| 英文摘要: | Arctic tundra ecosystems are warming almost twice as fast as the global average1. Permafrost thaw and the resulting release of greenhouse gases from decomposing soil organic carbon have the potential to accelerate climate warming2, 3. In recent decades, Arctic tundra ecosystems have changed rapidly4, including expansion of woody vegetation5, 6, in response to changing climate conditions. How such vegetation changes contribute to stabilization or destabilization of the permafrost is unknown. Here we present six years of field observations in a shrub removal experiment at a Siberian tundra site. Removing the shrub part of the vegetation initiated thawing of ice-rich permafrost, resulting in collapse of the originally elevated shrub patches into waterlogged depressions within five years. This thaw pond development shifted the plots from a methane sink into a methane source. The results of our field experiment demonstrate the importance of the vegetation cover for protection of the massive carbon reservoirs stored in the permafrost and illustrate the strong vulnerability of these tundra ecosystems to perturbations. If permafrost thawing can more frequently trigger such local permafrost collapse, methane-emitting wet depressions could become more abundant in the lowland tundra landscape, at the cost of permafrost-stabilizing low shrub vegetation. Arctic tundra ecosystems are characterized by permanently frozen ground (permafrost) covered by a soil layer, named the active layer, which thaws and refreezes every year. Permafrost thaw and the resulting release of carbon dioxide and methane by microbial decomposition of previously frozen organic carbon is considered one of the most significant potential feedbacks from terrestrial ecosystems to the climate system1, 2. Warming and thawing of permafrost have been observed in many locations in recent decades7. Although thawing depth is controlled to a large extent by the regional climate, non-climatic factors, such as changes in vegetation and hydrology, can strongly modify the response of permafrost to global warming8, 9. For a better understanding of the response of permafrost to climate change, it is important to determine how observed vegetation changes, for example, deciduous shrub expansion5, 6, contribute to stabilization or destabilization of the permafrost. We set up a shrub removal experiment at a Northeast-Siberian tundra site in 200710. The study site represents poorly drained lowland tundra underlain by thick continuous permafrost in the Low Arctic climate zone. We clipped off all aboveground biomass of the dominant deciduous shrub species Betula nana (dwarf birch) in five 10-m-diameter plots, leaving a few other shrubs, graminoids (grasses and sedges), mosses and lichens. The five control plots consisted of undisturbed B. nana-dominated vegetation. B. nana is a common tundra shrub species and is expected to benefit from climate warming11. The control and B. nana-removal plots were very similar at the start of the experiment (Supplementary Table 1). Here, we report on our field observations over six years during which the removal plots have undergone major changes. Shrub removal increased the thawing depth (active layer thickness) within one year (Fig. 1). This was consistent with larger ground heat fluxes in the removal plots10. In the following years the difference in active layer thickness between control and removal plots increased further, from a difference of 5 cm on average in 2008 to 15 cm in 2012 (Fig. 1; 2012: control = 25.8 ± 0.8 cm (mean ± s.e.m., n = 5 plots), removal = 40.8 ± 1.8 cm; F1,8 = 30.1, P = 0.001). In the control plots, active layer thickness was largest in 2011 (Fig. 1), which experienced an exceptionally wet and warm summer (Supplementary Table 2).
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