Worldroots

The Worldroots are extensions of the Oak of Ages, found within the heart of Athel Loren. By the time of the Forgotten Season, they had spread across many lands, creating a web that the spirits of the wood could traverse to reach faraway places. So did the great forest first discover the summerlands of Avelorn, and encounter the Elves of Ulthuan long ago.

Source

 * Warhammer Armies: Wood Elves (8th Edition) -- pg. 16