Mature Tree

Even seemingly non-trees, such as this cactus, count as Mature Trees

Mature Tree

Botanists can use hatchets and other cutting tools to log the forests of Eorzea and gather materials for use in crafting. Trees that are prime for logging are know as mature trees.

Finding Mature Trees

Mature trees are only discernible from other foliage when using the action   Triangulate. While this action is active, the location of mature trees is displayed on the minimap.
Once you have arrived at a mature tree, right-click it to begin logging.