Ratio of photosynthetically active to total incoming radiation above a Mediterranean deciduous oak forest

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 (R_s). There are only few reported PAR/R_s values worldwide concerning the Mediterranean climate and even fewer from rural or forest areas, especially at higher altitudes. Hourly PAR and R_s 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/R_s 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 R_s increases. Atmospheric clearness index, K_t, and actual water vapor pressure, e_a, are the key factors determining the ratio; however, relative humidity (RH) also seems to have an indirect effect by affecting K_t and e_a values. PAR/R_s changes from 0.468 to 0.455 as sky conditions change from clear to overcast and appears to increase with e_a. However, K_t affects the ratio when RH is lower than 60%, while e_a has a more obvious effect at more saturated atmospheric conditions.