BVOC fluxes and concentrations at a boreal forest site in Sweden: an overview of long-term observations at ICOS Norunda and the impacts of forest clear-cutting on BVOC emissions

Vegetation is the major source of volatile organic compounds (VOCs) in the atmosphere, which affect both air quality and climate. Long-term ecosystem-level data on biogenic VOC (BVOC) emissions, however, are limited. This limits assessment of impacts of short-term landscape-scale disturbances like clear-cutting, and seasonal-scale and interannual variation of emissions of boreal forests.

Here we present an overview of BVOC concentration and flux measurements and results spanning several years, leading up to the summer 2022 clear-cutting of a boreal forest located at the ICOS (Integrated Carbon Observation System) and ACTRIS (Aerosol, Clouds, and Traces Gases Research Infrastructure) station Norunda (located at 60°05′N, 17°29′E, ca. 30 km north of Uppsala) in Sweden. This managed boreal forest, between 80 and 120 years old, primarily consisted of a mix of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). Beginning in summer 2020, BVOC mixing ratios were measured using a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-ToF-MS) (Tofwerk AG, Thun, Switzerland). These Vocus measurements (at 10 Hz) were collected at 35 m on the station flux tower to determine BVOC fluxes using the eddy-covariance method. During several intensive BVOC sampling periods in 2020 and 2022, hourly adsorbent samples were also collected, at 37 and 60 m, for subsequent GC-MS analysis to determine compound-speciated BVOC concentrations. These samples were additionally used to estimate the changes in the fluxes of speciated monoterpene (MT) compounds using the surface-layer-gradient (SLG) and modified Bowen-ratio (MBR) methods.

Our results indicate a large variety of VOC compounds being emitted by the forest system, including among them terpenoids - e.g., isoprene, monoterpenes (MTs) and sesquiterpenes (SQTs). The most common MT compounds emitted were α-pinene and Δ3-carene. During the 2022 clearcut, MT emissions increased by more than an order of magnitude during active-cutting, with persisting MT emission increases from clearcut residue which continued for several months. In comparison, many BVOCs lacking storage reservoirs in plant tissues (e.g., isoprene) were relatively unaffected by active-cutting. Fluxes and the mixture of speciated MT compounds observed before, during, and post-cut are compared, and the additional total and speciated MT emissions due to clear-cutting are estimated. For context, in Sweden 69% of total land cover is forest, of which 84% is productive forest for clear-cut forestry (~58% of total land cover). From Swedish forestry information of yearly absolute timber removal and on-site residue volumes following typical clearcuts, we find that current Swedish boreal forest MT emission inventory estimates may be significantly underestimated.