How can I get past these trees?

Wait until you get past the guard first.  You can't do anything until then,
and the following hints might spoil your fun if you read them prematurely.

The first thing that you should do is talk to the people in the area to find
out something about them.

Someone will tell you that the trees used to be part of a musical group.

You're going to have to find some way to make the trees remember their past.

A musical instrument might help.

Have you done a good deed yet?

A suggestion or two is in your spell book.

There's no suitable place for planting trees here, so don't even try.

Why not polish a statue?

So it's a nice shiny gold.

As in the kind of gold that comes from lead.

Use the alchemist's magnet on the statue in the grassy meadow.  Then you'll
be given a box which contains a magic drum.