Establishing nitrogen critical loads in Alberta's Oil Sands: early responses of non-vascular plants

Background/Question/Methods

Nitrogen oxide (NOx) emissions are a byproduct of bitumen extraction in the oil-sands region of northeastern Alberta. Increased nitrogen (N) deposition from anthropogenic sources has the potential to alter N cycling, leading to potentially adverse ecosystem changes. Estimates of NOx emissions in the region are approximately 300 tpd and are expected to increase.  Ecosystem responses to N loading vary temporally, spatially, and by ecosystem type; therefore, the establishment of region-specific N critical loads is necessary. A five-year project was established to assess the impacts of experimentally applied nitrogen additions to jack pine forests, bogs and fens, to provide data to support development of critical loads for N in northeastern Alberta. Results presented here focus on the response of mature upland jack pine forest communities to over-canopy application of nitrogen. Five nitrogen-addition treatment sites (5, 10, 15, 20 and 25 kg nitrogen ha^-1 yr^-1) and one control site were established in 2011. Nitrogen is applied over the forest canopy by helicopter multiple times in the year. Non-vascular-plant primary and secondary chemistry responses to nitrogen treatments were quantified, as these are expected to be early indicators of a response to increased nitrogen loading. Non-vascular community composition, diversity and biomass were also measured. 

Results/Conclusions

After one year of nitrogen applications, the first response to enhanced nitrogen deposition in jack pine uplands was a significant increase in the nitrogen content of ground lichens (Cladina mitis, Cladina stellaris) and moss (Pleurozium schreberi) which dominate the understory. There was no significant difference in other chemical parameters in ground or epiphytic lichen or moss among treatments. There was also no significant effect of N treatment on non-vascular plant cover, diversity and biomass. The study is designed to include 5 years of nitrogen applications, with results examined annually and at the conclusion of applications. It is expected that “early-warning” indicators to these repeated low-dose applications of nitrogen will be identified by the end of the study.