Modern pollen assemblages from grazed vegetation in the western Pyrenees, France: a numerical tool for more precise reconstruction of past cultural landscapes

International audienceModern pollen assemblages from grazed vegetation in the Pyrenees Mountains (France) were studied with the aim of providing a calibrated model for reconstructing past pastoral activities. The modern analogues were selected to cover the major gradients of grazing pressure and degree of openness. The vegetation was surveyed by means of the synusial integrated method, assessing the structure and the patchiness of the pastoral phytoceonoses. A correlative model (Redundancy analysis) was devised relating 61 modern pollen spectra with 37 explanatory vegetation and land-use variables. It was shown that wooded, open grazed and nitrophilous sites are clearly separated from one another and that the model can be simplified using three relevant vegetation types as explanatory variables: dry heathland, semi-open oak forest and overgrazed community, respectively related to gradients of openness, soil richness and grazing pressure. When reconstructing past pastoral activities with fossil pollen spectra, it is important to consider scale-dependent influences of plant species. Low frequencies of well-dispersed taxa such as Artemisia, Chenopodiaceae, Plantago lanceolata and Plantago major/media must be interpreted with care since they reflect more regional, rather than local, input into the pastoral landscape. In contrast, the simultaneous occurrence of Asteroideae, Cichorioideae, Cirsium-type, Galium-type, Ranunculaceae, Stellaria-type and Potentilla-type pollen is clearly related to grazing on a local scale. Calculation of Davis indices also shows that Cichorioideae, Galium-type and Potentilla-type indicate the very local presence of the corresponding plants. These pastoral plant indicators may have a limited geographical validity, ie, mountainous regions with crystalline bedrock, which may indeed also provide the framework for the application to fossil spectra of the modern pollen/vegetation/land-use models presented here