arbor

Arbor, arboris, third declension, and feminine. That is the point worth remembering, because the -or ending elsewhere marks masculine agent nouns like auctor, victor and doctor, so the eye expects a masculine and gets it wrong. It is arbor alta, never arbor altus.

Latin treats trees as feminine as a class, which is why pinus, quercus and fagus are all feminine even when their endings look second-declension masculine. An older nominative arbos turns up in poetry and in Lucretius. English took arbour, arboreal and arboretum from it, and the word survives whole in Spanish arbol and, with the b softened away, in French arbre.