Unless the amount of trees (a graphics detail setting) can be enforced by the server there is no real fix to this.
If i'm flying with trees set to low and the server is using trees set to medium to calculate the collision boxes then i'll end up crashing into tress i can't see, if on the other hand i'm flying with a higher setting than the one used by the server i'll be able to fly through some of the trees i see (because they are not there in the server's "version" of what's rendered) and crash into others.
And since servers need to be populated and make an effort to cater to a variety of players and not just those who can crank up all the details to maximum, this can only mean one thing: the server forcing the players to fly with trees set to low no matter their own settings, because otherwise there will be a mismatch between players in what constitutes collision material.
I'm perfectly ok with this, i just have a feeling that as soon as we get a rough collision box outline for the main forested areas there will once again be people complaining about having to fly with lowered detail
