Indirect estimations and spatial variation in leaf area index of coniferous, deciduous and mixed forest stands in Forsmark and Laxemar

Two sites in Sweden are investigated for a potential deep repository of the nuclear waste, the Laxemar investigation area (57°5’N, 16°7’E) and the Forsmark investigation area (60°4’N, 18°2’E). In the characterisation of these sites, development of site descriptive models is an important part. Leaves are the main surface were an exchange of matter and energy between the atmosphere and the biosphere takes place, and leaf area index (LAI) of the vegetation cover is an important variable correlated to a number of ecophysiological parameters and hereby an important parameter in ecosystem models. In the investigation areas, LAI of boreal and temperate ecosystems were therefore estimated indirectly through optical measurements using the LAI-2000 (LI-COR, Cambridge UK) and TRAC (Tracing Radiation and Architecture of Canopies). On average, measured maximum LAI was .40 in Laxemar and .4 in Forsmark; minimum LAI was 1.65 in Laxemar and 1.97 in Forsmark. Forest inventory data showed that LAI is positively correlated with basal area, stand height, stand volume and breast height tree diameter. For the coniferous stands, there was also a linearly negative relationship with age. In the Laxemar investigation area, there were no significant relationships for LAI with a satellite derived kNN (kNearest Neighbor) data set with stand height, stand volume and stand age. The kNN data set can therefore not be used to extrapolate measured LAI over the Laxemar investigation area. There were significant relationships between LAI and the normalized difference vegetation index (NDVI) for coniferous, deciduous and mixed forest stands in the Laxemar investigation area. A NDVI image could be used to extrapolate LAI over the entire investigation area. For the Forsmark investigation area, effective LAI for all stands were correlated to NDVI and this relationship could then be used for extrapolation. The effective LAI image was afterwards corrected for average needle to shoot area ratio, woody to total area ratio and element-clumping index for coniferous, deciduous and mixed forest stands. NDVI modelled LAI was used to evaluate the LAI product from Moderate Resolution Imaging Spectroradiometer (MODIS), and the comparison indicated that MODIS LAI was neither correlated to LAI in Laxemar nor in Forsmark. MODIS LAI also was larger than NDVI modelled LAI and it showed large variations at both sites. It is therefore not recommended to use the MODIS LAI product for future LAI estimations in these small investigation areas.