Hi, unfortunately motion blur and velocity channel don't work with animated objects. We are working with ChaosGroup to implement it, but found several problems that are not solved yet.
There are two workarounds for that:
1) Convert Forest items to stand-alone instances using Forest Tools
Please note that is not possible to replicate the Forest animation modes, and instances will follow the animation of the source object. To simulate the random mode, you would create several Custom Objects with different animation offsets and scatter them.
2) In Forest 5 beta
, motion blur and velocity channel are supported for V-Ray proxies. Using this version, you would convert your animated objects to proxies and use them directly.
I hope that helps.