Wild Heath

The Wild Heath is the landscape that forms the borderland of Athel Loren. It is a boundary ever in motion, with the trees forever straining against the waystone fence that pens them in, these standing stones planted by the first Elves of Athel Loren. Even then, they knew the power of the forest, and sought to contain its fury.

On occasion, when summer is at its highest, the trees can even overwhelm the waystones and spill into the lands beyond. When this happens, the Spellsingers and Spellweavers of Athel Loren quickly loose their magics in order to coax the trees back to their proper home. They do this not to save the Bretonnian peasantry of the surrounding lands - though countless lives are doubtless preserved by these actions. Rather, they do it to maintain the tenuous balance by which Athel Loren exists. Though the instinct of the trees is to spread their roots across the world, the Elves seek to preserve the unity from which the forest draws its strength. Were its trees and spirits permitted to roam the world beyond, many would perish. Worse, those that survived would do so only by becoming as cruel and bloodthirsty as the most terrible servants of the Chaos Gods. Thus do the Wood Elves preserve the forest from itself by first preserving outsiders from the forest.

Source

 * Warhammer Armies: Wood Elves (8th Edition) -- pg. 13