Red spruce trees in the Central Appalachian Mountains have been recovering, in part, due to cleaner air over the last 25 years, with growth also being enhanced by warmer springs, and increased CO2.

See the source article by following the link below:
Red spruce trees in the Central Appalachian Mountains have been recovering, in part, due to cleaner air over the last 25 years, with growth also being enhanced by warmer springs, and increased CO2.