Question

Why are forests affect by war ?

Answer 1

In India forest department cut trees freely to meet British war needs.
In Java just before the Japanese occupied the region, the Dutch followed Scorched Earth Policy, destroying sawmills and burning huge piles of gaint teak logs so that they would not fall into Japanese hands.
The Japanese then exploited the forests recklessly for their own war industries, forcing forest villagers to cut down forests.
Many villagers used this opportunity to expand cultivation in the forest. After the war, it was difficult for the Indonesian forest service to get this land back.
In India people's need for agricultural land has brought them into conflict with the forest department's desire to control the land and exclude people from it.

Answer 2

Forests are un middle between the defence and the offence in a war thus will be used as a source of supply source of concealment (thinned) or place to ambush. A targeted ambush area is thus subject to massive defoliants from the air etc. Either way you look at it - Forests are collateral damage of war.