dropping the "s" from des if it precedes an adjective

Question

"l'hombre de grands arbres"  was my answer and it was wrong because I had omitted the "s" from des.  Can anyone explain why in this case one should keept the "s" on des?

Accepted Answer

L'ombre des grands arbres (of the big trees - de les), not l'ombre de grands arbres (of {some} big trees) - des, becomes de). The change from 'des' to 'de' only applies to the partitive article not when 'des' is the contraction of 'de les'
https://progress.lawlessfrench.com/revision/grammar/use-de-d-instead-of-des-in-front-of-adjectives-preceding-nouns-partitive-article?rts=%252Fsearch%253Fs%253Ddes%252Badjectives