A targeted metacollection of Nordic gardens reveals horticultural endemism of threatened woody plants

There is huge potential for bringing together international networks of Botanic Gardens and Arboreta and pooling their resources for coordinated action to conserve threatened species. Realising this potential requires effectively managing complex, largely independently acquired data for tens of thousands of species and many more individual plants with different wild or cultivated origins. Here we describe the establishment of a metacollection of threatened, temperate woody plant species held across 22 collections in Denmark, Finland, Greenland, Latvia, Norway, and Sweden. We align taxonomic data to the World Flora Online to identify priority taxa and then compare accession-level data using Botanic Garden Conservation International’s PlantSearch database to assess the conservation value of collections across the network. Taxonomic alignment reduced the numbers of apparently unique taxa recorded in names databases across the metacollection by 70% and numbers of threatened taxa by 36%. Going from names records to living plants, the metacollection includes 433 threatened taxa (a further reduction of 64%) represented by 3192 accessions spread unevenly across collections and taxa, 38% of which are wild or cultivated from wild origin. A third of the threatened taxa across the network are represented by single accessions (at a single site: horticultural endemics), most by three or fewer. Inconsistencies in data both inflate and hide rare species in collections. Taxonomic alignment and accession-level comparison reveals an urgent need to increase resilience through targeted exchange of conservation grade material. Through open data and standards, this regional metacollection can feed seamlessly into global conservation programmes.