Greetings Jaymie and thank you for your question. The following explanation is from a recent article written by Jeffrey O. Dawson, Professor of Tree Physiology, University of Illinois at Urbana-Champaign.
"The Advantages of Shedding Leaves in the Fall…Temperate deciduous broadleaves, as a whole, avoid having to pay the energy cost of maintenance respiration that would be required if they retained leaves year round. Winter leaf retention would not allow the broadleaved trees, with large masses of leaf tissue, to balance their winter loss of respired carbon (carbon dioxide). Although winter-hardened, dormant leaves of trees in cold temperate climes would have reduced levels of respiration in winter, temperatures are generally too low for an equivalent gain of carbon via photosynthesis. Fall color is associated with the process of leaf senescence leading up to the fall shedding of leaves."
"… What triggers these fall changes if not Jack Frost? A specific combination of shortening day length and cooling temperatures in autumn at a given locale is typically “sensed” by plant receptors resulting in the production of plant hormones that initiate leaf senescence. The initiation and timing of the various processes of leaf senescence are genetically controlled for tree populations of a rather narrow climatic zone."
Hope this answered your question!
