Chronic nitrogen additions and photosynthetic gas exchange in sugar maple: Instantaneous measurements and a stable isotope chronosequence

Although the deposition of atmospheric nitrogen (N) may increase rates of photosynthesis, it is unclear if these increases will be sustained into the future. To address this question we studied gas exchange in mature sugar maple (Acer saccharum) stands at four sites in northern Michigan. At each site plots receiving ambient deposition were compared to plots that have received experimental additions of 30 kg N ha^-1 yr^-1 since 1994. Two methods were used to assess gas exchange traits: analysis of δ¹³C in archived leaves collected annually from litter traps and instantaneous leaf level measurements.  Changes of 0.2-0.8‰ in δ¹³C of leaf litter were evident one year after N additions began and were largely maintained over the 12-year study period. While the effects were persistent, the response differed by site with average annual enrichment of 0.15-0.3‰ at three sites and average annual depletion of 0.4‰ at the fourth.  Instantaneous measurements of gas exchange complement the δ¹³C analysis and confirm the changes in gas exchange, with small increases (4.5%) in photosynthesis dominating the response at the three sites with enriched δ¹³C and a large increase (30%) in stomatal conductance dominating the gas exchange response at the site showing depletion in δ¹³C. These results suggest changes to gas exchange may be sustainable but that forests are heterogeneous in their response to N additions. Further, the aboveground growth increase observed in this study may not be the result of photosynthetic enhancement, but we hypothesize that it instead results of reallocation of C away from belowground pools.