Under the impression that the leaves are only green because the plant is producing chlorophyll. As the tree goes dormant, the loss of the chlor. makes the true color come out?
Do have less water transfer from leaf to tree, but the water-sap in the tree is the same amount all year, borrowing no extreme dry weather. The saying of sap raising and falling in a tree due to the seasons is a fallacy, there's a change of internal pressure in the tree.