Central to the green locations is the Tangle, an enormous, sprawling "forest" made of copper structures. The greenish tint the Tangle bears comes from verdigris (no, not the card Verdigris; it's the crust of copper sulfate or copper chloride that forms on copper after being exposed to air for long periods). The Tangle's inhabitants call this "the Molder" (hence cards like Molder Slug).
Below: Here's an early concept illustration by Vance Krovacs of the upper canopy of the Tangle. Note how metallic the tree-analog structures are here. Note also the scale of the piece: the small figure on the extreme left is human-sized (probably an elf).
Deep within the Tangle is a huge tree-structure that the Viridian Shaman call Tel-Jilad, the Tree of Tales. The surface of Tel-Jilad is carved with runes Tel-Jilad Stylus there by the Trolls of Tel-Jilad over hundreds of years.
Below: This is a later illustration of the Tangle by Mark Tedin. Note again the scale here - the cables connecting this stucture to other "trees" are basically rope bridges used by Viridian elves.