Although stable carbon isotopic composition (δ¹³C) of plants has been widely used to indicate different water regimes in terrestrial ecosystems over the past four decades, the changes in the plant δ¹³C value under waterlogging have not been sufficiently clarified. With the enhanced global warming in recent years, the increasing frequency and severity of river floods in Arctic regions lead to more waterlogging on willows that are widely distributed in river lowland. To investigate the δ¹³C changes in plants under different water conditions (including waterlogging), we measured the δ¹³C values in the leaves of willows with three species,Salix boganidensis,S. glauca, andS. pulchra, and also monitored changes in plant physiology, under several major flooding conditions in Northeastern Siberia. The foliar δ¹³C values of willows varied, ranging from −31.6 to −25.7‰ under the different hydrological status, which can be explained by: (i) under normal conditions, the foliar δ¹³C values decrease from dry (far from a river) to wet (along a river bank) areas; (ii) the δ¹³C values increase in frequently waterlogged areas owing to stomatal closure; and (iii) after prolonged flooding periods, the δ¹³C values again decrease, probably owing to the effects of not only the closure of stomata but also the reduction of foliar photosynthetic ability under long period of waterlogging. Based on these results, we predict that plant δ¹³C values are strongly influenced by plant physiological responses to diverse hydrological conditions, particularly the long periods of flooding, as occurs in Arctic regions.

Taiga–tundra boundary ecosystems are affected by climate change. Methane (CH₄) emissions in taiga–tundra boundary ecosystems have sparsely been evaluated from local to regional scales. We linked in situ CH₄ fluxes (2009–2016) with vegetation cover, and scaled these findings to estimate CH₄ emissions at a local scale (10 × 10 km) using high-resolution satellite images in an ecosystem on permafrost (Indigirka lowland, north-eastern Siberia). We defined nine vegetation classes, containing 71 species, of which 16 were dominant. Distribution patterns were affected by microtopographic height, thaw depth and soil moisture. The Indigirka lowland was covered by willow-dominated dense shrubland and cotton-sedge-dominated wetlands with sparse larch forests. In situ CH₄ emissions were high in wetlands. Lakes and rivers were CH₄ sources, while forest floors were mostly neutral in terms of CH₄ emission. Estimated local CH₄ emissions (37 mg m⁻² d⁻¹) were higher than those reported in similar studies. Our results indicate that: (i) sedge and emergent wetland ecosystems act as hot spots for CH₄ emissions, and (ii) sparse tree coverage does not regulate local CH₄ emissions and balance. Thus, larch growth and distribution, which are expected to change with climate, do not contribute to decreasing local CH₄ emissions.

The response of CH₄ emission from natural wetlands due to meteorological conditions is important because of its strong greenhouse effect. To understand the relationship between CH₄ flux and wetting, we observed interannual variations in chamber CH₄ flux, as well as the concentration, δ¹³C, and δD of dissolved CH₄ during the summer from 2009 to 2013 at the taiga–tundra boundary in the vicinity of Chokurdakh (70^∘37^′ N, 147^∘55^′ E), located on the lowlands of the Indigirka River in northeastern Siberia. We also conducted soil incubation experiments to interpret δ¹³C and δD of dissolved CH₄ and to investigate variations in CH₄ production and oxidation processes. Methane flux showed large interannual variations in wet areas of sphagnum mosses and sedges (36–140 mg CH₄ m⁻² day⁻¹ emitted). Increased CH₄ emission was recorded in the summer of 2011 when a wetting event with extreme precipitation occurred. Although water level decreased from 2011 to 2013, CH₄ emission remained relatively high in 2012, and increased further in 2013. Thaw depth became deeper from 2011 to 2013, which may partly explain the increase in CH₄ emission. Moreover, dissolved CH₄ concentration rose sharply by 1 order of magnitude from 2011 to 2012, and increased further from 2012 to 2013. Large variations in δ¹³C and δD of dissolved CH₄ were observed in 2011, and smaller variations were seen in 2012 and 2013, suggesting both enhancement of CH₄ production and less significance of CH₄ oxidation relative to the larger pool of dissolved CH₄. These multi-year effects of wetting on CH₄ dynamics may have been caused by continued soil reduction across multiple years following the wetting. Delayed activation of acetoclastic methanogenesis following soil reduction could also have contributed to the enhancement of CH₄ production. These processes suggest that duration of water saturation in the active layer can be important for predicting CH₄ emission following a wetting event in the permafrost ecosystem.