The Llarian Scar is the "heart" of a Mechanation. It is a special stone found in the living trees of the Mechana Forest. The properties of this stone, which are almost certainly magical, and the awareness of a Mechanation's body allows them to control it. However, the stones have no sentient properties while they are still in the tree. Each tree only has one stone, and the tree dies when the stone is removed.
Mechana trees don't grow like normal trees, and they only grow in the soil in the forest, which is magically charged by some source. If you take a Mechana tree out of the forest and try to plant it elsewhere, it will just die. Theoretically, you could grow one in potted soil of the forest, but eventually the soil would become unsuited to hold the tree and would just be like normal dirt.