Du pollen aux paysages : nouvelles méthodologies pour reconstituer les paysages

International audienceLes données polliniques fossiles offrent la possibilité de reconstituer l'histoire de la végétation à différentes échelles spatio-temporelles. Récemment, des modèles de la relation pollen-végétation qui prennent en compte les processus de dispersion et de déposition des pollens ont été développés pour interpréter et convertir les assemblages polliniques en pourcentage de couverture de plantes, groupes de plantes ou composantes paysagères pour une échelle régionale (≥ 50 km) et locale (≤ 2 km) autour des points d'enregistrements. Des reconstitutions quantitatives du couvert végétal dans le passé sont nécessaires pour résoudre les questions de recherche actuelles dans les domaines de l'histoire des climats, de l'archéologie et de la conservation (entretien) des paysages. Dans un premier volet, cet article exposera les nouveaux concepts, outils développés par le réseau POLLANDCAL (POLlen LANDscape CALibration) pour reconstituer le couvert végétal régional et local à partir des données polliniques fossiles en utilisant deux modèles : REVEALS (Regional Estimates of Vegetation Abundance from Large Lakes) et LOVE (Local Vegetation Estimates). Puis, dans un deuxième volet, nous illustrerons la mise en place de cette démarche quantitative sur le massif jurassien et les premiers résultats obtenu

MODELISATION DE LA RELATION ENTRE PLUIE POLLINIQUE ACTUELLE, VEGETATION ET PRATIQUES PASTORALES EN MOYENNE MONTAGNE (PYRENEES ET JURA)<br /><br />Application pour l'interprétation des données polliniques fossiles

The reliability of pollen analysis as a tool for the reconstruction of past vegetation and landscape depends on our knowledge of modern ecosystems. These reconstructions involve precise understanding of modern plant communities and how they are reflected in pollen assemblages.The calibration between present pollen deposition and vegetation has been studied with two approaches in pastoral areas of middle range mountains. In Pays Basque, the comparative approach differentiated well dispersed pollen taxa and a pool of pollen which indicates local grazing activities. These results were used to reconstruct local pastoral activity from fossil pollen records for the last 2000 years. Multiscale interpretation of pollen assemblages, use of multiproxy (pollen and non pollen palynomorphs) and complementary information from other fields (pastoral archaeology, history) give a better understanding of spatial dimensions and intensity of pastoral activities. In the Jura Mountains, Extended R-value model was used to estimate the relevant source area of pollen assemblages deposited in moss polsters (Radius of 300 m). Pollen productivity estimates were calculated for several taxa characteristics of woodland pasture landscape. This set of data is the first step for quantitative reconstruction of past landscape.These approaches aim to assess the structure and the composition of past landscape and to evaluate pastoral impact on past ecosystems. They will provide useful information for predictive models in various disciplines (climate modeling, nature conservancy...)La reconstitution de la flore et des paysages anciens, mais également celle des pratiques, à partir des spectres polliniques fossiles, dépendent de notre connaissance des écosystèmes actuels. Ces reconstitutions requièrent une meilleure compréhension des communautés végétales actuelles et de leur image pollinique.La calibration de la relation pluie pollinique actuelle / végétation a été étudiée selon deux approches dans des régions pastorales de moyenne montagne. Au Pays Basque, l'approche comparative a conduit à la discrimination des taxons transportés sur de longues distances et d'un ensemble d'indicateurs polliniques directement liés au pastoralisme. Dès lors, l'application de ces résultats aux données polliniques fossiles a permis de reconstruire à l'échelle locale les activités pastorales pour les 2000 dernières années. La lecture multi scalaire des données polliniques, l'utilisation combinée de plusieurs proxy (grain de pollen et microfossiles non polliniques) et la confrontation avec d'autres disciplines (archéologie pastorale, histoire) fournissent une meilleure lecture du fonctionnement des systèmes pastoraux, de leur dynamique et de leur signification. Parallèlement, le modèle Extended R-value a permis d'identifier la végétation à l'origine des grains de pollen conservés dans les mousses du paysage jurassien (rayon de 300m). Ainsi, les productions polliniques ont été estimées pour les taxons caractéristiques de ce paysage et constituent l'étape préliminaire pour simuler et reconstituer quantitativement les paysages anciens. Ces approches apportent désormais une lecture plus concrète de la structure et de la composition des paysages passés et de l'impact des pratiques pastorales sur l'environnement passé. Elles constituent des outils potentiels pour des modèles prédictifs (climat, écologie...

Estimating the Relevant Source Area of Pollen in the past cultural landscapes of southern Sweden -- A forward modelling approach

International audienceMiddle and Late Holocene in southern Sweden, in order to explore the possible effects of past changes in vegetation composition, openness and structure in terms of patch size and spatial distribution. The RSAP of small basins (bogs or lakes) in the past has to be estimated if quantitative reconstruction of past vegetation at the local spatial scale is to be achieved using Sugita's Landscape Reconstruction Algorithm (LRA). In this study we apply a forward modelling approach to estimate past RSAP using the computer simulation model HUMPOL. The landscape designs are based on past landscape maps produced using a combination of palaeobotanical, archaeological and historical data, and the area's geology and soil characteristics. Four time windows characterised by different landscape/land-use were selected, i.e. Early Neolithic, Late Bronze Age, Viking Age, and Middle Ages. We found that RSAP estimates for hypothetical past landscapes in Skåne differ by ca. 600 m to 1200 m between the selected time periods, whatever the size of the basin (lake or bog, 25- 250 m radius). The most probable explanation for the differences in RSAP between time slices is variable patch size and spatial distribution of patches in the landscape. The RSAPs vary between ca. 1200 and 2300 m for small basins (25 m and 70 m radius), and between ca. 2000 and 3000 m for larger basins (250 m radius). These values are within the range of earlier estimates of modern and past RSAPs for southern Scandinavia obtained using simulated or empirical data. These results suggest that, given the type of setting of that region in terms of taxa composition and traditional land-use, the RSAP for small-size lakes (25-250 m radius) will generally be in the range ca. 1200-3000 m. The forward modelling approach is found to be useful to assess the possible effects on RSAP of changes in vegetation/landscape characteristics between different periods of the past. Moreover, comparison of RSAP estimates obtained using both the forward and backward modelling approaches will be important to identify the most credible RSAP estimates for the past

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

A history of long-term human-environment interactions in the French Pyrenees inferred from the pollen data

International audienceOver the last decade, several research programs have been involved in studying the socioecological history of the Pyrenean Mountains using sedimentary records preserved in lakes and bogs. Their main focus was on understanding human exploitation of natural resources and its environmental consequences. Recovering these “memories” buried for thousands of years in sediments requires interdisciplinary efforts dealing with the analysis of a large number of bio-indicators. The study of those bio-indicators has become a multi-proxy process which combines the classicalstudy of fossil pollen and spores with macro-charcoal (size >150 m m) and nonpollen palynomorphs (algae, fungal spores, etc.) data

Palynologie et histoire des activités humaines en milieu montagnard. Bilan provisoire des recherches et nouvelles orientations méthodologiques sur le versant nord des Pyrénées

International audienceDans l'étude des relations environnement / sociétés en milieu montagnard, la palynologie se positionne aujourd'hui comme une discipline incontournable offrant la possibilité d'aborder la longue durée des processus d'anthropisation. Un examen critique de l'évolution de cette discipline par rapport à la problématique de l'anthropisation peut être mené à partir de l'exemple pyrénéen. S'il ressort que les acquis sont nombreux, ils ne s'accordent pas toutefois systématiquement aux nouvelles demandes formulées par un contexte scientifique résolument orienté vers l'interdisciplinarité. La variabilité des échelles d'analyses temporelles et spatiales constitue un écueil important, mais celui-ci semble pouvoir être contourné par le développement de nouvelles approches paléoécologiques reposant sur des analyses intégrées ainsi que sur la mise en place de référentiels fonctionnels. Nous présentons ici brièvement les résultats de quelques tentatives menées sur le territoire pyrénéen dans le cadre de différents programmes de recherches

Grazing activities and biodiversity history in the Pyrénées - new insights on high altitude ecosystems in the framework of a Human-Environment Observatory

International audienceReconstruction of the relationship between pastoral activities and vegetation history in the central Pyrenees demonstrates the importance of grazing pressure in the maintenance of floristic diversity in highland regions that have been abandone

Signals of tree volume and temperature in a high-resolution record of pollen accumulation rates in northern Finland

International audiencePollen accumulation rates (PARs) provide a potential proxy for quantitative tree volume (m3 ha 1) reconstruction with reliable absolute pollen productivity estimates (APPEs). We obtained APPEs for pine, spruce and birch at their range limits in northern Finland under two temperature periods ('warm' and 'cold') based on long-term pollen trap and tree volume records within a 14-km radius of each trap. APPEs (mean SE; 108 grains m 3 a 1) tend to be higher for the 'warm' periods (pine 123.8 24.4, birch 528.0 398.4, spruce 434.3 113.7) compared with the 'cold' periods (pine 95.5 37.3, birch 317.3 282.6, spruce 119.6 37.6), although the difference is only significant for spruce. Using an independent temperature record and the APPEs obtained, we reconstruct a low-frequency record of pine volume changes over the last 1000 years at Palomaa mire, where a high-resolution record of Pinus PARs is available. Five phases are distinguished in the reconstruction: moderate pine volume, AD 1080-1170; high volume, AD 1170-1340; low volume, AD 1340-1630; very low volume, AD 1630-1810; and rising pine volume, AD 1810- 1950. These phases do not coincide with periods of high or low June-July-August temperatures, and thus appear to reflect regional variations in tree volume, while high-frequency changes within each time-period block show variations in PARs in response to temperatur

Floristic diversity in the transition from traditional to modern land-use in southern Sweden A.D. 1800-2008

International audienceWe aim to provide a long-term ecological analysis of land-use and floristic diversity in the transition from traditional to modern land-use management in the time A.D. 1800-2008 in southern Sweden. We use the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model to quantify land-cover changes on a regional scale at 20-year intervals, based on the fossil pollen record. Floristic richness and evenness are estimated using palynological richness and the Shannon index applied to the REVEALS output, respectively. We identified a transition period of 60 years between 1880 and 1940 when the total tree cover increased and the tree composition changed from deciduous to coniferous dominance. Within the shrinking area of open land, arable land taxa expanded, while the number and coverage of herbs in the remaining grasslands decreased. The succession from open grasslands to more tree-covered habitats initially favoured palynological richness, which reached its highest values during the first 40 years of the transition period. The highest REVEALS-based evenness was recorded in the time of traditional land-use and at the beginning of the transition period, reflecting higher habitat diversity at these time intervals. Our results support a more dynamic ecosystem management that changes between traditional land-use and phases of succession (\40 years) to promote floristic diversity. We have developed and applied a palaeoecological methodology that contributes realistic estimates to be used in ecosystem management

Effects of land use and climate change on erosion intensity and sediment geochemistry at Lake Lehmilampi, Finland

International audienceThis paper aims to evaluate the possible relationships between erosion intensity and changes in climate and land use during the past 5.5 cal. k years at Lake Lehmilampi, eastern Finland. In this study we compare a detailed geochemical sediment record with (1) forest and land use history inferred from the first pollen and charcoal records from Lake Lehmilampi, and (2) existing archaeological surveys and independent proxy-records of climate change in the study region. The physical and geochemical sediment parameters examined include grain size analysis data and 23 chemical elements, determined with four selective extractions and ICP-MS. There are indications of possible human impact in the lake catchment as early as the Neolithic period, c. 3000-2550 bc, but the first undisputable signs are dated to 1800-100 bc. Cereal pollen reappears at c. ad 1700 and increases rapidly until c. ad 1950. The Holocene Thermal Maximum, its end c. 2000 bc, and the 'Medieval Climate Anomaly' were major climate events that had a prominent effect on erosion intensity, while human impact was a more significant factor during the period 3000 bc-ad 800 and from ad 1500 onwards. Although signs of changes in erosion intensity found in the sediment were small in this small catchment, they were significant enough to have a clear impact on the fraction of potentially mobile element species. This fraction increases with decreasing erosion intensity, which is probably related to a higher degree of chemical weathering and leaching during periods of decreased erosio