Just for the sake of everyone else. These trees are called Hedgetrees. They are located in the farthest reaches of the Twelveswood, near Gerolt, near Haughtpox Bloatbelly, etc. As a quest explains, these trees mark where the current hedge that protects Gridania is located. Basically like a magic wall that protects the forest. As some may recall in one of the Gridania GC quests, the Order of the Twin Adder are attempting to raise something called the Irmin Hedge (I maybe be wrong on this) that's a stronger version of the current hedge, but unfortunately are unable to. Anyway! These trees are explained to glow when people get near them.