Final answer:
The decline of the American chestnut tree is primarily caused by the Asian chestnut blight fungus, which spread through eastern North America. This fungus eliminated the American chestnut tree and led to structural changes in the forest.
Step-by-step explanation:
The widespread decline of the American chestnut tree is primarily caused by a fungus called Asian chestnut blight (Cryphonectria parasitica) that spread through eastern North America in the early 20th century. This fungus effectively eliminated the American chestnut tree, which was once common and comprised a significant portion of the forest canopy. The loss of the American chestnut tree led to structural changes in the forest and affected nutrient cycling.
Chestnut was replaced by oaks, which have a different decomposition rate. Additionally, several moth species that were specific to chestnut went extinct as a result. The impact of this fungus on the American chestnut tree highlights how pathogens can have a significant impact on an ecosystem.