Cold season emissions dominate the Arctic tundra methane budget.

Arctic terrestrial ecosystems are major global sources of methane (CH4); hence, it is important to understand the seasonal and climatic controls on CH4 emissions from these systems. Here, we report year-round CH4 emissions from Alaskan Arctic tundra eddy flux sites and regional fluxes derived from aircraft data. We find that emissions during the cold season (September to May) account for ≥ 50% of the annual CH4 flux, with the highest emissions from noninundated upland tundra. A major fraction of cold season emissions occur during the "zero curtain" period, when subsurface soil temperatures are poised near 0 °C. The zero curtain may persist longer than the growing season, and CH4 emissions are enhanced when the duration is extended by a deep thawed layer as can occur with thick snow cover. Regional scale fluxes of CH4 derived from aircraft data demonstrate the large spatial extent of late season CH4 emissions. Scaled to the circumpolar Arctic, cold season fluxes from tundra total 12 ± 5 (95% confidence interval) Tg CH4 y(-1), ∼ 25% of global emissions from extratropical wetlands, or ∼ 6% of total global wetland methane emissions. The dominance of late-season emissions, sensitivity to soil environmental conditions, and importance of dry tundra are not currently simulated in most global climate models. Because Arctic warming disproportionally impacts the cold season, our results suggest that higher cold-season CH4 emissions will result from observed and predicted increases in snow thickness, active layer depth, and soil temperature, representing important positive feedbacks on climate warming.

Paired comparison of water, energy and carbon exchanges over two young maritime pine stands (Pinus pinaster Ait.): effects of thinning and weeding in the early stage of tree growth.

The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO(2)), H(2)O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m(-2) on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R(E)). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m(-2), respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.