Please note that the tree and bush defense boni only apply only to Spearfighters, Soldiers, Mercenaries, Axe Warriors and Great Sword Warriors. All other units don't have that bonus. Also, there is some randomness involved (luck), when on the battlefield and you hover the mouse over the enemy troops, you'll see some attack values are listed as for example 2-4. I did notice though that when hiding my spearfighters behind a tree (+3 defensive bonus), they do live longer than in the open field.
So unless there is confirmation from more players saying the boni don't work, I'll set this as "Not a bug" for now, no offense please!