Tree Ring and Soil Chemistry Responses of Pinus Banksiana and Populus Tremuloides to Changes in Atmospheric Environments in the Oil Sands Region of Alberta, Canada.

The Athabasca oil sands region, the largest area for open-pit oil sands mining in Alberta, has received a large amount of air pollutants over the past several decades. The impact of such pollution on ecosystems in the region is poorly understood. In this study we tested two hypotheses: 1) long-term chronic air pollution would change tree ring and soil chemistry in the Athabasca oil sands region, and 2) tree ring and soil chemistry response to pollution would be tree species specific. We investigated tree ring and soil chemistry (d¹³C, d¹⁵N, and Ca:Al ratio) of Pinus banksiana (jack pine) and Populus tremuloides (trembling aspen, aspen) stands in two watersheds: NE7 and SM8; NE7 has been exposed to greater acid deposition due to its closeness to the mining area. Gradual decrease of tree ring d¹³C over time was found in both tree species in both watersheds. Tree ring d¹³C of aspen were more negative than that of jack pine while not different between watersheds. Between 1982 and 2009, d¹³C in tree ring decreased by 0.053‰ yr^-1 for jack pine and 0.054‰ yr^-1 for aspen (p<0.001 for both tree species). The d¹⁵N of tree rings was not different between tree species and watersheds. Nevertheless, the difference of d¹⁵N between watersheds increased in the most recent tree rings and d¹⁵N of the forest floor was lower in NE7 than in SM8, reflecting higher deposition rates of ¹⁵N-depleted N in NE7. The molar Ca:Al ratio in tree rings was lower (p<0.01) in jack pine than in aspen without a clear trend over time. Soil pH in the forest floor and Ca:Al ratio in the forest floor and mineral soil were lower in jack pine than in aspen stands (p<0.05) while no difference was found between two watersheds. We conclude that changes in tree ring d¹³C, but not d¹⁵N or Ca:Al ratio, reflected the long-term effects of air pollution in the Athabasca oil sands region.