Sezonowy przebieg wskaźnika wykorzystania wody (WUE) w lesie sosnowym

Assessment of the ecosystem productivity in relation to the amount of water used for such kind of processes have already been investigated for the different ecosystem types by many researchers. Water Use Efficiency (WUE) parameter shows seasonal variation due to changing weather conditions, which determine processes of photosynthesis, respiration and evapotranspiration, and because of biological factors variability. The analysis of the WUE dynamics in the forest ecosystem in Tuczno research station over the hydrological year 2012 were presented in the paper. WUE was calculated based on the daily CO2 and H CO2 O fluxes measured by means of the eddy covariance system (EC) installed over the forest canopy. The fluxes were quality checked in relation to the wind direction, friction velocity values and stationary. The data series used in this investigation were not gap-filled. By using the net value of both fluxes, a good indicator which describe the behavior of the ecosystem as the whole, was obtained. Among a number of weather condition factors, that affect the value of the tested indicator (WUE), mainly photosynthetic photon flux density (PPFD) and the air temperature were evaluated. Separate analysis has been done for daily courses of both CO2 and H2 O fluxes, as well as for WUE for each month of the analyzed period. The highest values of H2 O fluxes (FH) were observed in May and June (0.25 and 0.3 kg m-2s-1) and the lowest in the period from September to December (0.02–0.05 kg m-2s-1). The daily courses of water vapor fluxes revealed dependence to the measured PPFD fluxes, which determines in turn the intensity of the evapotranspiration process. The time courses of the daily 30-min average carbon dioxide fluxes (FC) were also highly variable in each month, similarly to the variances of the FH fluxes. The lowest values of the net F COC were measured in the period from October to December (net CO2 fluxes did not exceed 0.55g m-2 s-1), while in the period from April to July the net ecosystem productivity was the biggest (1.5–2.0 m-2s-1),). WUE reached its minimum in May (7 g [CO2] kg-1), [H2O]), what indicates that the evapotranspiration of the ecosystem was the least effective from the point of view of COCOD absorption from the atmosphere (the ecosystem had a high productivity but it lost a great amount of water at the same time). In contrary, higher WUE values were observed in the colder period of the year, when the amount of available radiant energy was limited. WUE reached its peak in September (nearly 45 g of [CO2] kg-1), [H2O] ) which reveals, that in autumn the decline of evapotranspiration rates were higher than rates of the net CO2 exchange decrease. It also highlights the high adaptability of Scots pine (main species in studied forest ecosystem – 99%), to described conditions. The designated water use efficiency (WUE) parameter, may be used as a good indicator of the current condition of the ecosystem.