Spatio-temporal variability and environmental controls of methane fluxes at the forest-tundra ecotone in the Fennoscandian mountains

We report on temporal and spatial variability in net methane (CH4) fluxes measured during the thaw period of 1999 and 2000 at three study sites along a c. 8° latitudinal gradient in the Fennoscandian mountain range and across the mountain birch‐tundra ecotone. All of the sites studied here were underlain by well‐drained mesic soils. In addition, we conducted warming experiments in the field to simulate future climate change. Our results show significant CH4 uptake at mesic sites spanning the forest‐tundra ecotone: on average 0.031 and 0.0065 mg CH4 m−2 h−1 during the 1999 and 2000 thaw periods, respectively, in Abisko (Sweden), and 0.019 and 0.032 mg CH4 m−2 h−1 during 2000 in Dovrefjell and Joatka (Norway), respectively. These values were both temporally and spatially highly variable, and multiple regression analysis of data from Abisko showed no consistent relationship with soil‐moisture status and temperature. Also, there was no consistent difference in CH4 fluxes between forest and tundra plots; our data, therefore, provide no support for the hypothesis that conversion of tundra to mountain birch forest, or vice versa, would result in a systematic change in the magnitude or direction of net CH4 fluxes in this region. Experimental warming treatments were associated with a 2.4 °C increase in soil temperatures (5 cm depth) in 1999 in Abisko, but no consistent soil warming was noted at any of the three field locations during 2000. In spite of this, there were significant treatment effects, principally early during the thaw period, with increased CH4 uptake compared with control (ambient) plots. These results suggest that direct effects of air warming on vegetation processes (e.g. transpiration, root exudation and nutrient assimilation) can influence CH4 fluxes even in predominantly methanotrophic environments. We conclude that net CH4 oxidation is significant in these cold, mesic soils and could be strengthened in an environmental change scenario involving a combination of (i) an increase in the length of the thaw period and (ii) increased mean temperatures during this period in combination with decreased soil‐moisture content.