Using spectral analysis to confront distributions of individual species with an overall periodic pattern in semi-arid vegetation

Contrasted and periodic vegetation patterns are observed throughout the semi-arid tropics. The relation between individual species and the 'overall' pattern has been investigated from a study site in North West Burkina Faso (West Africa), which displayed periodic woody patterns with varying levels of contrast and isotropy. The woody vegetation was described from two field plots (320 m by 320 m), within which woody individuals were either mapped or counted (in quadrats of 10 m by 10 m). A banded pattern of high intensity (tiger bush on hard sandstone, plot PTG) was compared with a less precise pattern on more favourable edaphic conditions (plot PSP). The periodic nature of the vegetation under study was directly addressed by interpreting spatial auto- and cross-covariance functions through spectra of spatial frequencies (spectral analysis). The overall pattern of vegetation was analysed from digitised aerial photographs, while distributions of individual species were characterised from field data. In both plots, the densest species (Combretum micranthum G. Don), though only dominant in PTG, had a spatial distribution that closely matched the overall pattern. Pterocarpus lucens Lepr was co-dominant in both plots although fairly independent on the periodic pattern. Several species displayed a positive link with the periodic pattern that was quite loose in PSP and tighter in PTG. This tighter link was concomitant with lower densities for most species. Indeed, only C. micranthum clearly benefited from the high intensity pattern with a density seven times higher in PTG than in PSP. Consequently, species richness and diversity were lower in PTG. A single species proved hence of overwhelming importance in accounting for the periodic pattern and its persistence through time. Spatial distributions of other species pointed rather towards individualistic responses to the opportunity/constraint provided by the periodic pattern in presence of different levels of edaphic and climatic stress.