As a backyard tree grower I am always looking to the leaves for clues to the current condition of the tree. Leaf curl on some of my “canary” trees will tell me that the trees need watering. Brown margins on the leaves might indicate too much fertilizer was applied. Chlorotic (yellowing) leaves, I’ve read, can be caused by too much or too little of one nutrient or another. My current quandary: What’s causing new growth to be pale? (I wish I had paid better attention in HORT101.)
It’s interesting to me that just as annular rings of a tree trunk indicate growing conditions over the life of the tree, the series of leaves in the current year often reflect growing conditions and nutrients applied (sometimes mis-applied) in the current growing season. A second flush of growth often show "forgiveness" for a mistake made in the spring.
One thing I find appealing about growing deciduous trees: they drop their leaves after they’ve left their clues to the over-all tree health. New growth promises to be better than the previous year, assuming I’ve read the leaves correctly.