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Eddy-covariance is a direct and accurate method to measure exchanges of greenhouse gases and energy between ecosystems and the atmosphere under good turbulent mixing conditions. Calm conditions can affect the accuracy of the eddy-covariance measurements by potential gas buildup below the measurement height. This study characterizes wind patterns and calm periods at Bílý Kříž Ecosystem Research Station, using four-year (2010-2013) wind data measured on an eddy-covariance tower at the site, located in Beskids Mountains, Czech Republic. The site is covered by young, dense Norway spruce forest on a 12.5° slope facing South. The results show that the prevailing wind direction was from S to SW direction (55 % frequency of occurrence), and to a much lesser extent from N, NE and NW but including the strongest winds above 12 m s-1, possibly during gale or storm events. Calm condition (u < 1 m s-1) accounted for 34 % of year time. Summer and Spring were the calmest seasons. Occurrence frequencies of calm conditions for the six-month periods from May to October (plant growing season) were higher than those for the remaining parts of the year (on average 39 % vs 28 % respectively). One should be aware of the calm periods in May-October when processing eddy-covariance data, since it is the time period of most contribution of forest ecosystem as a sink of CO2.

Accurate wind analyses using six wind classes and twelve splitting directions were done to deeply evaluate wind conditions on Štítná CzeCOS’s ecosystem station in the Czech Republic. The analysis brings information about contribution of different part of the research-forested area to the measurement of CO2 exchange between the studied ecosystem and atmosphere. Štítná station is located inside of a forest composed of Fagus sylvatica with age 114 years and the forest can be harvested sometime forward. Total of 82 % of the wind contributions speed are in the range of 1.0 - 6.0 m s-1 and the major accumulation of winds comes from SE (9 %), SSE (17 %) and S (14 %); 13 % were calm periods. The obtained information will be useful for evaluate the conditions of the use of the eddy covariance technique in the research area.