Seasonal and inter-annual variability of energy exchange above a boreal Scots pine forest

Abstract. Twelve-years of eddy-covariance measurements conducted above a boreal Scots pine forest in Hyytiälä, Southern Finland, were analyzed to assess the seasonal and inter-annual variability of surface conductance (g_s) and energy partitioning. The g_s had distinct annual course, driven by the seasonal cycle of the Scots pine. Low g_s (2–3 mm s⁻¹ in April) cause the sensible heat flux to peak in May–June while evapotranspiration takes over later in July–August when g_s is typically 5–7 mm s⁻¹. Hence, during normal years Bowen ratio decreases from 4–6 in April to 0.7–0.9 in August. Sensitivity of g_s to ambient vapor pressure deficit (D) was relatively constant but the reference value at D = 1 kPa varied seasonally and between years. Only two drought episodes when volumetric soil moisture content in upper mineral soil decreased below 0.15 m³ m⁻³ occurred during the period. Below this threshold value, transpiration was strongly reduced, which promoted sensible heat exchange increasing Bowen ratio to 3–4. Annual evapotranspiration varied between 218 and 361 mm and accounted between 50% and 90% of equilibrium evaporation. The forest floor contributed between 16 and 25% of the total evapotranspiration on annual scale. The fraction stayed similar over the observed range of environmental conditions including drought periods. The inter-annual variability of evapotranspiration could not be linked to any mean climate variable while the summertime sensible heat flux and net radiation were well explained by global radiation. The energy balance closure varied annually between 0.66 and 0.95 and had a distinct seasonal cycle with worse closure in spring when a large proportion of available energy is partitioned into sensible heat.