The influence of drought and thinning on leaf area index estimates from canopy transmittance method

Leaf area index (LAI) estimates from litterfall and from canopy transmittance measurements in photosynthetically active radiation (PAR) and total solar irradiance wave bands were compared in Turkey oak (Quercus cerris L) stands. The aim was to evaluate advantages and limits of the transmittance measurements method in estimating LAI and to determine whether the modifications due to silvicultural operations and to different climatic conditions affected the accuracy of this method. Data were collected in thinned and unthinned experimental plots established at two different locations: Valsavignone, in the Apennines, with a wet climate (''wet site''), and Caselli, near the Thyrrenian coast, with a longer and more severe summer dry period (''dry site''). Differences in stand density and LAI due to silvicultural operations led to significant differences in transmittance but did not affect light extinction coefficient (k). On the contrary, environmental constraints influenced canopy properties as transmittance and k. However, the variability in canopy properties do no limit the capability of canopy transmittance, measured in the PAR wave band, to be a good predictor (error < +/- 5%) of LAI at stand level. Some considerations are made about k and its value in PAR and total solar irradiance wave bands, and on variability in canopy structure of a given species in relation to drought.