For science it says leaves are plant organs. how is health of the tree dependent on the functioning of the tree's leaves?

Think of a Tree like a organic solar panel its leaves are cells basically what they do is turn the solar energy into food for themselves if the roots of the tree are too compressed the tree can get root rash and not have enough space to grow. It is important to always put trees in loose soil and let gravity do the work so that it will compact it at the right rate but, earthworms are a great way of fixing root rash because they will dig tunnels all around the roots for them to drink nitrates and nitrites in the soil for nutrients. The leaves turn a yellow color if the roots are that way or if there isn't enough phosphorus in the soil.