However, it is only occasionally that they concentrate and work great injury and damage in any one section.

At rare intervals, some very destructive insects may centre their work in one district.

They will kill a large number of trees in a short time.

They continue their destruction until some natural agency puts them to flight.
The fungi, on the other hand, develop slowly and work over long periods.

Sudden outbreaks of fungous diseases are unusual.
Heavy snows, lightning and wind storms also lay low many of the tree giants of the forest.