Elevated nitrogen (N) deposition in the bituminous sands region of northern Alberta, Canada is localized but expected to increase over time. Here we seek to determine the effects of above canopy N deposition on understorey vascular plants in a jack pine (Pinus banksiana) stand in a five-year experimental study. Aqueous N (ammonium nitrate) was applied four times annually (May through October) via helicopter above the canopy between 2011 and 2015 across a narrow but environmentally relevant N deposition gradient (0, 5, 10, 15, 20 and 25 kg N ha-1 yr-1). Changes in vascular plant species richness, diversity and total vascular cover were best explained by throughfall water flux, but the positive responses to precipitation decreased with increasing N application. Arctostaphylos uva-ursi and Maianthemum canadense showed positive cover increases in wet years; however, the positive cover expansion at ≥5 kg N ha-1 yr-1 treatments was suppressed relative to controls. Total cover expansion was muted in low precipitation years in treatments ≥10 kg N ha-1 yr-1. In contrast, Vaccinium vitis-idaea cover changes ≥10 kg N ha-1 yr-1 were consistently negative. There were no differences in soil net N mineralization rates, plant foliar N or NO3- leaching among treatments. We conjecture the extensive moss/lichen layer of the forest floor that accumulates most of incoming N in throughfall allows them to outcompete vascular plants for water during higher precipitation years, effectively reducing vascular cover expansion relative to controls. This work suggests the response of vascular plants in xeric jack pine ecosystems may interact with climate and these interactions should be considered in risk assessment studies.
Keywords: Biogeochemistry; Boreal; Cover; Ecology; Ericaceous; Experiment.
Copyright © 2022 Elsevier B.V. All rights reserved.