Little_ reaches the understory or forest floor

Forest floor = plants growing. Plants need sunlight to grow.

Answer : Little _sunlight_ reaches the understory or forest floor.

The emergent and canopy layers overshadow the shrubs and compete for sunlight to grow taller.

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen

It is useful for historians to write narratives so that history can be passed on from their point of view.  think about a story being told in third person, then think about it again being told in first person, by someone who actually experienced the event.