Animated opacity question

[Mikokariadis]

Been doing an exterior shot with lots of grass, trees and bushes. For the architectural content I am using scalpelmax in a sliced reveal.
But the trees/bushes are all vray proxys and scalpel has a hard time with that so I was thinking maybe I could use an animated directional (ground up) alpha to make the trees appear.

My question is, does this option make me have to calculate the light maps for every frame, mainly because "stuff" is going to start to pop out of thin air?
Could I be running into other sorts of problems with this option, mainly afecting objects just behind the alpha animated trees? Is this even possible?
Or should I just render another pass with just the trees and compose it in post?


Thank you in advance.

[SnipeyX]

My vote is do it in post, fearing the opacity would kill your render.