From Halopedia, the Halo wiki
Oak is a term used to refer to around four hundred separate species in the genus Quercus (Latin for "oak tree"), native to the northern hemisphere of Earth. The white oak is a notable member of the genus. Historically, they have been used in a variety of ways, from ship building to wine aging, and when the first human colonies were started, the first settlers saw fit to bring the oak tree to several new planets, quite possibly for its strength and resistance to attacks from insects and fungi. Oak trees were also transplanted by the Forerunners on Shield World 006.