The majority of Gillies Hill has been classified as a Semi-natural Ancient Woodland by the Woodland Trust using aerial surveys and by a follow-up Phase 1 habitat survey conducted in 2010. To date 21 Scotland & 38 United Kingdom ancient woodland indicator species have been documented on the hill demonstrating that the woodland's understory and sections of its tree cover have been cloaked in woodland for centuries; the earliest mapped confirmation of trees on the site dates back to the 1580s Pont map.