Leaf area index and normalized difference vegetation index as predictors of canopy characteristics and light interception by riparian species on the Lower Colorado River

Leaf area index (LAI) and normalized difference vegetation index (NDVI) were compared for riparian species along a 350 km stretch of the Lower Colorado River in the United States and Mexico. The species included two native trees, cottonwood (Populus fremontii) and willow (Salix gooddingii), and two salt-tolerant shrubs, saltcedar (Tamarix ramosissima) and arrowweed (Pluchia sericea), exhibiting large differences in leaf type and canopy architecture. LAI was measured with a Licor 2000 plant canopy analyzer calibrated against biomass measurements of LAI, whereas NDVI was measured by low-level aerial photography using a DyCam digital camera with Red (R)–Blue (B)–near infrared (NIR) bands. In addition, reflectance spectra were measured for leaf samples collected from plants in the field. Leaf samples of all species had similar reflectance spectra in the visible (VIS) and NIR, hence similar NDVI values, ranging from 0.62 to 0.72 (P>0.05). LAI values of field plants varied over a relatively narrow range, with mean values of 3.50, 3.28, 2.81 and 3.69 for cottonwood, willow, saltcedar and arrowweed, respectively. However, field plants showed distinct species differences in NDVI, with the following mean values: cottonwood (0.686), willow (0.600), saltcedar (0.473) and arrowweed (0.254) (all significantly different at P<0.05). Differences in NDVI among field plants could be explained by differences in the light extinction coefficient, k, for plant canopies, according to the formula: fIRs=(1−e−kLAI), where fIRs is the fraction of incident light intercepted by the canopy. At one extreme, cottonwood had broad leaves that faced the sun, and a calculated k of 1.25, whereas at the other extreme, arrowweed had linear leaves that were near to vertical, and had a k of 0.15. Ecophysiological implications of the differences among the species are discussed.