The process of leaves changing colors is essential for the life of broad leaf  trees. Trees begin removing nutrients from the leaves after the prime growing days are over. When there is not enough nutrients left to manufacture chlorophyll, sometime in September, the leaf begins changing color.
Chlorophyll is the amazing chemical that captures the suns energy, which gives the leaves their fundamental green color. The leaves contain precious minute quantities of minerals such as magnesium, nitrogen, calcium, phosphorus and potassium obtained from the soil.
If the essential nutrients located in the leaves were lost with leaf drop, the tree could not absorb enough replacement minerals through half frozen roots to  make new leaves in the spring. The tree, however, saves as much nutrients as possible before the leaf is killed by frost. By September, half of the leaf's nutrients have been removed to woody tissue for later use.
Therefore, the orderly process of shut down of summer activity which began in July, to the end result of fall colors, is actually the transfer of essential chemicals back to the tree. Once on the ground, the leaves also can aid in the growth of the tree by becoming mulch with the lawnmower.
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