DIRECT AND INDIRECT ESTIMATES OF LEAF-AREA INDEX (LAI) FOR LODGEPOLE AND LOBLOLLY-PINE STANDS

We compared direct and indirect estimates of leaf area index (LAI) for lodgepole and loblolly pine stands. Indirect estimates of LAI using radiative methods of the LI-COR LAI-2000 Plant Canopy Analyzer (PCA) did not correlate with allometric estimates for lodgepole pine, and correlated only weakly with litter-trap estimates for loblolly pine. The PCA consistently under-estimated LAI in lodgepole pine stands with high LAI, and over-estimated LAI in the loblolly pine stands with low LAI. We developed a physical model to test the hypothesis that the PCA may under-estimate LAI in high leaf area stands because of increased foliage overlap and, therefore, increased self-shading. Radiative estimates of LAI using the PCA for the physical model were consistently lower than allometric measures. Results from the physical model suggested that increased foliage overlap decreased the ability of the PCA to accurately estimate LAI. The relationship between allometric and radiative measures suggested an upper asymptote in LAI estimated using the PCA. The PCA may not accurately estimate LAI in stands of low or high leaf area index, and the bias or error associated with these estimates probably depends on species and canopy structure. A species specific correction factor will not necessarily correct bias in LAI estimates using the PCA.