Define deadwood in the context of these two sentences when populations are low, people use fallen tree limbs for fuel, which does not harm the trees. When populations grow rapidly, deadwood does not accumulate fast enough to provide fuel

Deadwood is woody debris in the forest that are no longer living including stems, shrubs, their partial remains, branches, twigs, but it exclude deadwood parts of living trees.

Explanation:

In simple words, deadwood is dead parts of trees in forests. Villagers use these dead fallen tree parts like branches, stems for fuel. When population is low, this need can be easily managed as their requirement is also less.

The speed at which trees become dead is slow and if does not match with the speed at which population grow then it won't serve their fuel purpose. So, when population grow, deadwood is not enough to serve their fuel purpose.

Deadwood is an play an important role in proper functioning of forest ecosystem.