Weather extremes and tree species shape soil greenhouse gas fluxes in an experimental fast-growing deciduous forest of air humidity manipulation

- Higher humidity and weather extremes are expected in northern regions.
- Soil CO2, CH4, and N2O fluxes were measured in forests at elevated humidity.
- Both increased humidity and weather extremes shape soil greenhouse gas (GHG) fluxes.
- The extent of change in GHG fluxes may differ between tree species.
- The results can be used to infer the future GHG dynamics in hemi-boreal forests.

Expected climate change in high latitudes includes increased air temperature, precipitation, and humidity in the coming decades. Simultaneously, climate extremes like heat waves and droughts become more frequent. In the Free-Air Humidity Manipulation (FAHM) experiment in silver birch (Betula pendula Roth) and hybrid aspen (Populus tremula L. × P. tremuloides Michx.) stands in Estonia, we focussed on two questions − how elevated humidity in high temperature and drought conditions influences soil greenhouse gas emissions, and what is the impact of tree species on greenhouse gas emissions.Soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes were measured using the static chamber method in the 3rd and 4th years of humidification, in 2010 and 2011 respectively. Soil temperature, soil water potential (SWP), relative humidity, and precipitation were monitored; tree and understory growth, litter fluxes, substrate-induced respiration, and basal respiration were measured. During the severe drought in summer 2011 (SWP fell below −250 kPa), aspen stands had higher CO2 emissions than birch stands and humidification increased CO2 emission for both tree species. Generally, methane consumption was higher in control than in humidified conditions. Humidification reduced N2O emission in aspen stands in 2010. Hence elevated humidity, heat wave and drought, and tree species significantly affected soil greenhouse gas emissions, however CH4 and N2O fluxes remained small in all cases. The results of this study can be used to infer the future greenhouse gas dynamics from mineral soils in northern forests and to forecast growth conditions for energy forestry in changing climate.

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