Hello to all!

I joined in the hopes that someone can help me with the title of a
horror movie from the 80s (I believe) that I've forgotten.

The plot involves a group of campers being hunted by a freakish
being in a dense forest.
I know, that covers the plot of probably about 72 movies, but wait... there's more.

One memorable scene involved a guy eating magic mushrooms and
climbing a huge redwood(?) tree.

Could anyone out there please help?

Many thanks in advance!