There are tons and tons of movies that have random big-breasted girls running hopelessly through the woods to their eventual death, so I haven't included them, per se.
So here's a list (because I love lists, as you know) of "famous" horror trees:

The Blair Witch Project...
Lots and lots of trees. And freaky little twig ornaments hanging around everywhere to wig our doomed trio out quite properly.

The Village...
Painting the trees near the perimeters of the woods where "Those we don't speak of" apparently roam. Lotta trees in this one, too.

Wrong Turn...
Two shots from the scenes where they were trying to hide from the mutant cannibalistic backwoods inbreds.

This is the scene right before she gets her head cut off. Nasty.

The Ring...
Just a bit of trivia: The tree with the fiery red leaves featured in the movie is a Japanese maple. The fruit of this tree is known as a "samara".

Also from The Ring...
The tree painted on the wall in Samara's hayloft bedroom.

Sleepy Hollow...
Now this is one creepy-ass tree. And a major part of the actual movie since the Headless Horseman literally leaps from within the tree on horseback. It was called, appropriately enough, The Tree of the Dead.

The horseman, sans head, in the grave under the tree roots.

Scream...
Nice imagery here. Poor Casey. But nice use of the tree!

Poltergeist...
One of my favorite trees in horror. I will never forget the first time I saw this film and the tree branch came smashing through the window. I didn't sleep for a week because of the tree in my own backyard scraping on the window.

Rob Zombie's Halloween...
Probably the most brutal scene in the film is the one where this poor kid is killed by young Michael with a huge tree branch. This is the end result.
Never bully Michael Myers, 'kay?

The Guardian...
Of all the movies that include creepy trees, this flick with its Druid-loving/worshiping nanny is the epitome of tree usage in horror. Bleeding trees & baby sacrifice.
Talk about tree hugging.

Also from The Guardian. Just like The Tree of the Dead in Sleepy Hollow, this tree bleeds when you cut it. Nice.

Final Destination 2...
Whether or not you actually like the FD film series, you have to admit they have some creative ways to kill people. Here's a nice way to get offed - logs through the windshield. Yikes.

The Evil Dead...
If I have to explain this picture then you certainly are no horror fan. Enough said.

Lord of the Rings: The Two Towers...
Not classified as horror, but truthfully those Ents were really ominous at times. So in this pic, the Ents were highly pissed. And pissed off tree-people that actually talk...that's just unnerving.
(The Battle Of Isengard, BTW.)
Looks like the wizard (Winter) from that Christmas special of long ago. Except Treebeard is green of course.
Like I said, talking trees are disturbing.

The Wizard of Oz...
When I was a kid, this freaking tree scared the shit out of me, even more than the Wicked Witch of the West. He was bitchy as hell - and he threw apples!
Gets my vote for the meanest damn tree in film.
* Sensing us, the trees tremble in their sleep,
The living leaves recoil before our fires,
Baring to us war-charred and broken branches,
And seeing theirs, we for our own destruction weep.
- Kathleen Raine, London Trees