Van Gunst, K.J. 2012. Forest Mortality in Lake Tahoe Basin from ...

14Leaf Area Index and Stocking IndexAll images were transformed to Normalized Difference Vegetation Index (NDVI), aratio of red to near-infrared bands which contains information about the health and extentof green vegetation in a pixel (Huete et al. 1997). NDVI is calculated using[1]where NIR=Near-Infrared wavelength, Landsat TM band 4 and VIS=Visible (Red)wavelength, Landsat TM band 3. We used regression modeling against field-measureddata to further transform NDVI into leaf area index (LAI) based on the nonlinear modelof Baret and Guyot (1991), which accounts for NDVI saturation when LAI increases tohigh values. This model uses the following equation:[2] –where VI equals NDVI, VI g equals NDVI value of bare soil, VI∞ equals asymptoticvalue of NDVI when LAI tends toward infinity, and K VI equals the extinction coefficientcontrolling slope of relationship between NDVI and LAI.For model calibration, we used the NDVI and LAI values from 30 field-sampled plotssampled with a LI-COR 2000 Plant Canopy Analyzer (LI-COR, Lincoln, Nebraska). andfound the NDVI value of 0.257 at bare soil (VI g ) in 5 areas in the study area almostcompletely dominated by bare soil and solved for the remaining two parameters: theasymptotic value of NDVI at 0.760 (VI∞ ) and the extinction rate between NDVI andLAI of 0.345 (K VI ). In many coniferous systems, the asymptotic value of NDVI isbetween 0.6 - 0.8 (White et al. 1997).