This plugin replaces most of Ascadian Isles's (not only, these plants may be found in other regions too) plant meshes with smoother and high-poly versions, along with correct collisions. Included are also optimized vanilla meshes, which look like original, but have a lower polycount and correct collisions. These can be found in the directory "lowpoly".
The below files must be deleted as they contain errors (and conflict with UC: Nature):
Meshes/f/flora_tree_01.NIF Meshes/f/flora_tree_02.NIF Meshes/f/flora_tree_03.nif Meshes/f/flora_tree_04.NIF Meshes/f/flora_tree_ai_05.NIF Meshes/f/flora_tree_ai_06.NIF
If you aren't going for an HD look, the low poly meshes seem to have odd collision issues (you can't walk through grass, etc) so you may want to skip it.