Ecological Applications, 21(7), 2011, pp. 2413–2424 Ó 2011 by the Ecological Society of America No evidence that chronic nitrogen additions increase photosynthesis in mature sugar maple forests A. F. TALHELM, 1,3 K. S. PREGITZER, 1 AND A. J. BURTON 2 1 College of Natural Resources, University of Idaho, Moscow, Idaho 83844 USA 2 Ecosystem Science Center, School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, Michigan 49931 USA Abstract. Atmospheric nitrogen (N) deposition can increase forest growth. Because N deposition commonly increases foliar N concentrations, it is thought that this increase in forest growth is a consequence of enhanced leaf-level photosynthesis. However, tests of this mechanism have been infrequent, and increases in photosynthesis have not been consistently observed in mature forests subject to chronic N deposition. In four mature northern hardwood forests in the north-central United States, chronic N additions (30 kg Nha 1 yr 1 as NaNO 3 for 14 years) have increased aboveground growth but have not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increases in growth, we hypothesized that the NO 3  additions increased foliar N concentrations and leaf-level photosynthesis in the dominant species in these forests (sugar maple, Acer saccharum). The NO 3  additions significantly increased foliar N. However, there was no significant difference between the ambient and þ NO 3  treatments in two seasons (2006–2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (lmol CO 2 s 1 g 1 N) was significantly decreased ( 13%) by NO 3  additions. Furthermore, we found no consistent NO 3  effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993–2007) and analyzed for d 13 C and d 18 O, isotopes that can be used together to integrate changes in photosynthesis over time. We observed a small but significant NO 3  effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number. These photosynthesis and leaf area data together suggest that NO 3  additions have not stimulated photosynthesis. There is no evidence that nutrient deficiencies have developed at these sites, so unlike other studies of photosynthesis in N-saturated forests, we cannot attribute the lack of a stimulation of photosynthesis to nutrient limitations. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation. Key words: Acer saccharum; atmospheric deposition; canopy tower measurements; global change; long- term experiment; nitrogen; northern hardwoods; stable isotopes; sugar maple. INTRODUCTION Around the world, human activities have greatly increased the availability of reactive nitrogen (N) (Galloway et al. 2008). Much of this anthropogenic N is moved into the atmosphere where it is transported away from emissions sources and later added to terrestrial ecosystems through atmospheric deposition (Galloway et al. 2008). Forecasts for the next century predict that as agricultural and industrial activities intensify around the world, terrestrial ecosystems could experience greater loads of atmospheric N deposition (Dentener et al. 2006). Consequently, it is important to understand the effects of chronic N deposition in order to predict how terrestrial ecosystems will function in the future. The widespread limitation of forest growth by N (LeBauer and Treseder 2008) suggests that in many areas enhanced rates of N deposition may increase forest productivity. For temperate forests in particular, in- creases in N deposition have led to many predictions (e.g., Aber et al. 1998, Ollinger et al. 2002) and observations of increased aboveground productivity and carbon (C) storage in forests (Nadelhoffer et al. 1999, Magnani et al. 2007, Pregitzer et al. 2008, Thomas et al. 2010). Increased foliar N concentrations are often reported in studies of N deposition effects on forests (e.g., Hutchinson et al. 1998, Baron et al. 2000, Bauer et al. 2004, Boggs et al. 2005, Elvir et al. 2005), and because of the key role of N in photosynthesis (Evans 1989), many models of the effects of N deposition on aboveground productivity explicitly include increases in Manuscript received 28 October 2010; revised 8 April 2011; accepted 15 April 2011. Corresponding Editor: A. R. Townsend. 3 E-mail: atalhelm@uidaho.edu 2413