The Frankincense Tree, or Boswellia Sacra, is the national tree of Oman. It is very valuable and has contributed to Oman's history largely, as it does not grow naturally in many other places. It is also a deciduous tree, and very fragrant when burnt. It is native to Oman, but also to Yemen. It is a tree that will grow in many environments, choosing to grow in rocky slopes and ravines where most trees cannot grow. Its average height is between 2 and 8 metres. On steep slopes, frankincense trees usually develop buttress roots, or plank roots, to stabilise the tree, since the roots are not made for this purpose. Frankincense trees have heavily adapted since they first appeared on earth 5,000 years ago.