A five-year experimental study was conducted to evaluate the response of an upland jack-pine (Pinus banksiana) forest to elevated levels of nitrogen (N) deposition applied as ammonium nitrate above the forest canopy in Northern Alberta.  N deposition in the region is expected to increase with industrial expansion of oil sands activities, and there is regional interest to set N critical loads for sensitive ecosystems in this boreal region. Jack-pine uplands represent one regional ecosystem that is expected to be sensitive to N deposition.

In this study, N was applied as NH₄NO₃ above a jack-pine canopy via helicopter, five times annually for five years (2010-2015) at dosages equivalent to 5, 10, 15, 20 and 25 kg N ha^-1 yr^-1. Over the course of the study, we observed that approximately 35% of the applied N was retained in the canopy while 65% reached understory vegetation dominated by lichens and mosses.

We measured a significant increase in tissue N concentrations of common ground lichens (Cladonia mitis and C. stellaris) and ground moss (Pleurozium schreberi) as well as epiphytic lichens (Hypogymnia physodes and Evernia mesomorpha).  On an annual basis, the applied N was primarily captured in the lichen and moss understory, where the predominant ground cover is C. mitis. In the highest treatments, N concentrations in C. mitis were 1.5-2.5 times greater than pre-treatment values. Peak N concentrations in the ground moss Pleurozium schreberi (1.4%) were consistent with values recorded in Europe at comparable N deposition rates, and indicate that a threshold of N saturation was reached 3 years into the treatment. N concentrations in vascular plants were not affected by the treatments, and we observed no changes in community composition for vascular and non-vascular plants.  Chlorophyll levels in C. mitis increased with N treatment, but there was no indication of toxicity or decline in C. mitis health, and decomposition and growth of the lichen remained consistent.