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

Boreal water courses are large emitters of carbon dioxide (CO₂) 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 CO₂ in these ditches and with variability in hydrology as the main temporal control. However, few studies have quantified ditch CO₂ 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 CO₂ dynamics in boreal regions.

Here, high-frequency (30 min) CO₂ 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 CO₂ concentrations displayed a clear seasonal pattern with higher CO₂ during summer than in spring and autumn. Concentrations were ranging from 0.41 to 3.99 mg C L^-1 (median: 1.69 mg C L^-1, corresponding to partial pressures (pCO₂) of 2553 μatm, IQR = 1.08 mg C L^-1). Strong diel cycles in CO₂ were developed during early summer, with daily amplitudes in the CO₂ reaching up to 2.1 mg C L^-1. These daily cycles in CO₂ were likely driven by aquatic primary production consuming CO₂ during daytime. In addition, individual high-flow events in response to rainfall had a major influence on the ditch CO₂ dynamics with generally a diluting effect, but the strength in the CO₂-discharge relationship varied among seasons and between events. It was evident from the study that growing season CO₂ 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 CO₂ emissions across boreal landscapes affected by clear-cut forestry.