Digitized maps of vascular plant species distributions in Denmark in the early 20th century

<b>Historic data are essential for quantifying changes in biodiversity and drivers thereof. A large part of such legacy data are stored undigitized in archives at e.g., natural history museums, and make up significant untapped resources for insights into past biodiversity patterns. In 1904, the Danish Botanical Society initiated the first systematic survey of species distribution within Denmark of the entire Danish flora. The largest part of the field surveys was conducted during the early decades of the 20th century. Results were published as black and white printed maps during the years 1931-1976, using dots for accurate records and hatchings for expert-assessed local abundance. We used automated georeferencing and an image classification algorithm to extract the distribution data from all published maps. Here, we present 1340 maps of historic distribution patterns of vascular plant species and subspecies in Denmark at a 10 x 10 km resolution. These maps represent the Danish flora of native and archaeophyte species known at the time, except a few of the most common native species that were not mapped. These digital maps can be used to assess historic patterns of species distributions and occupancy. By comparing with modern distribution data, species increases and decreases can be determined, contributing to large-scale synthesis of national, European or worldwide plant diversity changes in the modern era.</b><br><b>Data description</b><b>Data consists of 1340 shapefile maps collected in the zipfile "shp_maps".</b><b>Each map consists of 4 columns:</b><b>list_name refers to the species name</b><b>category (0 for "absence", 1 for "present", 2 for expert-estimated abundance "here and there", 3 for "absent, but has been found there in the past").</b><b>idgrid that links each observation to a standard grid map, "gridshpwgs".</b><b>geometry</b><b>Georeferenced image maps are in the folder "tiff_maps".</b><b>specieslist_info.csv includes overview of species names, shapefile names, image map names, taxonomy synonyms and comparisons to World Flora Online, Dansk Feltflora (1981). It also includes information of nativeness status and TBU publication.</b>translate.csv includes an overview of species names used in the modern systematic survey in Denmark, Atlas Flora Danica, with a translate column for the TBU mapped species - tbu_translatedname - which is also found in the specieslist.csv. Thus, this column should be used to link TBU taxonomy with the taxonomy used in Atlas Flora Danica (AFD).The file TBU_HansenDanskFeltflora_alignment includes explanations and alignment for the species mentioned in Hansen et al 1981 of the Danish Flora and the TBU mapped or text treated species.