The other problem is that non-native pests and diseases keep killing off trees. For example, the article mentions mountain ash as a species that's endangered. T

The other problem is that non-native pests and diseases keep killing off trees. For example, the article mentions mountain ash as a species that's endangered. They're suffering from a nasty fungus that came here from Asia - the Asian species of ash have evolved to defend themselves from it, but the European ones have no defences and can't survive infection.