Quantifying change in patterned semi-arid vegetation by Fourier analysis of digitized aerial photographs

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Abstract
Panchromatic aerial photographs from 1955 and 1985 (scale: 1:50 000) were used to quantify changes in semi-arid patterned vegetation caused by a succession of dry years in the early 1980s. The study site is located in the northwest part of Burkina Faso (West Africa), and features a plain with a savanna physiognomy and gentle slopes covered by tiger bush. Digitized data (pixel size of 3.15 m) covered a belt transect of 9 km by 1.5 km that has been divided into 315 m 2 square quadrats. Four reference quadrats were digitized with a pixel of 0.83 m, for comparison with high-resolution outlooks from 1994. Pattern quantification relied on spectral analysis by Fourier transform, that yielded dominant wavelengths (radial spectrum) and main orientations (angular spectrum). The vegetation in the plain displayed important changes that were related to the collapse of the herbaceous cover (and associated scattered trees), and its partial post-drought recovery. Such changes were quantified as a relative decline of small spatial wavelengths (<40 m), followed by a bounce-back of wavelengths in the range 25-50 m. An underlying pattern of fragmented bands of woodland was also revealed with a dominant orientation and a characteristic wavelength ranging from 80 m up to more than 105 m. On the gentle slopes, dense wooded bands that constituted tiger bush (with a wavelength in the range 60-80 m) displayed only minor changes through time. Spectral analysis by Fourier transform proved to be a suitable way to monitoring patterned semi-arid vegetation for which periodicity and orientation are important characteristics. Consistent diachronic comparisons are possible even with historical photographs of varying scale and quality.