Estimating leaf area index using onsite measurements of radiation fluxes

Modeling exchange processes between the earth surface and the atmosphere requires accurate parameters for the plant canopy development at all stages of the growing period. While meteorological parameters such as temperature, wind and water are often obtained with direct on-line measurements, obtaining consistent data of canopy leaf area index (LAI) can consume much off-line field sampling and data processing time. The LAI was evaluated at crop, grass, and forest sites located in different growing environments in the United States, using the normalized difference vegetation index (NDVI) derived with onsite hourly measurements of photosynthetically active radiation (PAR) and global solar radiation. Evaluations of LAI were made during the growing period from April to October in 2002, 2003, and 2004. As expected the maximum LAI generally coincided with periods of maximum PAR and NDVI, which occurred between July and early August. Maximum net PAR averaged 1050 µmol m^-2 s^-1 for corn, 900 µmol m^-2 s^-1 for soybean, 1000 µmol m^-2 s^-1 for grassland, and 900 µmol m^-2 s^-1 for forest; maximum NDVI averaged 0.8 for corn, 0.8 for soybean, 0.6 grassland, and 0.8 for forest; and maximum LAI averaged 5 m^-2 m^-2 for corn, 6 m^-2 m^-2 for soybean, less than 1 m^-2 m^-2 for grassland, and 5 m^-2 m^-2 for forest. The relation of LAI to NDVI was nearly a straight line with a slope of 5.0 for crop and forest canopies, and slope of 1.0 for the grasses. Estimates of LAI compared quite reasonably with direct measurements for corn and soybean canopies. These results suggest the possibility of using real-time single point measurements of radiation properties to obtain LAI parameters for routine models of ecosystems.