What causes pioneer grasses and weeds that have grown in an abandoned field to die?

Tall  plant block  out sunlight  causes the  pioneer  grasses  and  weeds  that  have grown  in  an abandoned  field  to  die . These  is  because   these  tall plant  has  dense  canopy which  led low  amount of direct sunlight  being  received  by  the  grass  and weeds,  that  is  less  than  six hours  of direct  sunlight   per  day.

Maybe haha