What effect does increased air pollution generally have on trees in the United States?

Increased air pollution has a variety of negative effects on trees, which can significantly impact their overall health. Trees absorb pollutants from the air, which can lead to stress and a range of health issues, such as reduced photosynthesis, susceptibility to disease, and even death. The pollutants, like sulfur dioxide and nitrogen oxides, can cause leaf burn or chlorosis, impairing a tree's ability to produce food and grow effectively.

While the other options mention aspects such as growth rates, carbon fixation, and biodiversity, increased air pollution doesn't enhance these characteristics. Instead, it tends to limit the growth and health of trees and other vegetation. Adverse health impacts are a direct consequence of the way trees interact with a polluted atmosphere, making it the most accurate reflection of the effects of increased air pollution on trees in the United States.