| Atmospheric nitrogen (N) deposition is known to impact forests in a variety of ways ranging from increased growth and photosynthesis to needle necrosis. More than half of the growing-season N deposition flux at the Niwot Ridge Long-Term Ecological Research (LTER) site's subalpine forest (Niwot Forest) is of anthropogenic origin. Additionally, annual inorganic N deposition and the concentration of nitrate (NO3-) in wet deposition have been increasing at Niwot Ridge since the early 1980's. Previously, N fertilization studies investigating forest responses to increased N deposition have primarily dealt with deposition loading to the soil. However, some studies indicate that forest canopies retain a substantial portion of atmospherically-deposited N before this N reaches the soil in throughfall solutions. In the present study, canopy N uptake (CNU), as well as its influence on photosynthesis, was investigated.; At the Niwot Forest, growing-season throughfall fluxes of ammonium (NH 4+) and NO3- are markedly lower (∼70% lower) than fluxes in wet plus dry deposition, indicating that the forest canopy takes up atmospherically-deposited N. This uptake was found to be driven by diffusion of NH4+ and NO3 - into foliage and canopy woody tissues.; Although the canopy is taking up atmospherically-deposited N, Engelmann spruce foliar N content is relatively low at the Niwot Forest. This low foliar N content contributes to low rates of light-saturated photosynthesis and maximum carboxylation, which is consistent with N-limited production of photosynthetic proteins involved in the light harvesting and Calvin cycle reactions. Further, a linear dependence of maximum carboxylation on needle N content was found and N solutions that had been directly applied to foliage at Niwot Forest spruce branches induced a 12% greater photosynthetic efficiency (i.e., proportion of absorbed light utilized by photosynthesis).; The low foliar N content and strong coupling of photosynthetic parameters to foliar N content show that the Niwot Forest has yet to reach a state of N saturation. Noting that the Niwot Forest has some of the largest N deposition fluxes in the central Rockies, our CNU and photosynthetic parameter results showing "pre-N-saturation" conditions should prove useful in addressing N critical loads criteria for Rocky Mountain region subalpine forests. |
