How does cutting down only mature trees in a forest extend the life of the forest

Cutting down only mature trees allows more resources to be available for   the younger growing trees and extends the life of the forest.

Explanation:

Older an taller trees grow to a height and forms a canopy over the forest floor.This cuts the maximum amount of sunlight from reaching the under growths.Thus the shorter and younger trees lie in shade and cannot grow properly due to lack of sunlight.On the other hand the trees could penetrate deeper into the soil consuming up a lot of nutrient and the competition for resources becomes maximum.When taller and older trees are cut, the canopy is removed and sunlight couldl penetrate to the forest floor and the stress on soil is  also reduced , thus allows the growth of younger trees which in turn extends the life of the forest.