Shen, MG, Chen, J, Zhu, XL, Tang, YH (2009). Yellow flowers can decrease NDVI and EVI values: evidence from a field experiment in an alpine meadow. CANADIAN JOURNAL OF REMOTE SENSING, 35(2), 99-106.
The enhanced vegetation index (EVI) and the normalized difference vegetation index (NDVI) have proven to be useful in a wide range of studies on vegetation status. NDVI and EVI were designed taking into account variable atmospheric conditions and soil backgrounds and a wide range of sensor view and sun angles. However, these two indices may suffer from flower effects if there is a considerable flower fraction in the canopy. To evaluate quantitatively the flower cover effects on NDVI and EVI, we conducted a field experiment in an alpine meadow where the July flowers were predominantly yellow because Halerpestes tricuspis (Ranunculaceae) was in flower. We investigated the responses of these two indices to variations in the flower cover by adopting regression analysis based on the in situ data. The results showed the following: (i) the yellow flower of H. tricuspis significantly increased the red band canopy reflectance, but without apparent variations in near-infrared or blue band reflectance; (ii) the increase in red band reflectance significantly reduced the NDVI and EVI values; and (iii) the EVI was more sensitive than the NDVI to changes in the flower cover. Furthermore, clear decreases were observed in the NDVI and EVI values of the Moderate Resolution Imaging Spectroradiometer (MODIS) product for the study site caused by flower effects. These results suggest that there is a need to examine the effects of flower cover on the NDVI and EVI when they are used for types of vegetation that have profuse seasonal blooms.