Development of the Alberta oil sands region has increased local nitrogen emission rates, raising concerns over the eutrophication of forest and peatland plant communities. The objective of this report was to conduct a literature review on the effects of nitrogen deposition in forest and peatland plant communities, discuss the potential effects of nitrogen deposition in the oil sands region and provide recommendations for a biomonitoring program.
Increases in nitrogen deposition rates have been linked to changes in plant community composition, plant tissue chemistry, and soil nitrogen cycling in forests throughout Europe and North America. In the boreal forest, nitrogen-induced changes in plant community composition are characterized by increased abundance of nitrophilous grasses, forbs, and deciduous shrubs, and decreased abundance of feathermosses, lichens, and ericaceous shrubs. Nitrogen addition experiments indicate that floristic changes can occur with nitrogen additions as low as 5 kg N ha- 1 yr-1. High nitrogen deposition rates increase nitrogen concentrations in plant tissue, which may inhibit plant growth and increase the incidence of plant pathogens. Plant tissue nitrogen and amino acid concentrations increase in response to nitrogen additions of 10 kg N ha-1 yr-1 or more. Nitrogen enrichment is also indicated by increased nitrate leaching, which can deplete soils of base cations and cause soil acidification, root damage, and nutrient imbalances in vegetation. Recommended critical loads for nitrogen in forests range from 5 to 10 kg N ha-1 yr-1 for coniferous forests and 10 to 20 kg N ha-1 yr-1 for deciduous forests.
Exceedance of critical nitrogen loads in bogs and poor fens initially increases and then decreases Sphagnum growth, and increases the abundance of sedges, forbs, and selected ericaceous shrubs. Nitrogen saturation of bogs and poor fens may occur at deposition rates of 12 to 18 kg N ha-1 yr-1. Rich fens appear to be less sensitive to eutrophication than ombrogenous peatlands. Nitrogen enrichment in rich fens increases the growth of brown mosses and sedges but does not appear to cause significant changes in plant community composition, although long- term studies are scarce. Recommended critical loads for peatlands range from 5 to 10 kg N ha-1 yr-1 for bogs, 10 to 20 kg N ha-1 yr-1 for poor fens, and 15 to 25 kg N ha-1 yr-1 for rich fens.
Nitrogen-sensitive vegetation types in the oil sands region include jack pine-lichen, white spruce-feathermoss, black spruce-Labrador tea, bog, and poor fen communities. Exceedance of critical nitrogen loads in these communities may result in losses of characteristic understory species such as lichens, mosses, small forbs, and dwarf shrubs. Wooded fens may be subject to decreases in plant species diversity where critical nitrogen loads are exceeded. Traditional plant species harvested by aboriginal communities occur mainly on rich forest sites and do not appear to be threatened by eutrophication.
Modeled nitrogen deposition rates in the oil sands region suggest critical load exceedances at sites located in close proximity to emissions sources. Based on current nitrogen emission rates, modeled deposition rates range from 65 kg N ha-1 yr-1 near emissions sources, to less than 5 kg N ha-1 yr-1 at remote sites. Projected increases in nitrogen emissions over the next decade will likely expand the area affected by nitrogen enrichment. Assessment of the effects of nitrogen deposition on plant communities in the oil sands region should include validation of modeled nitrogen deposition rates, mapping of critical load exceedances, long term monitoring of permanent plots, and nitrogen addition experiments. Recommended response variables for a biomonitoring program include plant tissue chemistry, plant community composition, and soil nitrogen status. Nitrogen indicator species, hypothetical critical loads, and further recommendations for a biomonitoring program are discussed.