The jungle gives defense bonus because it offers great camoflauge, but Evergreen Trees allow you basically hide behind them altogether. And that is the problem in a way, in trying to create a power for them - they already work as a "power" in offering line of sight blocking.
Maybe the way to go here is with a TIMBER power that could potentially crush (deal damage with a single attack die for the all the trees and 2 attack dice for the biggest one) all adjacent figures when it is destroyed.
