The Quenya words for tree are alda (a pair of trees aldu; plural aldar) and ornë.
The Sindarin words for tree are galadh (plural gelaidh) and orn (plural yrn).
These are basic words for tree, there are other words for specific types of trees.
As I'm mentioning Illunse plurals, I thought that I'd add Quenya and Sindarin plurals. To construct most Quenya plurals a suffix of -r or -i is added. Most Sindarin nouns are made plural by changing the vowels in the word, similar to the English words man and men.