Assessing spatio-temporal variations in plant phenology using Fourier analysis on NDVI time series: results from a dry savannah environment in Namibia

Time series of Normalized Difference Vegetation Index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR) were used to capture plant phenology in Etosha National Park, a dry savannah environment in Namibia. Data from two consecutive growing periods with different precipitation conditions were included to study impacts of inter-seasonal rainfall variations on a highly water-limited ecosystem. Additionally, a contemporary reference map with four major vegetation units was used to compare phenology between plant formations. Phenological attributes were acquired for both seasons using Fourier analysis. Parameters were calculated for the entire study area and further stratified with respect to the mapping units of the reference. Vegetation growth was found to vary significantly between the two periods in accordance with available rainfall data. Additionally, separability of vegetation entities based on Fourier parameters was weak due to within-class scattering and was commonly outranged by inter-seasonal variations. Finally, discrimination of cover types was tested by combining selected Fourier parameters in a clustering procedure. Spatial class distribution was compared to the reference statistically and only a moderate correspondence was discovered. We conclude that Fourier-based NDVI attributes are limited for cover-type discrimination across space and time, as they only quantify certain aspects of plant phenology and seem to be largely altered by the actual rainfall situation.