Not literally turn them off, of course. When camera is close to the forest, leaves and branches automatically become transparent in a small area where the camera is looking.
The problem is, that area is indeed very small and often can not encompass your entire army. As soon as the battle starts, confusion starts as you many units (yours and enemies) and engagements are hard to see from the trees. You constantly have to move camera around and lose precious time because you have to zoom in/zoom out and try to find perfect angle to see what's happening.
And then you have to move to another are of the map to do the same thing. It's not a problem on maps where there are just patches of trees, but on heavily wooded maps, things can become chaotic very easy. It's such a problem that I avoid heavily wooded areas even with Wood Elves. In Single Player mode, it's not a huge problem because you can always pause, but in multiplayer it's a huge issue.
So, I would propose a solution, a simple ON/OFF toggle, which, if turned on, makes all leaves, branches etc... on the entire map transparent, and not just in the small area camera is looking at.
Sure, you could go to tactical map, but unit cards are not visible there.
It would alleviate the confusion for the most part and make battles on heavily wooded maps more enjoyable.