摘要
With the aim of addressing the broader issues of land use, climate change and energy crisis, eddy covariance measurements of energy and CO2 exchange from a bioenergy crop (reed canary grass, Phalaris arundinaceae, L.) cultivated on a drained organic soil (a cutover peatland) in eastern Finland were initiated in the spring of 2004. Based on the climatically diverse dataset from the 2004 to 2010 period, the objectives of the work presented here are to characterize the interannual variability in water vapor exchange and to understand the linkage between energy and CO2 exchange from this perennial crop during two extreme growing seasons. Correcting the measured soil heat flux by accounting for the change in heat storage above the heat flux plates helped close the energy balance at this site. Interannual variability in ecosystem processes of energy and CO2 exchange were attributed primarily to marked differences in the amount and distribution of seasonal precipitation. Differences in the phonological development of the crop during seasons with contrasting climatic conditions were reflected in the normalized difference vegetation index (NDVI) estimated from the radiation instruments installed on the tower. Wet years were characterized by an even distribution of seasonal precipitation, low to moderate air and soil temperatures, lower solar and net radiation intensities (owing to reduced number of bright sunshine hours and therefore, higher amount of diffuse sky radiation), moderate to saturated soil moisture conditions and lower vapor pressure deficit. These climatic conditions resulted in high bulk surface conductance (gs), high evapotranspiration (ET) and low sensible heat flux with a peak seasonal Bowen ratio (¦Â = 0.1). These conditions were favourable for a high uptake of atmospheric CO2. Dry years, on the contrary, were marked by long dry spells during important phases of crop growth, climatic and soil moisture stress leading to high evaporative demand, low gs values, reduced evapotranspiration and high sensible heat flux (¦Â = 0.3-0.4). On a seasonal basis, the ET losses during a dry year were 13 % lower compared to those during a wet year. The corresponding reduction in gross ecosystem productivity (GEP), however, was to the extent of 21 % . Owing to the ability of this perennial crop to sequester large amounts of atmospheric carbon into its above- and below-ground biomass, the water use efficiency (defined as the slope of the linear regression of monthly values of GEP against ET) of this cultivation system was found to be 9.1 g CO2 per kg of H2O lost as ET. The results stemming from this work further support our earlier conclusions that this bioenergy system is a suitable land use option on drained and abandoned cutover peatlands with a high potential for offsetting CO2 load to the atmosphere.