It seems to me that a very important element in the falling leaf scenario might be the physical protection of the tree. Since most plants in a cold or temperate climate go into dormancy when the cold dry air develops it would seem that that would prevent transpiration. I have a number of plants that when they go dormant do not loose their leaves but don't seem to suffer from drying out. It seems to me that most deciduous trees loose their leaves to protect against the damage done by snow fall. For example I have two magnolias, one looses its leaves each winter and the other doesn't. Every year the one that doesn't loose it's leaves looses several major branches from snow; the other, none. However, neither suffers from the dry cold weather. The Magnolia Grande Flora is a more tropical plant (what I have) so it developed in a climate where snow is not an issue, therefore, it doesn't need to drop it's leaves. Any thoughts?
I have never had a magnolia, but I know they occupy a unique place in evolution. Bear in mind also that your two specimens are hybrids (as our most of our garden plants) and don't, sadly, tell us much about evolution.
Thank you for your fascinating comment! (Anything that sends me back to my primary sources for more information is interesting.)