The PCs find a map marked with a symbol that looks like an ornate X. Arcana checks reveal that it is the Vrksah, the tiefling symbol of the Crossed Tree. Nature checks reveal that wherever two living treetrunks cross is a weak place; here a gate may naturally form, connecting to a similar crossed-tree configuration somewhere else in the world. Thus, a farmer may go for a walk in his own orchard, and, before he realizes that he has left his property, find himself on an entirely different continent. No one knows the locations of more than a handful of crossed tree gates, except perhaps the druids.
Crossed trees can be used as a way for the party to collect fast-travel shortcuts to important parts of the world. They can be an alternative or supplement to 4e teleportation circles, perhaps one more suitable for primal parties. Less prosaically, they may also lead the party to haunted fey groves, shadow forests where trees with human faces scream and mock, or trapped apple orchards where adventurers enter but never return: or if they do, they come back sown with evil seeds.