Investigating relationships between Landsat ETM+ sensor data and leaf area index in a boreal conifer forest

The aim of this paper is to investigate the feasibility of using Landsat ETM+ data for the determination of leaf area index (LAI). The investigation is prompted by the need for obtaining spatially distributed data on LAI to be used as input for carbon modelling of northern boreal forests. Detailed field data have been collected in a coniferous forest area in Uppland, central Sweden, dominated by Norway spruce and Scots pine. A forest canopy reflectance model (Kuusk and Nilson, 2000) has been used to simulate stand reflectances in the Landsat ETM+ wavelength bands as a means of investigating the theoretical reflectance response to LAI changes. The analysis shows that the response to changes in LAI is strongest in the visible wavelength bands, particularly Channel 3, whereas only weak response is noted in the NIR band and for some vegetation indices [simple ratio (SR) and NDVI]. Modelled reflectances are influenced by various other factors, particularly ground reflectance and leaf biochemical properties. Observed reflectances from the Landsat ETM+ sensor have been compared with reflectance modelling results and with field-based LAI estimates. The results indicate that LAI estimation using inverse canopy reflectance modelling may be difficult, given the large number of input parameters required and the current level of uncertainty in these parameters. Statistical relationships between LAI and observed ETM+ reflectances are strongest in ETM+ Channel 7.