot be considered to be controlled solely by rising temperatures.

Highlights

The boreal forest in North America is largely underlain by discontinuous and sporadic permafrost us cri (Helbig et al, 2016) and contains huge stores of soil organic carbon (Tarnocai et al, 2009).Ecosystem properties are known to influence many components of the physical processes involved in permafrost thaw (Loranty et al 2018)
The simulations matched the observations of evolving soil temperature in both unburned and burned black spruce sites at Mosquito Creek, and resolved the differences in profiles down to 1 m
Tended to over-estimate surface soil moisture (0.05 m depth). Both model and data suggested lower soil moisture values in the unburned sites (Figure 2, top panel), the model values were systematically larger than the observed values at 0.05 and 0.15 m depth

Summary

Introduction

The boreal forest in North America is largely underlain by discontinuous and sporadic permafrost us cri (Helbig et al, 2016) and contains huge stores of soil organic carbon (Tarnocai et al, 2009).Ecosystem properties are known to influence many components of the physical processes involved in permafrost thaw (Loranty et al 2018). Soil organic matter has a similar influence as mosses on thermal conductivity and permafrost thaw (Johnson et al, 2013). Permafrost beneath the boreal forest is sensitive to environmental change because it is close to its climatic limit, and may only exist as it is protected by these ecosystem interactions (Shur and Jorgenson, 2007). Permafrost-ecosystem feedbacks are poorly understood, related to disturbance and vegetation-active layer thickness interactions (Grosse et al, 2016). The complexity of these interactions means there are disagreements between model simulations of current permafrost extent and its climate sensitivity (Koven et al, 2013; McGuire et al, 2016)

Methods

Results

Conclusion