Red spruce was once a valued timber species but was threatened by acid deposition, which reduced its growth and increased mortality for this sensitive species. A Northern Research Station (NRS) scientist and partners set out to establish whether red spruce is rebounding 20 years after Clean Air Act mandates, and if so, why?

The decline of red spruce is one of the best documented cases of acid deposition damage to a sensitive species. Indeed, research on red spruce decline was used as part of the scientific justification behind reductions in pollution mandated by the 1990 Amendments to the Clean Air Act. After decades of reductions in acid deposition, monitoring has documented some improvements in environmental quality, but no evidence of increased red spruce growth associated with pollution reductions had been documented. Researchers from the NRS and the University of Vermont collected wood samples from 658 red spruce in 52 plots across five northeastern states to evaluate recent growth. They found that more than 75 percent of trees and 90 percent of plots exhibited increased growth since 2001. Growth was negatively associated with nitrogen deposition and positively associated with higher temperatures in the fall, winter, and spring that reduce winter injury and extend the growing season. Negative relations between growth and nitrogen have decreased in recent years as deposition levels have declined. This study offers unique evidence that red spruce are rebounding from historical acid deposition stress and that the species is poised for enhanced management for valued timber and habitat provisions and other ecosystem services.