Which art style was characterized by simple forms made from natural materials?

It’s hard to say, I don’t know any styles that fit this exact description but I can give my two best guesses.

Environmental Art
- uses natural materials from the art piece’s environment as the media and basis for the artwork. Often used to make a statement about nature or environmental conditions.
(think Walter De Maria’s “Lightening Field”)

Art Nouveau
- usually an applied art that uses nature-inspired forms but not necessarily made from natura materials.
(think Alphonse Mucha’s “Daydream”)

sorry i couldn’t offer something more specific. hope you find what you’re looking for!

The king of france loved the italian renaissance so much that he all of the above