AN ELEMENTAL AND DENDROCHRONOLOGICAL ASSESSMENT OF TREMBLING ASPEN (POPULUS TREMULOIDES) AND WHITE SPRUCE (PICEA GLAUCA) SURROUNDING THE ATHABASCA OIL SANDS REGION IN ALBERTA, CANADA

Abstract

The Athabasca Oil Sands Region (AOSR) is a renowned source of air pollution in Alberta, Canada. Oil sands activities within the AOSR release a diverse set of contaminants into the atmosphere, which deposit onto surrounding boreal forest ecosystem components. Previous research has illustrated that airborne pollutant enrichment is occurring within a 50 km radius of the AOSR. Concentrations of these pollutants increase at areas in closer proximity to the AOSR. Boreal forest vegetation that lives within this 50 km zone of pollutant enrichment may be susceptible to alteration or adverse influences by these pollutants. To date, the impact of AOSR emissions on surrounding boreal forest species is largely unknown. Therefore, the objective of this study was to assess the impact of AOSR aerial deposition on two endemic tree species: trembling aspen (Populus tremuloides) and white spruce (Picea glauca). The radial growth and elemental composition of white spruce and trembling aspen stands, and associated soils, were compared at three study sites: a Control site upwind from the AOSR, to represent baseline radial growth and tree/soil elemental composition within the region, and two downwind sites, one at the edge of the AOSR and the second approximately 50 km from the oil sand facilities. At each site plant and soil samples and tree ring cores were collected and analyzed for radial growth and elemental composition using dendrochronology, soil science, dendrochemistry, X-Ray Fluorescence (XRF), and X-ray Absorbance Near Edge Structure (XANES) spectroscopy. These methods were utilized to assess whether trembling aspen and white spruce radial growth and tree chemistry were altered at sites downwind from the AOSR, and to determine if soil characteristics were similar and comparable across sites. White spruce stands from the two downwind sites showed a decline in radial growth compared to the Control site. A gradual and persistent decline in radial growth occurred for the last decade of growth at both downwind sites in white spruce, illustrating at threshold point in time of adverse impacts on the trees. More severe radial-growth declines occurred at the site closest to the AOSR. Radial growth did not change per site for trembling aspen, illustrating a different radial-growth response per species. Greater Mn fluorescence, using XRF, occurred in the majority of trembling aspen and white spruce tree components (leaves, trees, roots, cores) and topsoil at the downwind sites compared to the Control site. Topsoil Mn enrichment increased at sites in closer proximity to the AOSR, where radial-growth declines were most severe and aerial deposition from the AOSR is most concentrated. Excess Mn is known to be toxic to trees, suggesting that Mn enrichment may be causing a decline in radial growth in white spruce at the downwind sites. Manganese was elevated in the majority of trembling aspen tree media compared to white spruce potentially explaining different radial-growth responses per tree species at the downwind sites. XANES revealed the speciation of Mn to be the same in all white spruce tree materials (Mn(II)) but different in the leaf litter (Mn-oxides) at Site 0 km. Other elements observed did not show a distinct pattern, as seen with Mn. These results suggest that Mn may be a potential contaminant of concern to white spruce downwind and at a minimum of a 50 km radius from the AOSR. Aerial deposition from the AOSR may be adversely impacting nearby white spruce trees, which could lead to losses of resources to local forestry companies. Trembling aspen appears to be more resilient to Mn deposition as it did not illustrate the same radial-growth declines as white spruce. It is recommended that the AOSR monitor boreal forest ecosystem components downwind and, especially within a 50 km zone of their facilities to better understand aerial-fallout accumulations and any further adverse impacts from ongoing emission releases.