Reduction of the Ambient UV-B Radiation in the High-Arctic IncreasesFv/Fmin Salix arctica and Vaccinium uliginosum and Reduces Stomatal Conductance and Internal CO2Concentration in Salix arctica

This study investigates effects of reducing the ambient UV radiation on gas exchange and chlorophyll fluorescence of two shrub species, Salix arctica and Vaccinium uliginosum, in a high arctic heath in Northeast Greenland in July and August. On two sites films, Mylar and Lexan, were used to reduce UV-B radiation and UV-B + A radiation, respectively. A UV transparent film, Teflon, and no film were used as controls. Field measurements showed that the plants under Teflon, Mylar, and Lexan received approximately 91, 39, and 17% of the ambient UV-B irradiance, respectively. Reduced UV radiation increased maximal photochemical efficiency (F v /F m ) in both species. The responses varied in significance according to species, sites and time of growing season. Net assimilation (P n ), measured as net CO2 uptake, was not significantly affected. But over the whole growing season stomatal conductance and intercellular CO2concentration were decreased by both UV treatments. The underlying mechanisms for these results are discussed. PAR-irradiance had a negative influence on the absolute values of F v /F m . A positive correlation was found between F v /F m and P n measured at ambient CO2-level. It is concluded that it cannot be excluded, that the observed short-term effects could result in long-term negative effects on growth and survival for the investigated species.