Funny coincidence, a couple of months ago I finished putting together a visual reference of "exotic" trees using pictures found online. Someday I'll use it to show players the strange and fantastical forests they are walking through.
Which brings up one of the issues I have with outdoor scenics. There are so many different kinds of trees, how do you you pick which ones to make, and where do you stop? Even when you settle on one, how many variations do you need to avoid oddly homogenous forests? And how many thematic versions (normal, dead, fallen, lightning-struck, snowy, spooky) do you make of the same tree?
Another issue is playability. Scale height trees are an obstacle to seeing the playing surface and to reaching and handling miniatures, especially with full foliage.
From a gaming perspective, my preference would be for freestanding tree trunks no more than two inches tall. I'll use flight stands for characters or monsters who climb.
If diorama-level trees are a must, perhaps one solution would be "foliage tops" that could be easily added to and removed from the trunks. Sold separately, multiple types of foliage in various basic shapes could go on the same trunks.
Favorite Pieces: Chasms, 45 Degree Passages, River & Lava Banks, Ledges, Large Curved Walls, Elevation Arch