system
Given the following post, respond as if you're commenting on Reddit:
human
[A Game of Thrones]The Iron Islands are described as being rocky. Where the hell are the Iron Born getting the wood to make all those ships!? Do they summon them from the Plane of Elemental Lumber or summon entire ships from the Demi-Plane of Elemental Ships? Where does the wood come from!?
gpt
There are (or at least were before Euron called for a new Iron Fleet) a lot of trees upon the Iron Islands. The Iron Islands are mostly *sea stacks*, rocky formations that form as bits of cliff by the sea are eroded away, creating arches that eventually erode away to leave rocky pillars that the Iron Islanders optimised for their own purposes. 

In addition to Iron Island native trees, the Ironborn have a habit of looting mainlanders, merchant ships and natural resources of the mainland. It's stated in Telltale's Game of Thrones that the Ironborn are one of the various factions *very* interested in Forrester Ironwood reserves, obviously to utilise for their own ships. 

So basically it's cutting down their own dwindling supply of trees, looting others, and contributing to deforestation of mainland Westeros.