If you have Heroes of Horror there's a table of 100 "that ain't right" things to include that are generally creepy and easily can fall into forest settings. Other things that I like:

--random dilapidated shrine and thousands/millions of (harmless) tiny spiders crawling around the area. Lots of webs in the branches and a few husks of small birds and whatnot. (This actually happened to me on a countryside stroll in western Japan once, the ground appeared to be moving because it was crawling with spiders over EVERYTHING.)

--recently vacated campsite. The bedroll and shelter are still up but the fire is cooling embers and the campers are nowhere to be found.

--thick fog suddenly rises up and all sound is severely muffled, vision including dark vision or heat vision are basically worthless.

--compass (if present) starts spinning wildly. If the party presses on it returns to normal but if they stick around it continues to act strangely for a few minutes then settles firmly on "north". Can the group really trust it now, though?

--phantasm reenacts some scene, then disappears. It doesn't react to the PCs at all, plays out once, and doesn't happen again for at least 24 hours.

--dead silence. If you've ever been in natural forest when this happens, you know all you need to know about it.

--animal exodus. ALL animals running pell-mell definitely AWAY from something or TOWARD something.

--domesticated animals without owners. Not feral animals, just regular animals without their people. A house cat meowing for food; a small flock of sheep milling around; a few cattle just chilling in a small clearing; a friendly dog.