Drought Periodic drought directly affects trees by weakening or killing some species and releasing others. The skirmish line between a glade and its associated dry forest community gradually moves in favor of the forest in good years, and in favor of the glade in times of drought. Periods of drought alone would maintain some dry woodlands and glades, but usually predisposes a community to fire. Mammals, Insects and Disease Animals can influence the species composition of a forest. Large grazing and browsing animals, such as deer, beavers and domestic livestock, can have serious direct impact on a forest.
Smaller mammals seem to favor seeds.
Beavers build dams and can cause localized flooding, further changing the mix of tree types in a forest. A handful of imported insects and diseases have had a tremendous impact on the forests of the Eastern United States. The chestnut blight totally wiped out the American chestnut from millions of acres and is currently endangering the Ozark chinquapin. Dutch elm disease greatly reduced the abundance of elms in bottomland forests throughout Missouri. The gypsy moth isn't here in force, yet, but it will probably affect our forests.