Harmonic Analysis of Long-term Modis Time-series Data for Vegetation Dynamics in Hawaii

Long-term MODIS vegetation index records were used to extract regularly-repeating seasonal and interannual greenness cycles in Hawaiian ecosystems using harmonic analysis. With two vegetation indices, NDVI and EVI, the MODIS system provided an opportunity to combine the two measures and create a hybrid approach to the leaf phenology study in a diversity of Hawaiian ecosystems. Despite data noise caused by variable meteorological conditions, the analysis effectively summarized the amplitudes and the frequencies of a series of sinusoidal harmonic terms based on 16-day vegetation index composite data. The amplitude of annual greenness quickly increased with mean annual precipitation (MAP) up to a 1000-1500mm range and declined rapidly thereafter. In wetter environments, where MAP was higher than 2000mm, NDVI values became saturated and EVI records showed that the amplitude of annual and biannual greenness cycles decreased gradually with MAP. MAP was also significantly correlated with the frequency of the first two predominant harmonics, the primary and the secondary harmonic terms. As MAP increased, the frequency of greenness cycles (or harmonic terms) increased. This pattern was observed among 12 land cover types selected from the Hawaii Gap Analysis Program’s land cover map. These land cover types were categorized into three phenological groups based on their mean frequency of greenness cycles and MAP.