What are UK native trees and should we favour these over ornamental or exotic tree species? The term native trees refers to those species that colonised the UK at the end of the most recent ice age - i.e. more than 10,000 years ago. But there are also trees that we term 'naturalised' species, which moved into the British Isles after that time. The topic of native trees is an interesting one as it is now important not only to safeguard our existing native trees but also to widen the number of tree species in order to extend the range available to wildlife and safeguard tree cover in the future.