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The terrestrial net ecosystem productivity (NEP) has increased during the past three decades, but the mechanisms responsible are still unclear. We analyzed 17 years (2001–2017) of eddy‐covariance measurements of NEP, evapotranspiration (ET) and light and water use efficiency from a boreal coniferous forest in Southern Finland for trends and inter‐annual variability (IAV). The forest was a mean annual carbon sink (252 [±42] gC m‐2a‐1), and NEP increased at rate +6.4–7.0 gC m‐2a‐1 (or ca. +2.5% a‐1) during the period. This was attributed to the increasing gross‐primary productivity GPP and occurred without detectable change in ET. The start of annual carbon uptake period was advanced by 0.7 d a‐1, and increase in GPP and NEP outside the main growing season contributed ca. one‐third and one‐fourth of the annual trend, respectively. Meteorological factors were responsible for the IAV of fluxes but did not explain the long‐term trends. The growing season GPP trend was strongest in ample light during the peak growing season. Using a multi‐layer ecosystem model, we showed that direct CO2 fertilization effect diminishes when moving from leaf to ecosystem, and only 30–40% of the observed ecosystem GPP increase could be attributed to CO2. The increasing trend in leaf‐area index (LAI), stimulated by forest thinning in 2002, was the main driver of the enhanced GPP and NEP of the mid‐rotation managed forest. It also compensated for the decrease of mean leaf stomatal conductance with increasing CO2 and LAI, explaining the apparent proportionality between observed GPP and CO2 trends. The results emphasize that attributing trends to their physical and physiological drivers is challenged by strong IAV, and uncertainty of LAI and species composition changes due to the dynamic flux footprint. The results enlighten the underlying mechanisms responsible for the increasing terrestrial carbon uptake in the boreal zone. [ABSTRACT FROM AUTHOR]

Helicopter based simultaneous measurements of broadband surface albedo and the effective leaf area index (LAIeff) were carried out in subarctic area of Finnish Lapland in spring 2008, 2009, and 2010 under varying illumination and snow cover conditions. Vertical profile measurements show that the found relationship between albedo and LAIeff seems to be rather independent of the flight altitude and therefore the footprint scale. Actually, flights above 500 m in altitude revealed low variations of the surface albedo approaching an aerial average at 1 km, meaning that a footprint of 20 km is representative of the landscape. The albedo of the area was beta distributed, and without LAIeff values below 0.25, the average albedo value of the area would decrease from 0.49 to 0.44 showing the albedo sensitivity to sparse vegetation. The results agreed with the photon recollision probability based model PARAS and the MODIS satellite albedo product MCD43A3. However, differences between satellite based and airborne albedo values were noticed, which could be explained by a difference in footprint size and/or the strong local heterogeneity as certain flights were operated on specific targets. Plain Language Summary: Helicopter based measurements were used to assess how much a forest stand laying over a snow slab reduces the surface albedo at high latitudes where the sun zenith angle is large and shadow cast is always important. The effect is amplified in the case of sparse vegetation as there is less mutual shadowing. Model results and satellite observations are found in good agreement with the airborne data sets. Key Points: Surface albedo and effective leaf area index (LAI) can be measured at fine resolution and landscape scale simultaneously using helicopterSurface albedo and effective LAI are coherently retrieved based on a photon recollision probability based modelAirborne and satellite‐based surface albedo show a good agreement [ABSTRACT FROM AUTHOR]