What does 'thick trees' mean?

Question

"C'mon, follow me -- any more firs' years? Mind yer step, now! Firs' years follow me!"
Slipping and stumbling, they followed Hagrid down what seemed to be a steep, narrow path. It was so dark on either side of them that Harry thought there must be thick trees there. Nobody spoke much. Neville, the boy who kept losing his toad, sniffed once or twice.
(Harry Potter and the Sorcerer's Stone)

Does ‘thick trees’ mean many trees whose trunks have big diameters; or does it mean dense forest?

Answer 1

In context it means "dense forest" -- there are many trees close together, making passage through them difficult.

I think it's more common to say "thick forest" than "thick trees", and that would also make more literal sense, as it is not the individual trees that are thick, but rather their "grouping".