You can build a lot of things from Lego, the most successful builds in my opinion are builds that present the real world in a way that mimics it well.
When building a forest we will want to produce a lot of rounded and asymmetrical lines so that we can create a look of a real forest, I found for you many works that show a variety of interesting techniques on how to create the asymmetry that we will need for the forest.
You will also see in the works a variety of trees and rocks in different shapes and shades that each is built from different parts that create a look of a real forest.

Feel free to see the amazing works of designers from around the world that I found for you:


The Magic Swan Geese A walk through the woods Mountainside Meadow (Main) The Chase! FG Full 1 Maison d'Assurancetourix My Tropical Paradise LEGO Landscape CCC XIV - Forest Camp Faerie Forest (Aerial) Waterfall Left Spring2 (LoR) Northward Bound. CCCXII A Hunters Fire CCCXII Robin of Locksleys Camp JRC3, The hideout at Finder's Falls. (Main) My Tropical Paradise Fall is coming! Swebrick Masterbuilder (5 of 5) Axe Tree Tutorial (48 of 48) Kowe'on Sanctuary Endor