[Every plant species contains several growth/shape variations]

Alpenrose
Alpine Aster
Alpine Cyclamen
Alpine Hawkweed
Alpine Lady's-Mantle
Alpine Sow-Thristle
Black Yarrow
Brown Knapweed
Common Agrimony
Common Cottongrass
Creeping Pine
Curly Dock
European Ash
Fairy Foxglove
Fireweed
Foxglove
Great Masterwort
Hogweed
Martagon Lily
Monk's Hood
Perennial Cornflower
Rose Daphne
Sessile Oak
Spiked Rampion
White Asphodel
Wig Knapweed
Wolf's Bane

Photos: Chunky, GIMP, Photoshop
Plants: frogocomics

Answer: This is a common problem users had. Basically after exporting, WorldPainter fixes blocks that "don't belong", such as floating sand, etc. However, it also removes grass and plants that are floating, thus ruining the trees. A fix is to go to the WorldPainter advanced settings. It is usually located at C:\Program Files\WorldPainter\worldpainter.vmoptions. Run Notepad WITH ADMINISTRATOR PERMISSIONS and open the file. If you can't find it, make sure the Open File dialog is set to All Files.

Add the following line:
-Dorg.pepsoft.worldpainter.enforceBlockRules=false

WorldPainter should not apply any fixups now. But the problem is that once you go into the world, grass on trees will start dropping. This can be fixed through NBT. Use your favourite NBT editor and go to a random world. (Any World!) Go to level.dat of that world, and navigate to Data\GameRules. Copy the GameRules package. Now open the level.dat of the world containing CubeLeaf trees. Copy the gamerules to the level.dat. Now go into the GameRules package and set "randomTickSpeed" to 0.

Although seemingly complex, it'll get easier after a few tries.