ESTIMATION OF DECIDUOUS FOREST LEAF-AREA INDEX USING DIRECT AND INDIRECT METHODS

This study evaluated one semi-direct and three indirect methods for es timating leaf area index (LAI) by comparing these estimates with direc t estimates derived from litter collection. The semi-direct method use s a thin metallic needle to count a number of contacts across fresh li tter layers. One indirect method is based on the penetration of diffus e global radiation measured over the course of a day. The second indir ect method uses the LAI-2000 plant canopy analyser (PCA) which measure s diffuse light penetration from five different sky sectors simultaneo usly. The third indirect method uses the ''Demon'' portable light sens or to measure the penetration of direct beam sunlight at different zen ith angles over the course of half a day. The Poisson model of gap fre quency was applied to estimate plant area index (PAI) from observed tr ansmittances using the second and third methods. Litter collection fro m II temperate deciduous forests gave values of LAI ranging from 1.7 t o 7.5. Estimates based on the needle method showed a significant linea r relationship with LAI values obtained from litter collections but we re systematically lower (by 6-37%). PAI estimates using all three indi rect techniques (fixed light sensor system, LAI-2000 and Demon) showed a strong linear relationship with LAI derived from litter collection. Differences, averaged over all forest stands, between PAI estimates f rom each of the three indirect methods and LAI from litter collections were below 2%. if we consider that LAI=PAI-WAI (wood area index) then , all three indirect methods underestimated LAI by an additional facto r close to the value of WAI. One reason could be a local clumping of a rchitectural canopy components: in particular, the spatial disposition s of branchlets and leaves are not independent, leading to a non-rando m relationship between the distributions of these two canopy component s.