%0 Research article
%T Reduced simple ratio better than NDVI for estimating LAI in Finnish pine and spruce stands
%A Stenberg, Pauline
%A Rautiainen, Miina
%A Manninen, Terhikki
%A Voipio, Pekka
%A Smolander, Heikki
%D 2004
%J Silva Fennica
%V 38
%N 1
%R doi:10.14214/sf.431
%U https://silvafennica.fi/article/431
%X Estimation of leaf area index (LAI) using spectral vegetation indices  (SVIs) was studied based on data from 683 plots on two Scots pine and  Norway spruce dominated sites in Finland. The SVIs studied included the  normalised difference vegetation index (NDVI), the simple ratio (SR),  and the reduced simple ratio (RSR), and were calculated from Landsat ETM  images of the two sites. Regular grids of size 1 km2 with gridpoints  placed at 50 m intervals were established at the sites and measurements  of LAI using the LAI-2000 instrument were taken at the gridpoints.  SVI-LAI relationships were examined at plot scale, where the plots were  defined as circular areas of radius 70 m around each gridpoint. Plotwise  mean LAI was computed as a weighted average of LAI readings taken  around the gridpoints belonging to the plot. Mean LAI for the plots  ranged from 0.36 to 3.72 (hemisurface area). All of the studied SVIs  showed fair positive correlation with LAI but RSR responded more  dynamically to LAI than did SR or NDVI. Especially NDVI showed poor  sensitivity to changes in LAI. RSR explained 63% of the variation in LAI  when all plots were included (n = 683) and the coefficient of  determination rose to 75% when data was restricted to homogeneous plots  (n = 381). Maps of estimated LAI using RSR showed good agreement with  maps of measured LAI for the two sites.