It's an alpha-texture super-sampling issue, see https://en.wikipedia.org/wiki/Alpha_to_coverage for what Skyrim does NOT, and here for what Skyrim does: https://www.polycount.com/forum/archive/index.php/t-69528.html (same as they did in Oblivion). Look how the tree-textures are hand-tuned by the Bethesda artists: Above is normal filtering, below is original special filtering. Except hand-managing foilage-textures and leave it to everyones taste, it's difficult to identify "foilage" automatically and apply a different alpha-channel filtering. I'm open to suggestions how to identify foilage-textures though.
