Population/ ecosystem question

Question : http://imgur.com/a/6Gac4  (part a )
                 http://imgur.com/a/wHegf     (part b )

Hi, I don't quite get part b ii). It says when the canopy closes, what happens to the ground vegetation. The answer would be that there is less growth in the ground vegetation but how can that be? Since you don't have the canopy blocking the sun, surely more sunlight is available to pass through to the lower vegetation for more growth, no?

For III regarding to the shrew population, the effect is that there will be fewer of them. Why? I thought they only feed on the lower vegetation, why will they emigrate/ die?

For c) I don't understand why the falling of leaves can create a more stable population, the answer says that 'No sudden drop in 1963-64/population stable
Shrub / field / ground layers able to grow during Spring
before canopy closes/more leaf litter so more nutrients' . I don't quite get this.

Thanks