A Wood Elf army can always place an additional forest (use a Citadel Wood) on the battlefield. This is done during deployment, before any units are deployed, and must be placed wholly in your half of the battlefield. This forest is not mysterious terrain - declare its type when you place it. If you cannot fit the forest on your half of the table, move other terrain features by the shortest distance necessary so that the forest can be placed. If you still cannot place the forest, or if the scenario means that the Wood Elf army does not have a table half in which to deploy, this additional forest is not used in this battle.