Comment on bg-2021-110

doi:10.5194/bg-2021-110-rc2

Abstract

Abstract. Large-herbivore grazing has been shown to substantially alter tundra soil and vegetation properties as well as carbon fluxes, yet observational evidence to quantify the impact of herbivore introduction into Arctic permafrost ecosystems remains sparse. In this study we investigated growing-season CO2 and CH4 fluxes with flux chambers on a former wet tussock tundra inside Pleistocene Park, a landscape experiment in northeast Siberia with a 22-year history of grazing. Reference data for an undisturbed system were collected on a nearby ungrazed tussock tundra. Linked to a reduction in soil moisture, topsoil temperatures at the grazed site reacted 1Â order of magnitude faster to changes in air temperatures compared to the ungrazed site and were significantly higher, and the difference strongly decreased with depth. Overall, both GPPÂ (gross primary productivity, i.e.,Â CO2 uptake by photosynthesis) and RecoÂ (ecosystem respiration, i.e.,Â CO2 release from the ecosystem) were significantly higher at the grazed site with notable variations across plots at each site. The increases in CO2 component fluxes largely compensated for each other, leaving NEEÂ (net ecosystem exchange) similar across grazed and ungrazed sites for the observation period. Soil moisture and CH4 fluxes at the grazed site decreased over the observation period, while in contrast the constantly waterlogged soils at the ungrazed site kept CH4 fluxes at significantly higher levels. Our results indicate that grazing of large herbivores may promote topsoil warming and drying, in this way effectively accelerating CO2 turnover while decreasing methane emissions in the summer months of peak ecosystem activity. Since we lack quantitative information on the pre-treatment status of the grazed ecosystem, however, these findings need to be considered qualitative trends for the peak growing season, and absolute differences between treatments are subject to elevated uncertainty. Moreover, our experiment did not include autumn and winter fluxes, and thus no inferences can be made for the annual NEE and CH4 budgets in tundra ecosystems.

Highlights

In the context of global climate change, surface air temperatures in polar regions have been shown to rise about twice as fast as the global mean in the past (Overland et al, 2015)
We investigated the impact of long-term grazing disturbance on a previously wet tussock tundra ecosystem underlain by permafrost in the Siberian Arctic using flux-chamber observations over 2.5 weeks during the growing season in summer 2019
Over the past 22 years, introduction of large herds of herbivores in the context of the so-called Pleistocene Park 470 experiment has altered vegetation and soil properties within the affected area, this way initiating an ongoing transformation from a water-logged, overgrown tussock tundra towards a drier ecosystem featuring more turf-like vegetation

Summary

Introduction

In the context of global climate change, surface air temperatures in polar regions have been shown to rise about twice as fast as the global mean in the past (Overland et al, 2015). Since this trend is expected to continue in the future, Northern hemisphere permafrost ecosystems are at an exceptional risk for degradation. The Arctic permafrost region stores about 50% 20 of the belowground organic carbon stocks on Earth (Hugelius et al, 2014), with an estimated pool of organic C between 1307 Gt and 1672 Gt (Hugelius et al, 2014; Tarnocai et al, 2009).

Methods

Results

Discussion

Conclusion