Dynamics of savanna vegetation in relation to fire use in Madagascar: time series analysis of remote sensing images.

Summary Although fire is recognized as an influential factor in savanna vegetation dynamics, its role is not clearly defined. This thesis addresses the problem of studying the relationship between fire use and vegetation dynamics. The approach chosen is based on the analysis of time series of remote sensing images at medium spatial resolution. The savannas studied are located in the Marovoay watershed in northwestern Madagascar. As there is no consensus on the methods to be used, the savannahs of Madagascar offer a particular context, due to the very pronounced degradation of the vegetation cover and the changes sought, to test existing methods and propose new ones. The first objective of this work is to identify the fire regime through the monitoring of spatio-temporal variations of burned areas in the savannah. For this purpose, a method of mapping burned areas was developed: it is based on the calculation of an annual indicator indicating the passage of a fire during the dry season and a seasonal indicator translating the period of passage of the fire. This method, applied to the study site, produced a temporal series of data used to characterize the fire regime based on two parameters, the period of occurrence and the frequency of fire passage. In parallel, the second objective is to characterize vegetation dynamics by analyzing spatio-temporal variations in plant activity. Two approaches for detecting changes, based on NDVI time series processing, were tested. The first one is based on the analysis of inter-annual variations of a phenological indicator translating plant activity during the growth phase of savannahs. The second uses a temporal decomposition technique to extract the trend of an NDVI series. In both cases, the results allowed to characterize the vegetation dynamics through three classes of evolution of the vegetation activity (progressive, regressive or stable series). These results were evaluated by comparison with those from change detection techniques based on diachronic analysis of high spatial resolution images. Finally, in the last step of the work, we studied the relationships between information on fire regimes and vegetation dynamics using multivariate regression models. The objective is to estimate the importance and role of fire in vegetation dynamics. The results led to three conclusions: a) Fire is a factor in the maintenance of savannahs. b) In situations where pressure from anthropogenic activities is low, fire, particularly through the frequency of its use, is a determining factor in vegetation dynamics. c) In other situations, the interpretation of the results is complex and difficult, most likely due to the interaction of multiple anthropogenic factors.