Responses of sugar maple and hemlock seedlings to elevated carbon dioxide under altered above- and belowground nitrogen sources

Various human-induced changes to the atmosphere have caused carbon dioxide (CO ₂), nitrogen dioxide (NO ₂) and nitrate deposition (NO3 ^-) to increase in many regions of the world. The goal of this study was to examine the simultaneous influence of these three factors on tree seedlings. We used open-top chambers to fumigate sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) with ambient or elevated CO ₂ and NO ₂ (elevated concentrations were 760 ppm and 40 ppb, respectively). In addition, we applied an artificial wet deposition of 30 kg ha ^-1 year ^-1 NO3 ^- to half of the open-top chambers. After two growing seasons, hemlocks showed a stimulation of growth under elevated CO ₂, but the addition of elevated NO ₂ or NO ₃ ^- eliminated this effect. In contrast, sugar maple seedlings showed no growth enhancement under elevated CO ₂ alone and decreased growth in the presence of NO ₂ or NO3 ^-, and the combined treatments of elevated CO ₂ with increased NO ₂ or NO ₃ ^- were similar to control plants. Elevated CO ₂ induced changes in the leaf characteristics of both species, including decreased specific leaf area, decreased N and increased C:N. The effects of elevated CO ₂, NO ₂ and NO3 ^- on growth were not additive and treatments that singly had no effect often modified the effects of other treatments. The growth of both maple and hemlock seedlings under the full combination of treatments (CO ₂+NO ₂+NO3 ^-) was similar to that of seedlings grown under control conditions, suggesting that models predicting increased seedling growth under future atmospheric conditions may be overestimating the growth and carbon storage potential of young trees.