I live in Houston, TX and have a sweet gum tree in our yard. It is large and mature, but it has lots of leaves turning yellow and brown and dropping. Since we have not had any cool weather yet, I think that it is too early for this to be occuring. I do not see any issue with the trunk nor has there been any other stress issue. The leaves that are turning yellow and dropping are mostly from the lower 2/3 of the tree. The top is still green. Is this normal, and should I be fertilizing this tree?