Subject: New tree vs mulched mature tree. Net environmental benefit?

I have read in this site that a young tree produces more oxygen net than a mature tree due to respiration, 
bough damage and other factors.  However, I am interested to know if there is an optimum time for a tree 
species at which it would be better to kill the tree, mulch it and use the mulch to assist other trees and 
plants to grow, whilst at the same time replacing the original tree with a new one of the same species.
The mulched plants would have a better survival rate (especially in hot climates such as Australia) and a 
factor could be applied to this by someone with more knowledge than myself.
Would the mulch eventually break down and provide soil improvement, and assist the plants with minerals, 
not just moisture retention?

Assume that the tree has been felled and mulched using modern harvesting methods, so there will be 
some  carbon input due to fossil fuels for saws, grinders and transport.
The calculation would be done over say 20 years.

Re: New tree vs mulched mature tree. Net environmental benefit?