deciduous and evergreen species of Magnolia

[chezfenny1]

I am working on a poster for an assignment, my topic is the Botany of Magnolias. I am interested in knowing why there are both deciduous and evergreen species within the genus. It would be extremely helpful for me to have a reference to a work that explains this. I have done a great deal of web searching using Google scholar etc and looked at Jim Gardiner's book on Magnolias but just cant locate an answer.

I'd appreciate you thoughts on this

[chezfenny1]

I wondered whether the different species had become either deciduous or evergreen according to the climate in which they developed, but is it temperature or humidity or both that affect that development?