Arable-weed flora and its pollen representation: A case study from the eastern part of France
Résumé
Local pollen deposition in human-influenced vegetation types is studied with the aim of establishing the relationship between arable-weed vegetation and its pollen representation as an aid in interpreting human presence and impact on vegetation from pollen assemblages. The study area is located in the Franche-Comté region, eastern part of France. The presence of plants within 100 m2 plots and their pollen representation in moss samples within the same plots was recorded from different crops and annual fallows. Patterns in the vegetation and pollen assemblages were investigated separately using principal coordinate analysis (PCoA). Procrustean co-inertia analysis (PCIA) was then used to test whether two configurations of sites in the PCoA plots were similar to one another. The relationship between plant and pollen presences was quantified using association, over- and under-representation indices. Each taxon was related to its mode of pollination and characteristics of the flowers, as well as to the size of its pollen grain. Six groups of relevés are defined on the basis of species composition (arable-weed flora of acid or basic soil, etc.). PCIA analyses revealed that the general patterns found in the vegetation data are the same with different levels of determination (species or pollen types). On the other hand, the numerical analyses of the modern pollen data-set not allow us to distinguish all the groups. Only the first one (of acid soil) can be distinguished from the others (with few differences nevertheless). The patterns found with the representation indices are explained by flower characteristics (mode of pollination, etc.), which all have an important influence on their pollen production and dispersal properties. The discrepancies found between the two data-sets (vegetation and modern pollen assemblages) may be explained by these characteristics (pollen production, dispersal and preservation of individual taxa as well as by land-use practices). If the identification of our groups of vegetation relevés from their pollen assemblages is not possible, we can nevertheless identify pollen indicators of arable areas classically found in northern countries (Cerealia, Centaurea cyanus, Polygonum aviculare-Type, Fallopia, etc.) and some others which seem to be present rather in more southern countries (Papaver rhoeas-Group, Sinapis, Kickxia, Scleranthus, Euphorbia and Valerianella).