Carbon dioxide dynamics in a boreal forest ditch affected by clear-cut forestry

Abstract

<p>Boreal water courses are large emitters of carbon dioxide (CO<sub>2</sub>) to the atmosphere. In Sweden, a high share of these water courses are man-made ditches, created to improve drainage and increase forest productivity. Previous studies from boreal regions have mainly suggested that terrestrial sources sustain the CO<sub>2</sub> in these ditches and with variability in hydrology as the main temporal control. However, few studies have quantified ditch CO<sub>2</sub> dynamics in harvested catchments. An altered hydrology, increased nutrient export and light availability upon forest harvest are all factors that potentially can change the main source control. Thus, there is a strong need to better understand how clear-cut forestry affects the ditch CO<sub>2</sub> dynamics in boreal regions.</p><p>Here, high-frequency (30 min) CO<sub>2</sub> concentration dynamics together with other hydro-chemical variables were studied in a forest ditch draining a fully harvested catchment in the Trollberget Experimental Area, northern Sweden. Data were collected during the snow-free season from May to October. Ditch CO<sub>2</sub> concentrations displayed a clear seasonal pattern with higher CO<sub>2</sub> during summer than in spring and autumn. Concentrations were ranging from 0.41 to 3.99 mg C L<sup>-1</sup> (median: 1.69 mg C L<sup>-1</sup>, corresponding to partial pressures (pCO<sub>2</sub>) of 2553 μatm, IQR = 1.08 mg C L<sup>-1</sup>). Strong diel cycles in CO<sub>2</sub> were developed during early summer, with daily amplitudes in the CO<sub>2</sub> reaching up to 2.1 mg C L<sup>-1</sup>. These daily cycles in CO<sub>2</sub> were likely driven by aquatic primary production consuming CO<sub>2</sub> during daytime. In addition, individual high-flow events in response to rainfall had a major influence on the ditch CO<sub>2</sub> dynamics with generally a diluting effect, but the strength in the CO<sub>2</sub>-discharge relationship varied among seasons and between events. It was evident from the study that growing season CO<sub>2</sub> dynamics in forest ditches affected by clear-cut forestry are high and controlled by a combination of hydrological and biological factors. These high dynamics and the associated controls need to be considered when scaling ditch CO<sub>2</sub> emissions across boreal landscapes affected by clear-cut forestry.</p>