Evaluating the effects of leaf characteristics on spectral signatures of savannah woody species on remotely sensed imagery

Abstract

Woody species on savannahs provide nutrition to wildlife and livestock ungulates. Understanding of woody leaf reflectance would benefit the application of remote sensing in analysis of these rangelands. This study sought to establish the influence of the leaf characteristics of size (leaf form) and chlorophyll content on the spectral reflectance characteristics of savannah woody species. The ability of optical remotely-sensed imagery to identify these two leaf characteristics and use them to determine the woody content of savannahs was then assessed. Two species that represented narrow leaf and broad leaf savannah woody species were studied: Acacia tortilis (renamed Vachellia tortilis) and Ziziphus mucronata, respectively. Forty seven woody individuals representing these species were sampled in north-western South Africa. Chlorophyll content, leaf area index (LAI) and spectral reflectance were determined in-situ using a chlorophyll meter, a canopy analyser and spectroradiometer, respectively. A SPOT 6 NAOMI image acquired at a time of year when grass reflectance was excluded from the spectral signature of non-senescent vegetation was used. The image data were converted to reflectance (%), and the green and near infrared (NIR) reflectance of the field-sampled trees on the image were correlated with the in-situ data. The results showed that the woody species differed significantly in their chlorophyll content and green reflectance, but only the NIR reflectance of the broad leaf species correlated strongly with a leaf characteristic, LAI. From the results it is concluded that LAI is the more reliable leaf characteristic for analysing the characteristics of savannahs in terms of woody content.