Background/Question/Methods - Nitrogen (N) and sulfur (S) deposition in forest systems can have positive or negative impacts on tree growth.  The growth of many forests is limited by N availability, and therefore N and S deposition may stimulate growth.  However, N and S additions can sometimes be greater than that required by trees and negatively impact tree health.  In cases where N and S deposition exceeds the critical acid load of a site, tree health and growth may be reduced directly and indirectly due to soil nutrient imbalances caused by leaching of base cations from the soil.  To determine the relationship between N and S deposition and tree growth, analyses comparing the growth of sugar maple and red spruce and critical load exceedances (positive and negative) were conducted.  Sugar maple and red spruce were selected as the test species because both have been shown to experience decline in areas of high N and S deposition.  Critical loads were calculated using the simple mass balance model and three different levels of protection to tree health (Bc/Al soil solution ratio = 0.6, 1.2 and 10.0), and were compared against 2002 N and S deposition to estimate critical load exceedance.  Annual tree growth estimates for sugar maple and red spruce were obtained from the U.S. Forest Service Forest Inventory and Analysis (FIA) database.  Multivariate regression analyses were conducted using a quadratic functional form to test for an inverted U-shaped relationship between tree growth and critical load exceedance. 
Results/Conclusions - Although none of coefficients for the quadratic curve were significant at the p=0.05 level, the estimated parameters for both species suggested an inverted U-shaped relationship between tree growth and critical load exceedance.  In addition, the data revealed that the highest protection level critical load (Bc/Al=10.0) best described the relationship between N and S deposition and the growth of sugar maple and red spruce.  Considering the implications of the results across the ranges of the two species, tree health may have been compromised in at least 80% of plots in 13 of the 24 states where sugar maple is found and in 100% of plots in 5 of the 8 states where red spruce grows.  These results suggest that the health of sugar maple and red spruce may be compromised in multiple locations throughout their natural ranges.