Résumé
Morocco is rich in temporary pools which harbour numerous rare plant species. Long-term conservation of such threatened plant communities should be based on the understanding of their past dynamics. Despite conditions unfavourable to pollen preservation, surface sediments of acidic temporary pools are shown to contain pollen assemblages likely to allow vegetation reconstruction. Knowledge of the modern relationships between pollen and vegetation is, however, necessary for interpreting fossil data in terms of past vegetation. Surface pollen assemblages and floristic surveys of a temporary pool in Benslimane forest, western Morocco, are compared in order to evaluate the pollen record of the local hydrophytic vegetation. Floristic surveys were carried out for 12
years (1996–2008) along two crossing permanent transects. A set of 21 surface-sediment samples, taken along the same transects in 2007, were analysed for pollen. The spatial relationships between vegetation and pollen assemblages are explored by means of multivariate analyses, statistical tests and linear regressions. The calculation of representation indices moreover allows proposing quantitative ways for pollen-based plant-abundance reconstruction.
Results reveal that the vegetation structure along the hydrological gradient is well recorded in the pollen assemblages, with: (1) a marginal zone characterised by terrestrial taxa and rare amphibious taxa (
Elatine,
Pilularia), (2) an intermediate zone of amphibious taxa (
Alisma-type,
Illecebrum/Paronychia,
Isoetes velata-type), and (3) a central zone of aquatics (
Myriophyllum alterniflorum,
Ranunculus-type). The best correlation between the pollen record and total pool vegetation was found in the centre of the pool, which supports the reliability of the study of a single core from the centre of the pool for the reconstruction of the past dynamics of the local hydrophytic vegetation. Both the qualitative ‘community’ approach (representation indices and indicator pollen taxa) and the quantitative ‘taxa’ approach (correction factors) suggest that reconstructions of past populations can be achieved from a few taxa, namely
Isoetes velata-type,
Myriophyllum alterniflorum and
Ranunculus-type. For these taxa, regression parameters (slope and y-intercept) have been calculated between pollen percentages and plant percentages in present vegetation, and between pollen influxes and plant abundances, respectively. These parameters can be extended to interpret fossil data from other temporary pools within the same region to reconstruct their relative and absolute past plant abundances.