Biases in the frequency of fruits and seeds in modern fluvial sediments in NW Italy: the key to interpret analogous fossil assemblages

Quantitative taphonomic analyses of recent fruit and seed assemblages (carpological assemblages or carpodeposits) are essential to allow for more accurate interpretation of fossil carpodeposits. To this end, extensive taphonomic analyses were undertaken in two modern, small, fluvial catchment basins (Ca’ Viettone and Valtorta-Rivara) in northwestern Italy that are characterized by different vegetation types. Quantitative data from vegetational surveys and carpodeposit analyses were compared using a standardized graphic representation (Plant Community Scenario, PCS). The contents of the carpodeposits clearly differentiate the different types of vegetation in each basin. Moreover, carpological assemblages from the same basin have a similar signature. Comparison of all samples indicates a relationship between the standing vegetation and the PCS reconstruction based on carpological analysis. The bedload carpodeposits studied seem to characterize, at least qualitatively, the vegetation of the entire basin rather than just the area adjacent to the sample site. When differences in taxonomic frequency in the standing vegetation and in the carpological assemblages are evaluated, patterns in the over- and underrepresentation of certain types of fruits and seeds become apparent. Such patterns are quantified by a bias index for diaspores of each taxon, derived from empirical observations and applicable to bedload carpodeposits. Factors biasing representation in an assemblage include disseminule size and woodiness. In addition, the mode of dispersal seems to be very important: anemochorous (wind dispersed), endozoochorous (animal dispersal via excrement), and myrmecochorous (ant dispersed) diaspores are clearly overrepresented.