Abstract Body

Since the settlement of the first cities, humans have selected trees to provide natural resources and beautify urban landscapes. Ancient and modern trade facilitated species movement, often far from native habitats and climates. At the same time, advances in horticultural technology enabled breeding species into new cultivars and varieties suitable for myriad uses.

After millennia of urbanization, with urban areas covering 2% of land mass yet hosting half of humanity, it is reasonable to ask: how many tree species have humans successfully retained and introduced into the world’s urban areas? how can we transform the world’s cities into biodiverse havens for trees?

We collated public tree species data from 473 urban areas in 73 countries, and 21 Koppen-Geiger climate zones. Tree species data was consolidated from 13 million occurrence records from tree inventories and the scientific literature, and complemented with data on their conservation status (IUCN) and invasion potential (GRIIS).

The global urban areas investigated contained a diverse tree flora (n=4,734 species) which represent about 8% of all known tree species, 29% of genera and 68% of families that contain trees. Extrapolation of observed species richness further suggests that an additional 3,738 tree species might be present in other urban areas, making up to 14% of the known global tree flora.

Around 8% of tree species recorded (n=372) are listed as invasive in at least one country, thus signaling their potential for biological invasion if spreading from cities to rural areas. On the other hand, conservation status data highlighted 179 species as “vulnerable” in the wild, 122 species “endangered”, 62 species “critically endangered”. Six tree species found in the world’s urban areas are thought to be “extinct in the wild”.

Cities are often consider foci of species extinctions, yet our analysis shows that the world’s cities can host a diverse tree flora. Whilst trade in ornamental plants has facilitated the spread of invasive species in the past, we also suggest that urban environments offer untapped opportunities to advance global biological conservation efforts for wild tree species and other plants in the future.