Meaning: Forests consist of many trees.
Choice of Words: Forests: large areas of land covered with trees. Full: containing a lot of something.
Meaning: This means the same as the first sentence, but it emphasizes that there are a lot of trees in forests.
Choice of Words: Many means a lot. Trees are tall plants with trunks and branches. Forests are large areas of land covered in trees.
Meaning: This means the same as the first two sentences, but it focuses on the fact that forests are places where you can find many trees.
Choice of Words: Places are areas where you can be or go. Lots means a lot. Trees are tall plants with trunks and branches.