Here's a mod that makes your Skyrim forests all dense and lovely

pcgamer.com:

In videogames sometimes «forests» have like two or three dozen trees. That's too few trees. Or they have lots of trees, but the trees are really widely spaced out. Twenty or thirty feet apart. Obviously, all of these forests are nonsense. We won't even begin to address the forests where the trees are too dense to travel through except on the main path the developers want you to follow.

For all its strengths, Skyrim is a game without good forests. Too few trees, and they're too small. No more! Building on work done in earlier mods, Fabled Forests for Skyrim Special Edition gives you «tall, dense, FPS-friendly forests to get lost in.» 

Modder Kojilama offers up two versions: One is just nice, dense, lush woodlands with tall trees. Not so tall that you can't have sunlight through the trees, however. Fabled forests also tweaks the sizes of the trees a bit, randomly, so that there's diversity in how they show up.

The other, Fabled Forests of Mythic Proportions, uses huge trees akin to those giants that populate the west coast of North America, turning your woodlands into «a dark and inescapable labyrinth.» These are forest so dense, with trees so huge, that they're maze-like and dark at the roots. It transforms the area around Riverbend to be nearly unrecognizable.

What I think is specifically cool here is that the tweaks are from multiple other mods. The tree placements are drawn from other modders' work—Traverse the Ulvenwald and Nature of the Wild Lands. The tree models are from Happy Little Trees, but reworked to be taller and with deeper coloration.

The mod even has support for popular mod Seasons of Skyrim, which lets the seasons change in your now very rich forests.

You can find Fabled Forests on Nexusmods. Again, it's