Sophie Leguédois

Jean-Paul Party

Jean-Luc Dupouey

Thierry Gauquelin

Jean-Claude Gégout

Caroline Lecareux

Vincent Badeau

Simon Rizetto

Anne Probst

The vegetation map of France going numerical: a new harmonised national geographical database

Keywords: Numerical mapping, Geographical Information System, digitalisation, harmonisation, scan, vegetation series, vector, potential vegetation

In this paper we present the digitalisation of the map of the vegetation of France edited in 64 sheets by the CNRS between 1947 and 1991. The geographical covers and the databases built during this work are gathered in a geographical database called "Base de Données Géographique de la VÉGétation de la France" (BDGveg_FR). The main covers show respectively the vegetation succession stages, as a georeferenced scan, at 1/200,000, and the harmonised map of the potential vegetation, in a vector format, at 1/1,000,000. The harmonised map of the potential vegetation is linked with a national 6-level typology synthesised from the keys of the 64 sheets. The BDGveg_FR is unique because of its period of time, its local and national scales, its exhaustive cover, and its information on the plant associations. Thus, it is actually complementary to the other databases on the French vegetation presently available. It is particularly well appropriate to assess the impact of global changes (e.g. climate, atmospheric pollution) on ecosystem behaviour.

Introduction

Between 1947 and 1990, the "Centre National de la Recherche Scientifique" (CNRS) published 64 paper sheets which form a vegetation map of the metropolitan French territory. This vegetation map draws two kinds of information: the succession stages at the date of the survey and the natural potential vegetation, i.e. the vegetation that should develop considering the partly irreversible environmental changes (see Ozenda, 1986;[START_REF] Härdtle | On the theoretical concept of the potential natural vegetation and proposals for an up-to-date modification[END_REF]. This paper map is a unique source of information for the French territory with its detailed scale, the period of time it concerns, its exhaustive cover, and the vegetation data it provides [START_REF] Rey | A history of the vegetation mapping in France[END_REF]. In this note we present the digitalisation of the vegetation map of the CNRS, the geographical covers, and the databases produced. A detailed description of the mapping process, the maps produced as well as several applications is available in [START_REF] Leguédois | The vegetation map of the CNRS going numerical: the geographical database of the vegetation of France[END_REF].

Mapping sources

Each sheet of the vegetation map of the CNRS includes:  the map sensu stricto at 1/200,000 which gives the succession stages at the time of the survey;  a series of seven insets at 1/1,250,000 characterising the agricultural and ecological conditions of the mapped territory and, among them, the botanical inset which represents the natural potential vegetation;  a detailed key specific to the sheet.

The global coherence of the vegetation map of the CNRS has been ensured [START_REF] Gauquelin | A history of the service for the vegetation mapping of France[END_REF] by: (i) a relatively short surveying period (80 % of the 64 sheets were edited between 1963 et 1985), (ii) the small number of main authors (19 authors are responsible for 94 % of the sheets), (iii) the centralised management (the " Service de la Carte de Végétation" created in 1945 in Toulouse) and, (iv) a common mapping method even though the mapping rationales have been adapted with the authors and the periods. The cornerstone of this classification is the succession which actually reflects the different stages leading to the climax vegetation. However some mapped vegetation series represent only one single stage of the succession like pine forests or some oak forests in lowland. Moreover, depending on the authors, the mapped natural potential vegetation can reflect either the site natural conditions only, or the site natural conditions plus the durable man-made changes (i.e. pollution, soil erosion, drainage, species naturalisation like Maritime Pine in the Landes). Despite these mapping inconsistencies inherent to each classification system, the data provided by the vegetation map of the CNRS is a unique, reliable, detailed, rare , and exhaustive source of information for the considered period.

Digitalisation and harmonisation

A set of digitalisation and harmonisation procedures have been followed to produce the different items constituting the BDGveg_FR.

The map sensu stricto as well as the insets (except the relief inset) of the 64 sheets have been scanned at high resolution (600 dpi) and then patched as well as georeferenced to produce seven continuous raster covers for the whole metropolitan French territory. The georeferenced scan at 1/200,000 issued from the map sensu stricto, is the succession stage cover which reproduces the original paper map. The six others georeferenced scans are secondary covers which represent the 1/1,250,000 insets of the paper map: botanical data, soil conditions, soil use, agriculture, climate, and agricultural restrictions.

The harmonised vector cover of the natural potential vegetation at 1/1,000,000 is based on the 1/1,250,000 botanical insets of each of the 64 sheets of the paper map of the CNRS. Because of the inconsistencies of classification between the different sheets (see previous section) a harmonisation work was needed. This harmonisation has been done in two steps: firstly the synthesis of a national typology based on the legends of the 64 maps at 1/200,000 and, secondly, the digitalisation of the botanical insets. The general idea of the synthesis of the typology was to homogenise, as efficiently as possible at a national level, the information of the 64 sheets while trying to stay closer to the original legends. It resulted in a national nested 6-level typology of the vegetation of France. The digitalisation of the natural potential vegetation cover results from the vectorisation, the patching, and the georeferencing of the 64 botanical insets. The precision of the georeferencing (from 100 to 800 m depending on the considered region) agrees with a use at 1/1,000,000. Each polygon of the natural potential vegetation cover has been assigned to one type of vegetation as classified in the 5 th level of the typology. The chosen type of vegetation and the mapping limits have been checked against the original paper maps of the CNRS and others vegetation maps at the local scale (more particularly [START_REF] Ozenda | The vegetation successions of the Alpine area and their comparisons with other phytogeographical systems[END_REF][START_REF] Ozenda | Sketch of a map of the potential vegetation of France at 1/1,500,000[END_REF].

Results and discussion

All the digital information produced during this work has been gathered in a geographical database, called "Base de Données de la VÉGétation de la France" (BDGveg_FR). The BDGveg_FR contains:

 a 1/200,000 georeferenced scan of the succession stages which is a testimony of the vegetation of the years 1940-1990;  a harmonised 1/1,000,000 vector cover of the potential vegetation (Fig. 1) with descriptive data for each mapping unit; this cover has been checked and verified by comparison with other maps (see previous section) and the floristic heterogeneity of each mapping unit has been quantified;  a database of the 6-level typology (Fig. 2) of the French vegetation which can be joined to the harmonised potential vegetation cover at the 5 th level (Vegetation Mapping Units or VMU);  six secondary covers as georeferenced scans which correspond to the insets of the paper map (soil, land cover, agriculture, climate, local potential vegetation) as well as 10 vector covers which regroup data extracted from the botanical inset (main secondary tree species, distribution area of some Mediterranean species, and drainage level);  a database with the main metadata for each paper sheet (authors, collaborators, publication date, number, name, extent).

The vegetation information of the BDGveg_FR is more synthetic than the distribution data of species gathered in floristic geographical databases [START_REF] Brisse | European vegetation survey. La banque de données phytosociologiques SOPHY[END_REF][START_REF] Drapier | The ecological database of the French national forest survey (IFN) [In French][END_REF][START_REF] Gégout | EcoPlant: A forest site database linking floristic data with soil and climate variables[END_REF].

Conclusion and perspectives

The BDGveg_FR is a novel geographical database on the French vegetation which fills a gap in the available databases by the period of time its covers , its local and national scales (1/200,000 and 1/1,000,000), its exhaustive cover, and the information it provides on the plant associations. The BDGveg_FR could be used to analyse long-term and large-scale impacts of global changes like climate, atmospheric pollution or tree species substitution as well to design sampling schemes. An equivalence between the EUNIS European classification and the VMUs has already been established. It enables assessment of the impacts of long range atmospheric pollution at the European level [START_REF] Probst | National Focal Centre report: France. In Modelling and Mapping of Atmospherically-induced Ecosystem Impacts in Europe[END_REF]. On an applied point of view, this database could be used to characterise the biogeographical and ecological situation of naturalist studies or environmental impact assessments as well as to help vegetation mapping by remote sensing. For example, current stages of vegetation could be obtained by combining the harmonised map of potential vegetation with the Corine Land Cover database 1 . It is also an attractive teaching material. [START_REF] Leguédois | The vegetation map of the CNRS going numerical: the geographical database of the vegetation of France[END_REF]. The detailed legend is shown in Fig. 2.

Fig. 2: Detailed legend of the harmonised map of potential vegetation of France [START_REF] Leguédois | The vegetation map of the CNRS going numerical: the geographical database of the vegetation of France[END_REF]. Only VMUs depicted on the map (Fig. 1) are presented here.

1Fig. 1 :

 1 Fig. 1: Harmonised map of the French natural potential vegetation[START_REF] Leguédois | The vegetation map of the CNRS going numerical: the geographical database of the vegetation of France[END_REF]. The detailed legend is shown in Fig.2.

Acknowledgements

The authors are grateful to Ed Rowe who kindly and skilfully check the English translation of the detailed legend.

Abbreviations

BDGveg_FR = Base de Données Géographique de la VÉGétation de la France (geographical database of the vegetation of France) CNRS = Centre National de la Recherche Scientifique (French national centre for scientific research) VMU = Vegetation Mapping Units