How's your leaf material made?
Have you tried overriding materials?
Since flicker only occurs on leaves, test, if a single leaf behaves same. If so, post it here.
If not, isolate whole tree crown & test again.
If issue persists, change environment/lighting, then Camera/tone mapping...
PS
you can render even smaller size, so you can test more, faster - if this flicker occurs, it will stay obvious