Proposed Canon
The Stupid Meal says that fliers cannot be attacked by non-fliers over "water, mountains or heavy trees", suggesting that there are more than one category of forest.Erf-b1-p39 Different forests have been termed "heavy" or "dense."
Other units may be able to enter forests, but suffer penalties to movement.
All non-forest units may suffer combat penalties in forests.
Forests may give a bonus to hiding.