Gap fraction based estimation of LAI in Scots pine stands subjected to experimental removal of branches and stems

We compared estimates of leaf area index obtained by the LAI-2000 plant canopy analyzer (LAI(PCA)) to direct estimates of LAI ("true" LAI) obtained through allometric relationships. This was done for two Scots pine (Pinus sylvestris L.) stands, where LAI was gradually reduced by removing branches and whole trees. LAI (defined on a hemisurface area basis) decreased from 2.24 to 0.50 in the branch removal experiment and from 1.58 to 0.29 in the tree removal experiment. The aim of the study was to analyse the variation in the ratio of the LAI-2000 estimate to the true LAI (LAI(PCA)/LAI) with changes in stand structure and total leaf area. In the tree removal plot, which had a smaller proportion of woody (branch) area, LAIPCA/LAI remained fairly stable (0.63-0.69) and was smaller than that in the branch removal plot, where LAI(PCA)/LAI increased from 0.76 to 1.16 along with the decrease in leaf area and a subsequent increase in woody (stem) area. The ratio of LAI(PCA) to the plant area index (PAI) differed less between plots but remained higher in the branch removal plot (increasing from 0.56 to 0.69) than in the tree removal plot, where it varied between 0.55 and 0.60. Results were analysed with the help of a theoretical canopy radiation model, which can be inverted to give LAI based on the gap fraction values measured by the LAI-2000 and stand structural parameters. Model-inverted LAI agreed well with directly measured LAI, suggesting that the model is a useful tool for correcting bias in the LAI-2000 estimates because of grouping of leaf area and the contribution of woody area.