So I watched this video last night and Ryan mentions that it is important to leave some old needles on two needle pines due to the hormones they produce.
I have not heard this before and thought that there may be some ideas as to why this is important?http://www.youtube.com/watch?v=t7ngvzuN-F4
For tree in training or the tree that is thin or weak area we do not remove all of the old needle. Take off old needles that grow downward. This is the same basic principle.