The amount of solar radiation absorbed, reflected or transmitted by vegetation depends on the radiation distribution and the canopy properties. This study of the optical properties of an oak forest under foliated and non-foliated conditions is attempted in order to better understand radiation use and exchange. Canopy absorptivity, reflectivity and transmittivity to global solar and photosynthetically active radiation (S and PAR, respectively) are considered. To estimate the forest optical properties, radiation measurements above and below the canopy were taken in the middle of a 0.97 km2 natural, slightly sloped, 50-year-old, oak forest in the Northern Peloponese, Greece.

It was found that the greatest part of the incident S is absorbed by the foliated or non-foliated canopy. In the first case, absorptivity is close to 0.7, with reflectivity and transmittivity sharing the remaining portion. Before leaf appearance, although reflectivity may be only a little lower, transmittivity is considerably higher, allowing more than 40% of the incoming radiation to reach the ground. PAR is absorbed to an even higher degree by the green elements of the canopy, allowing only a 5% radiation loss, via transmission, since canopy reflectivity is practically zero. The ground contribution to the canopy short-wave radiation balance is small during the dense foliation period, but it becomes considerable during the leafless period. Especially when considering the balance in the PAR region, the ground has no effect.