"2. The leaves on the trees hold some of the heat close to the ground & colder equals more condensation."
I had first hand experience with the phenomenon about 2 weeks ago. My buddy camped in the open without trees to speak of, and he awoke with frost on his down bag (d'oh!)
and his water bottles were partially frozen. Now here's the proof in the pudding... I was no more than 12 feet away partially enclosed on one side by a Sugar Sumac and on the other by an Alligator Juniper. No frost, no ice, no worries! his thoery was that he would catch the sunlight first thing, while I would freeze in the shade. Ha, sucka!