A simplified methodology for the correction of Leaf Area Index (LAI) measurements obtained by ceptometer with reference to Pinus Portuguese forests

Forest leaf area index (LAI) is a very important structural parameter, controlling many biological and physiological processes associated with vegetation on the Earth’s surface. Its accurate estimation is extremely important for forest managers. There are a widely range of methods for its estimation. One of them is using ground-based instruments like the sunfleck ceptometers, with advantages because it is not a destructive method and is very easy to apply. Although, when applied to Pinus, the most important forest specie in Portugal, LAI is underestimated by these instruments, mainly because of its foliage clumping. Trying to correct this underestimation, some previous studies proposed the multiplication of the measured LAI value by a constant correction factor, while others have only proposed practical advice for a correct use of these instruments. Thus, although unquestionably a practical method for recording LAI, use of the ceptomer is problematic. Age of threes from this database ranged from 74 to 14 years old, representing heterogeneous Pinus stands, at it is usual in Portugal. As assumed by some authors, the LAI obtained from allometric equations is less problematic and was be used to generate a reference LAI in order to develop a new methodology for correcting ceptometer measurements, even considering that is not immune to error because it is site- and specie- specific. This way, LAI data from 102 sampling plots, established across all North of Portugal, representative of the Pinus pinaster forest in Portugal, was collected. This database was divided in 82 sampling plots for the adjustment of the LAI ceptometer correction model and 20 were used for the model validation. This new correction methodology does not assume a constant correction value, unlike previous ones, and indicates that the bias (the difference between the ceptometer and the reference LAI) is estimated as a function of the basal area per unit ground area and that this bias is summed to the estimated value. Being the basal area an easily structural obtainable from forest inventories, this new methodology can be easily applied. The observed LAI of this database ranged from 6.4 to 0.34, once again showing the heterogeneity of the Portuguese Pinus forest. Also in general the research has demonstrated a significant difference in the accuracy of LAI as estimated by ceptometer in relation to reference values, thus measurements for Pinus should be corrected. This study has shown that the estimated Pinus LAI needs a correction and that the new approach improves the LAI ceptometer estimations.