Good question! This question is an actually an entire topic. For example, see this article.
So we have four patterns here:
The short answer is that they are all correct grammatically. However, they have slightly different nuances.
"は" is called the topic particle for good reason; it marks what the speaker's statement is about. Without any context, (1) could be a statement about the environment from a book. The statement describes what simply happens. On the other hand, (2) is a statement about the tree itself. An example usage is, 石は浮かないが木は浮く (Stones don't float, but trees float). Here, we compare stones and trees. The statement isn't about what happens generally; it specifically compares stones and trees, the topics.
Now, alone, (3) seems slightly strange alone. Note that as "浮きやすい" is an adjective, the whole sentence seems to describe the tree, which wouldn't make sense if tree weren't the topic. On the other hand, it could be used as in "なぜ、海水では人が浮きやすいのでしょうか". Here, the topic is seawater; not people.
(4) alone is a natural statement about trees themselves. It describes trees, saying trees float easily.
I hope this helps a little bit!