Conservation of threatened relict trees through living ex situ collections: lessons from the global survey of the genus Zelkova (Ulmaceae)

Maintaining living ex situ collections is one of the key conservation methods in botanic gardens worldwide. Despite of the existence of many other conservation approaches used nowadays, it offers for many endangered plants an important insurance policy for the future, especially for rare and threatened relict trees. The aim of this research was to investigate the global extent of living ex situ collections, to assess and discuss their viability and inform the development of conservation approaches that respond to latest global conservation challenges. We used as a model taxon the tree genus Zelkova (Ulmaceae). The genus includes six prominent Tertiary relict trees which survived the last glaciation in disjunct and isolated refugial regions. Our comprehensive worldwide survey shows that the majority of botanic institutions with Zelkova collections are in countries with a strong horticultural tradition and not in locations of their origin. More importantly, the acutely threatened Zelkova species are not the most represented in collections, and thus safeguarded through ex situ conservation. Less than 20% of the ex situ collections surveyed contain plant material of known wild provenance while the majority (90%) of collections are generally very small (1–10 trees). Botanic gardens and arboreta particularly in regions where iconic relict trees naturally occur should play a vital role in the conservation of these species. The coordination of conservation efforts between gardens has to be enhanced to prioritise action for the most threatened relict trees. Large scale genetic studies should be undertaken, ideally at genus level, in order to verify or clarify the provenance of ex situ collections of relict trees in cultivation. For the most threatened relict tree genera, well-coordinated specialist groups should be created.