Calibration of two indirect methods for estimating the leaf area index in Eucalyptus plantations

The leaf area index (LAI) has been used as a forestry management tool, since it can be used as a reference for the performance of biophysical and physiological processes. Several indirect methods used to determine LAI have been used, but few studies address the calibration of these methodologies at different ages of the forest crop cycle. The LAI-2000 is an indirect method of LAI measurement whose principle is based on the balance between the radiation reaching inside of the canopy and the radiation on the top of it. Another widely used indirect method is the hemispherical photographs that capture the patterns of obstruction and light penetration in the canopy in which the canopy architecture and the foliage area can be quantified. The objective of this study was to compare the indirect methods LAI-2000 and hemispheric photos with the direct method of LAI obtained by destructive sampling of leaf biomass (real LAI), and to evaluate settings and use conditions of the equipments used by both methods. The evaluations were performed in clonal plantations of Eucalyptus grandis x urophylla at different ages and located in southern Bahia, Brazil. The control of the photos exposure was a decisive factor for the use of hemispheric photos. Moreover the opening of the device cover between 270 and 360 and the diffuse cloudy radiation conditions were essential for the proper use of LAI-2000. Both methods showed positive correlations with real LAI, however, there was an underestimation of 68% of the hemispheric photos and 51% of the LAI-2000 in relation to the values obtained by the direct method. Thus, when the interest is the absolute value of LAI, the calibration using destructive methods of leaf biomass is required.