Hi, are you using 3D or 3D trees ? (i'm not sure seeing the images). What is your V-Ray version ?
For 3D trees, please try converting the trees to stand-alone objects using Forest Tools
. Does the problem solve or not ?
For 2D trees you can do a similar test collapsing the Forest object to a mesh.
This test will help to identify if the problem is caused by Forest or V-Ray. Thanks.