Biosphere-atmosphere exchanges of carbon, water and energy
A combined Integrated ICOS atmosphere-ecosystem station is located in the center of KCS (Fig 1). ICOS is a pan-European research infrastructure with the mission to produce standardized, high-precision, and long-term observations of greenhouse gases and to facilitate research to understand the carbon cycle (www.icos-cp.eu/). The ICOS Svartberget infrastructure has been in operation since 2014 within ICOS Sweden. The atmosphere and ecosystem stations were officially labelled and designated as ICOS Europe stations in 2017 and 2019, respectively. All data collected are quality controlled by central thematic centers and made openly available via the ICOS Carbon Portal (https://data.icos-cp.eu/portal).
The atmosphere station conducts measurements of concentrations of CO2, carbon monoxide (CO), methane (CH4), as well as air temperature and humidity at three levels (35, 85, 150 m height). At 150 m, concentrations of14CO2 and13CO2 are also measured with lower time resolution (i.e. by periodic flask sampling). The atmospheric concentration measurements have a footprint of several hundred kilometers and in combination with other atmospheric measurement stations in Northern Europe capture subcontinental scale conditions.
The ecosystem station provides measurements of CO2, water (i.e. evapotranspiration), and energy fluxes using the eddy covariance (EC) technique. These measurements (conducted at 34.5 m height) have a footprint of several hundred meters representing the specific ecosystem conditions in the vicinity of the tower, including catchments C2 and C7. In addition, key meteorological (i.e. radiation, temperature, wind, humidity, precipitation) and soil environmental (temperature, moisture, heat flux and water table level) variables are continuously monitored on the tower and along four soil depth profiles, respectively. Permanent sample plots for inventories of tree and understory vegetation biomass, species, leaf area, phenology and leaf chemistry, as well as litter fall give additional information on the biotic ecosystem properties.
The ICOS infrastructure also serves as a platform for establishing and connecting external research projects. For instance, the installation of two more EC systems at 60 and 85m height along the ICOS tower provides additional estimates of CO2, CH4, water and energy exchanges at the landscape scale (i.e. few km radius), roughly spanning the area of the KCS (Chi et al., 2019). Integration of these landscape EC measurements with aquatic fluxes of carbon species via stream runoff has resulted in a first estimate of the net landscape carbon balance (NLCB) for the KCS (Chi et al., 2020). In addition, the combination of EC and sapflow measurements around the ICOS tower has provided an opportunity to partition the forest water cycle components (Kozii et al., 2020). Furthermore, the concentration measurement profile including several levels along the 150 m tall tower has enabled investigations of atmospheric organic pollutants (Bidleman et al. 2017), as well as water isotope and mercury dynamics. The ICOS tower structure also hosts multispectral sensors, phenology cameras and UAVs within the SITES-Spectral infrastructure to collect spectral data for estimating ecosystem vegetation properties at various spatial and temporal scales.