Is tree pluralized in Chinese?
There is no plural form for the non-human Chinese nouns. 树 can be singular or plural. 树木 is a general term for trees (any tree; all trees)
To indicate tree  in the plural form, you need to add plural counting words like 'two, few, some', or a plural pronoun like 'these, those'
一棵樹 - a tree
那棵樹 - that tree
幾棵樹 - a few trees
那些樹 - those trees
樹木 is a general term for trees. It refers to all trees in general. e.g. 樹木的歷史大概有 4.2億年 - The history of trees is about 420 million years
Similarly:
一隻野獸 - a wild animal
那隻野獸 - that animal
那些野獸 - those animals
幾隻野獸 - a few wild animals
野獸 - wild animals in general

I see trees - 我看到樹/ 我看到樹木
I see the trees - 我看到那些樹 / 我看到那些樹木
I see the big red trees - 我看到那些大紅樹
I see many trees - 我看見很多樹/ 我看見很多樹木
I see both trees - 兩棵樹我都看到了

There is no plural form for the non-human Chinese nouns, but for a human noun, you can add 們, as in 人們，醫生們，士兵們 (people, doctors, soldiers)