punctuation - Do I need an apostrophe in "These trees’ roots"?

Do I need an apostrophe in "These trees’ roots"?

For example,

"Wow!!! These trees' roots are so long!"

In this example the speaker can see both roots and the trees themselves.

Answer

Yes, you need the apostrophe. -s' denotes possession of some thing or things by multiple owners. The roots belong to the trees; the trees own the roots. Therefore, the roots are the trees' roots.