Though at first sight these artefacts looks like an material issue, it might be caused by a duplicated distribution of used items (as V-Ray's transparency limits can be reached in that case easily). I would suggest you to double check if there are not multiple areas in use covering tha same space.
If your issue persist, would you please send us your scene through WeTransfer to email@example.com to analyse it here? Also please specify which 3ds Max / Forest Pack / V-Ray release are you using. Thanks in advance.