and what does moss have to do with this? I dont get it all: What other ways?
Okay, most areas with lots of plants have
trees.
Now,
trees aren't the only method of plants possible. There are many non-tree plants. We've named some already. For my example, let's take moss.
Now, moss is a plant. It is not impossible that at some point in the past,
trees weren't around. At such a time, it's also probable that you had something like moss all over the land instead of trees. Moss isn't particularly rooted. It can pretty much grow on top of rocks and things with like a quarter inch of dust on them.
Other plants don't even need that. Most water plants are free floating. Hence: rooted is not a requisite for plants, hence trees are not rooted because they need to be to perform photosynthesis, hence modeling trees by requiring they be rooted to perform photosynthesis isn't correct.