Chorological data for the main European woody species
The data are organized as a set of ESRI shapefiles (*.shp, *.shx, *.dbf, *.prj files) mapping the distribution ranges of the main European tree and shrub species. For each species and in some cases subspecies, one or more shapefiles have been created containing: a) polygon features (name suffix “plg”), which define continuous areas of occupancy of the species range and b) point features (name suffix “pnt”), which identify more fragmented and isolated populations. For species with reported synanthropic occurrences outside the natural range, an additional point and/or polygon shapefile has also been created (suffix “syn”). Polygon borders delimiting species ranges are generalized across the mainland and sea boundaries. This offers the possibility to mask sea areas or to clip and extract the terrestrial range parts using GIS data layers of the users' choice. An additional version of polygon ranges are clipped with a coastline (name suffix “clip”), which have been derived from Natural Earth dataset "Admin 0 - Countries" 1:50M version 4.1.0 (https://www.naturalearthdata.com). Finally, an accompanying text document is included with the data, which provides more details on methodology and a list of all mapped species with related file names, taxonomical delimitation of the mapped species and references used to compile the respective chorological dataset.
Steps to reproduce
The chorological maps found in printed publications were digitalized with a photo scanner and converted into high resolution images. Then, all the map images collected from physical books, from e-books in PDF or DJVU formats, or from web pages were imported into GIS software and geo-referenced by identifying control points on a coordinate grid (when it was present) or on prominent geographical features of large rivers, coastlines or country boundaries. In order to avoid excessive image distortions, the geo-referencing process was performed in the original projection of the source map, or in a similar one in cases where the projection was unknown. Then, the existing point and polygon geodatabases were imported and intersected with the digitalized maps. The general base maps were compared and evaluated with the more updated and detailed ones at country level and with available point/polygon data. Additionally, a digital elevation model (DEM) was used as background to provide information on the orography of the mapped areas. Finally, by comparing, evaluating and synthesizing the information of all different sources, continuous areas of occupancy of the species were drawn as polygons. Single or small concentrations of occurrence locations separated from the main continuity of the species range were considered as isolated populations and digitalized as point features instead of polygons. For those plant species occurring also outside the native ranges, the distribution area was digitalized separately as introduced and naturalized range (synanthropic). Moreover, when detailed information was available, their ranges were mapped separately.