What will happen if trees are removed from an ecosystem?

Well, first off, it's not gonna happen, so don't worry about that. Trees, or more accurately, the photosynthesis they carry out, convert carbon dioxide to oxygen. It might sound odd, but yes, trees and plants breathe IN what YOU breathe OUT. Every time you exhale, you give a tree or a plant the breathe of life. Trees are not the only providers of oxygen, but they certainly help. River also provide oxygen. They depend on us. We depend on them.Luckily, they can be renewed (planted from seeds) and grow almost anywhere. There are more trees in North America (Amazon) now than there were in 1492.