LAUSR

Photosynthetically active radiation (PAR) is an important parameter in ecological research. However, it is not routinely measured and often is estimated as a constant ratio of the incoming solar shortwave radiation (Rs). There are only few reported PAR/Rs values worldwide concerning the Mediterranean climate and even fewer from rural or forest areas, especially at higher altitudes. Hourly PAR and Rs flux densities were measured above a deciduous oak forest in Greece from 1999 to 2005, and their relationship was investigated under various conditions. Results show that the annual mean hourly PAR/Rs is 0.454, ranging from 0.443 in spring to 0.478 in autumn, with intermediate values in summer (0.454) and winter (0.459). The ratio increases with dew point temperature but decreases as solar elevation angle or Rs increases. Atmospheric clearness index, Kt, and actual water vapor pressure, ea, are the key factors determining the ratio; however, relative humidity (RH) also seems to have an indirect effect by affecting Kt and ea values. PAR/Rs changes from 0.468 to 0.455 as sky conditions change from clear to overcast and appears to increase with ea. However, Kt affects the ratio when RH is lower than 60%, while ea has a more obvious effect at more saturated atmospheric conditions.