Using them as is for a game a pathed game can be difficult - unless you want units to wander between them.
However there are a few simple fixes:
1. Customize the trees, give them larger pathing maps.
2. Customize underbrush with pathing - like making a copy of the shrubs and giving them the pathing of a barrel to fill in gaps in the pathing.
Many of the features you can optimize via the Object Editor like not allowing trees to be placed in water, giving them the pathing of say lumber mill or even castle - then plopping in a huge circle on your map - what happens is that none will end up in your water, on your cliffs and they will be spaced wide apart. Go back to the editor and rest the pathing and the allowed placement. Now you can go back and place trees inside of the "woodland" and even put trees in the water.
Another issue is the bottom of the trees. They have ragged bottoms. I assume that Blizzard meant for that to look like tall grasses at the base.
There are two fixes. Either select the tree (or using Ctrl+Shift+~ select all of a type) lower them all via Ctrl Page Down. OR select and copy your trees - plop them down in an empty spot then use advanced Replace doodads. You can replace the selected ones with a dead version. Then take the stumps and place them where the trees are standing holding down Shift as you place them - or you can just remove their pathing altogether via the Object Editor. to place them.
If you remove the pathing of any doodad you can stack it exactly on top of another doodad with the exact same pathing. Go to the editor and reset the pathing the doodad will move to its center. Works rather well if you are stacking say trees with their dead versions, or bricks or walls or columns and you need them to precisely stack up. Even if you raise a doodad up and then return its pathing it will center into its pathing without "falling" down".
I personally do not like the size of most of the doodads. Most of the buildings when placed alongside a living units being say the Blood Mage are too short. The doorways for say the cathedral, or the buildings is way too small. So I have gone through and modified all of my doodads, building units and what not increasing their sizes to make them more realistically proportioned.
Along with that I have made "giant" versions of the trees (not the mushrooms) and "seedling" versions. Giant versions are nearly twice the size and have a larger pathing (Min Scale 2.5 Max Scale 3.5/4.00), seedlings are as tall as say The blood mage to about hip high. (Min Scale 0.10 Max Scale 0.80). I also have taller "boarder trees" (Min Scale 4 - 5.5 Max Scale 6 - 7.5) with 99,999.00 hit points - they can be harvested but take like forever to fall - those I use the pathing of say the Arcane Observatory - its a very large pathing and prevent both ground and air units - very useful along the edges of the map to "hide" the end of the world.
Tinting: One can get a lot more variety by tinting trees. Once can take say the Ashenvale Canopy tree and by reducing the red and the green get a bluish coloring. or make them darker so they blend in better with the landscape.
Yes there are tree models out there, say the ones found in the Ultimate Terraining Map - they are great for pretty pictures and as accents for landscapes in playable maps - however they are not harvestable (having no death animation).
