Our forests, wild and urban, are dying.

See this Dec. 24, 2012 New York Times article by Sandra Blakeslee about New Mexico's Los Alamos Lab's effort to study the problem.
Note: Aerosols and gaseous substances abound in the same air that is changing our climate.
Many of these contaminants are phytotoxic pollutants, such as black carbon soot, sulfur dioxide, and ozone which kill or injure trees.

Photo: A pile of leaf fall in a street gutter in Flagstaff AZ, November 4, 2012. A wide variation of coloration and leaf health can be seen. DT Lange