Anthropogenic atmospheric emissions and subsequent deposition of nitrogen (N) can affect N-sensitive habitats and lead to shifts in plant species community composition. This study assessed the effects of N deposition on plant community composition for Jack pine forests across northwestern Canada and across a smaller subset of sites surrounding the Athabasca Oil Sands Region (AOSR) using 'gradient forest' analysis. Predictor influence on community composition varied depending on the scale of the study and relatively
distinct thresholds were identified for different plant groups. In the larger scale study, a total deposited nitrogen (TDN) threshold of 1.5 – 3 kg N ha-1yr-1 was well suited to protect predominantly lichen species, consistent with lichen-based critical loads from other studies. Across the smaller scale study, a TDN threshold of 5.6 kg N ha-1yr-1 was primarily associated with vascular species changepoints but did include some important N-indicator lichen and bryophyte species.
Author Keywords: critical loads, gradientForest, Jack pine, Nitrogen deposition, species composition