Abstract
Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space-for-time’ approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg) is significantly lower at shrub (2.98 ± 0.48 kg m−2) and forest (2.04 ± 0.25 kg m−2) plots than at heath plots (7.03 ± 0.79 kg m−2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.
Highlights
Northern high latitudes, north of 60° over land, and across the Arctic Ocean, have warmed by between 1–4 °C since 1960, and at a rate substantially greater than the global mean (Serreze & Francis, 2006; Hansen et al, 2010; Serreze & Barry, 2011)
This emphasizes a close link between the dominance of nonericaceous woody species present in a community and the amount of C stored in the soil
We have shown that the changes in soil organic carbon (SOC) over ecotones hold true at the landscape scale, and are similar in contrasting climatic contexts
Summary
North of 60° over land, and across the Arctic Ocean, have warmed by between 1–4 °C since 1960, and at a rate substantially greater than the global mean (Serreze & Francis, 2006; Hansen et al, 2010; Serreze & Barry, 2011). Empirical data from field studies is providing growing evidence that specific relationships exist between the vegetation type and biomass in arctic and boreal ecosystems and the amount of C stored in the soil (Wilmking et al, 2006; Kane & Vogel, 2009; Hartley et al, 2012) These do not conform to the positive relationships between productivity and C storage predicted by global C cycle models (Cramer et al, 2001; Qian et al, 2010; Todd-Brown et al, 2014). The increase of woody shrub cover in arctic systems occurs over a gradient from low densities to dominance over time (Myers-Smith et al, 2011; Elmendorf et al, 2012b) and it is important to understand the effect on C storage of this more subtle change as well as the larger-scale differences between forest and tundra.
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