Pollen distribution in surface sediments of the northern Lower Medjerda valley (northeastern Tunisia)

Past Mediterranean vegetation dynamics are commonly reconstructed using fossil pollen records. However, the interpretation of pollen records in terms of vegetation, climate and land-use changes is often limited due to complex pollen-vegetation relationships. So far, such relationships have been poorly examined in the semi-arid regions of Northern Africa. This work aims to document the modern pollen assemblages that reflect the main vegetation types associated to different land-uses in the northeastern Tunisia. For that, we collected 29 modern terrestrial surface samples in the Lower Medjerda valley and compared the pollen spectra to remotely sensed vegetation data. This original approach, coupling pollen and remotely sensed land cover data, presents some limitations and errors affecting both datasets although it gives promising results. Multivariate analyses show that the composition of pollen samples collected within the Mediterranean maquis may be strongly influenced by adjacent land covers such as conifer woodland or open vegetation. In contrast, pollen spectra associated with land-uses corresponding to heterogeneous agricultural areas and open landscape, mainly irrigated lands, have a distinct signature. However, close to water bodies, pollen spectra reflect predominantly the characteristic halophytic vegetation whatever the surrounding land cover types. We also examined the pollen composition of sediment core-tops from the lagoon Sidi Ali Mekki, to evaluate the ability of pollen sequences to record past vegetation and land-use histories in the Ghar el Melh region. We found that core pollen signal is mainly imprinted by the predominant local vegetation of Sidi Ali Mekki shore although regional and extra-regional pollen inputs appear stronger than in modern surface samples.