!"Birçok" yeşil ağaç"lar" gördüm.
√Birçok yeşil ağaç gördüm.
√Yeşil ağaçlar gördüm.
(I saw lots of green trees.)
As "birçok" means "lots of", we can't use the plural suffix "lar".They have the same meaning.But in general, people usually oversee and some even don't know this small grammer rule.