One of the dominant tree species in the temperate deciduous forest in the eastern part of the United States used to be the American chestnut. Now, the dominant tree species are mixed oak and hickory trees and other hardwood trees. What is the reason for this change?

The chestnut blight.

Explanation:

The chestnut blight is a fungal disease, caused by the Cryphonectria parasitica, a parasitic fungus of the chestnut trees. It was introduced in America in the early 1900s by imported Asian chestnut trees.

There have been estimations that there were about 3 - 4 million chestnut trees in the eastern North America, and about 25% of them were the American chestnut. After the blight, there were roughly 100 trees left.

Answer: Chestnut trees were infected by a pathogen, the Cryphonectria parasitica in the early 20th Century.

Explanation:

Cryphonectria parasitica is a pathogenic fungus native to Asia that was introduced into North America in the first decade of the 20th century. The resulting disease it caused is known as chestnut blight and is characterized by brown patches which eventually becomes a hollow blight until it surrounds the width of the tree and kills all tissue above the blight. The disease wiped out the Chestnut population due to its ease of contamination through wind-borne fungal spores and by rain splash action.