General:
* Woodlands
* Forests
* Timber
* Arbor (a specific type of tree)
* Vegetation
* Flora
* Greenwood (poetic)
* Growth
Specific types of trees:
* Evergreens
* Deciduous trees
* Conifers
* Hardwoods
* Softwoods
Figurative language:
* Giants
* Sentinels
* Guardians
Informal:
* Bushes (though not technically trees)
* Plants
* Greens
* Wood
The best word to use depends on the specific situation and the tone you want to convey.