I have played around with this and seems like we can't have thin beaches (1 pixel) but instead the beach zone (rgb 170,170,170) needs to be at least 4 pixels thick.
I created a beach zone of 2 pixels thick and then a 1 pixel thick transition between beach and sandy grass and it got rid of most of the trees. But I still had an odd one here and there. Going to try a 3 pixel beach and 1 pixel transition beach to sandy grass tonight and see if that does it. Not real happy about that since it creates 3/8 of a mile thick beach zone but if that is what it takes.
Might also try doing 2 pixels of beach and 3 pixels of beach to sandy grass transition then have the grass zone beyond that. But I think that is the issue.