Using tree rings of red spruce in the Central Appalachian Mountains to explore growth trends before and after the Clean Air Act

Background/Question/Methods

Acid deposition caused by pollutant emissions (NO_x and SO₂) has detrimental effects on ecosystem structure and function. Acid deposition causes cation leaching from leaves and soils, reduced soil pH, and reductions in frost hardiness in some tree species. Red spruce (Picea rubens Sarg.) trees are considered to be an iconic indicator species for the effects of acid deposition. Since the Clean Air Act (1970), pollutant emissions have declined nationally by ~58% from the highest recorded emissions in 1973. We analyzed annual growth rings of red spruce trees growing in the central Appalachian Mountains to examine the growth response to historical changes in pollution. Twenty red spruce trees were cored at each of three forested sites along an 800 km latitudinal transect in eastern West Virginia from which basal area increment (BAI) was calculated. On a subset of five trees δ¹³C was measured on bulk wood on an annual time scale and converted to D¹³C to account for changes in atmospheric δ¹³C. To determine environmental drivers of BAI and D¹³C from 1940-2013, we used a multivariate analysis with local climatic variables (e.g. mean annual precipitation and temperature), U.S. SO₂ and NO_x emissions, and atmospheric CO₂ concentrations.

Results/Conclusions

We observed that red spruce tree BAI increased from ~1900 to 1940, after which there were large declines in growth. However, at all three red spruce stands, BAI increased after the Clean Air Act. We used a third order polynomial function to estimate the year when the trend changed from a decline to an increase in BAI. For two red spruce sites this change occurred ~1986 and in the third site ~1992. The rates of decline between 1940 and the mid-1980s were not different between sites; nor was the rate of recovery after the mid-1980s. Preliminary isotopic data from one of the sites indicates a sharp transition in D¹³C, where there is a strong enrichment in the heavier isotope prior to 1989, after which there is a reversal in D¹³C, indicating a strong depletion in the heavier isotope. These isotopic results are consistent with those found for red cedar trees in a close-by site, which suggests that recovery from acidic deposition involved increased stomatal conductance and photosynthesis. Our data suggest a similar response by red spruce trees and a potentially more widespread response to reduced acid deposition since the Clean Air Act.