If you place a helo over a forest, the “change altitude” button will be grayed out. Makes sense – if the helo were to fly low, the trees would get in the way.
But you can still do it! The button is not grayed out when flying TO a forest, and you can queue an altitude change order for a helo sent to a forest. The helo will dutifully descend in the middle of the trees. The helo will be able to fight infantry depending on the minimum range of its weapons, and it will also be shielded from AA unless approached from a direction where the forest is thin.
Positioning is key to getting any advantage out of this niche gimmick. A cannon helo surrounded by forests on all sides but one, overlooking a field shorter than 1.5km, can be fairly difficult to destroy as any AA that can target it can also be shot at in return. Supercobras, back when they had anti-plane missiles, could be “landed” in deep forests so that they are hidden from sight and immune to both AA and tank shots while still capable of shooting at passing planes.
If you order a helo to land from this position, it will “slide” in a direction until it leaves the forest, and land there. This is most useful for exiting deep forest, as a normal move will cause your helo to lift and fly over the forest, potentially exposing it to AA.
As with many tricks here, all credit for this one goes to nande.