Our knowledge of the critical ecosystem services trees and forests play as a sink for anthropogenically perturbed contaminants such as mercury has developed rapidly over the past two decades. For mercury we have learned that the foliar assimilation of elemental mercury from the atmosphere represents the major deposition pathway of mercury to terrestrial systems. We have also learned that not all this mercury remains in foliage and some is transported down trees within the phloem. A fraction of this mercury is continually translocated from phloem to the hydroactive xylem (sapwood) and this has been suggested in some studies to be stored long-term in the growth rings of trees. Since, trees take up more mercury when there is more in the atmosphere, many studies have used the mercury stored in the growth rings of trees as historical archives for past atmospheric elemental mercury concentrations. However, there are studies that have reported differences in growth rings and reported local historical usage (inventories) of mercury, which have lead to questions in regards to the use of growth rings as atmospheric mercury archives. Evidence points towards the effectiveness of particular trees as historical archives for atmospheric mercury being species dependent.
Stable mercury isotopes are a very powerful tool for biogeochemists studying the environmental cycling of mercury because these analyses provide unique information that has the potential to trace both sources of mercury and the transformation (reaction/processes) that the mercury has undergone in a given sample. That is because specific reactions impart specific changes (fractionations) in the normal make up of mercury stable isotopes. Dr. McLagan recently published a study that used stable mercury to reveal critical information relating to both the use of trees to decipher historical industrial mercury usage (archiving) and the internal processing of mercury within trees. It is my intent to explore this much further to understand: (i) how and why certain species are better archiving species than others? (ii) What compounds and structures are responsible for sorbing and transporting mercury within trees? (iii) What processes affect the makeup of mercury within trees? (iv) What are the role of soils and fungi in this cycling of mercury in within trees? (v) What additional archiving related information can we learn from this method?
The FEWA Lab is building exciting collaborative projects within Canada (Southern Ontario) and internationally (South America, Central America, and USA) to explore these questions, including involvement in the incredible SPRUCE Project that will assess the impacts of climate change on boreal peat/forest ecosystems. In addition to applying mercury stable isotope analyses, these projects will look into tree physiological characterization and modelling components.