Greenhouse gas emissions from boreal inland waters unchanged after forest harvesting

Abstract

Abstract. Forestry practices generally result in an increased export of carbon and nitrogen to downstream aquatic systems.Although these losses affect the greenhouse gas budget of managed forests, it is unknown if they modify greenhouse gas 10 emissions of recipient aquatic systems. To assess this question, we quantified atmospheric fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) of humic lakes and their inlet streams in four boreal catchments of which two were treated with forest clear-cuts followed by site preparation (18% and 44% of the catchment area) using a Before/AfterControl/Impact-experiment. We measured atmospheric gas fluxes and hydrological and physicochemical water characteristics in hillslope groundwater, along stream transects and at multiple locations in lakes at 2-hourly to biweekly intervals throughout 15 the summer season over a four year period. We found that the treatment did not significantly change greenhouse gas emissions from streams or lakes within three years of the treatment, despite significant increases of CO2 and CH4 concentrations in hillslope groundwater. Our results highlight the importance of the riparian zone-stream continuum as effective biogeochemical buffers and wind shelters to prevent greenhouse gases leaching from forest clear-cuts and evasion via downstream inland waters. These findings are representative for low productive forests located in relatively flat landscapes where forestry 20 practices cause only a limited initial impact on catchment hydrology and biogeochemistry.

Listening to air–water gas exchange in running waters

“…Flux chamber k 600 estimates were compared to independently measured k 600 values using a propane gas injection technique, available for four subreaches per stream, where k estimated from propane loss downstream was converted to k 600 for standardized comparison among gases (Jähne et al ). The arithmetic mean k 600 across three flux chambers located in each subreach correlated strongly with subreach specific k 600 values from propane‐injection experiments, but underestimated subreach specific k 600 by 39% (linear regression, slope = 0.61, p < 0.001, R 2 = 0.58, n = 42, Klaus et al ). Flux chamber k 600 estimates were also assessed on potential bias by chamber‐induced turbulence (Lorke et al ) by comparative ADV measurements with and without the chamber placed in a stream on the campus of Umeå University in October 2018 (Supporting Information Text S4).…”

“…Each study site was sampled four to eight times between May 2014 and April 2015 to cover a wide range of water temperature (0–17°C) and discharge (0.2–113.8 L s −1 , Table ). The covered range in discharge corresponded to the 8 th –94 th , 1 st –97 th , and 17 th –85 th percentile of hourly discharge measurements during 01 April–31 October in 2012–2015 in Lillsjölidtjärnen, Struptjärn, and Övre Björntjärn, respectively (Klaus et al ).…”

“…The regional hydrology is characterized by pronounced spring floods in April–May, summer and winter low flow periods, and autumn storms in September–November. For more details on the streams, see Klaus et al ().…”

“…Air–water gas exchange velocities were measured using a static gas flux chamber following the protocol by Klaus et al () unless declared elsewise below. Briefly, the chamber had an elongated hexagonal cross‐section with rounded edges to minimize flow disturbance, mounted to a tripod and placed on the water surface with its side walls extending 2 cm into the water.…”

Headwater lakes and their influence on downstream discharge

“…Only a few studies have directly addressed how lakes modify catchment‐scale responses to environmental change; however, existing research suggests that lakes may exert a strong influence (Klaus et al ). For example, headwater lakes can moderate the influence of forest harvesting on downstream thermal regimes, suggesting that catchments with lakes may be more resilient to forest cover disturbance than those without (Mellina et al ).…”