Elemental concentrations in tree-rings from red and white oak trees at six sites across Southern Ontario, Canada, were assessed to determine whether they passively record changes in geochemical cycling in the presence of environmental stress. Periods of stress were defined as sustained periods with elevated δ13C values in tree-rings relative to atmospheric CO2 during the same period. In some trees, nutrient concentrations (Ca, Mg, Mn) were erratic during historic periods of stress while chemically similar non-nutrients (Ba, Sr) and the anthropogenic pollutant Pb were not. Tree-ring concentrations of Ca and Sr were related to bedrock type and leachable concentrations in the soil. In contrast, tree-ring concentrations of Mg were not related to bedrock type, although Mg concentration in the soil leachate was. Tree-ring Mn, Ba and Pb concentrations were not related to bedrock type or soil concentrations, but were inversely related to soil pH. Erratic behavior of nutrient elements during historic periods of stress suggests that some nutrient concentrations in the environment were not always passively recorded by tree-rings.
Red and white oak trees were sampled at six sites across Southern Ontario, Canada (Fig. 1). These sites were selected to cover a broad range of soil and bedrock types as well as varying distances from urban pollution sources such as Hamilton, Toronto and Sudbury (Fig. 1 and Table 1). Forests at each site are mixed coniferous and deciduous remnant second growth stands. The sites are all located within the natural range for red and white oak, and while not the dominant species at every stand, they are abundant. When possible, white oak was sampled preferentially to red oak because it has preferred characteristics for dendrochemical analysis (Cutter and Guyette, 1993). Red oak is less preferred due to higher heartwood permeability and moisture content (Cutter and Guyette, 1993).
Between three and ten dominant, visibly healthy trees were sampled at each site for potential chemical analysis. From each tree, eight cores were taken, two from each of four positions located at 90° intervals around the tree at breast height. Cores were taken using a 5 mm diameter increment corer. The corer was washed with ethanol followed by deionized water between trees and rinsed with deionized water between cores from the same tree. From these sampled trees, one to three trees from each site were selected for chemical analysis after an examination of the extracted core.
Additional trees were sampled at each site for cross-dating purposes. These cores were air-dried, mounted and sanded with increasing grit number sandpaper until individual growth rings were easily visible. Cores from the same site were cross-dated according to the principles of Stokes and Smiley (1968). Distinctive ring-width patterns were identified in the cross-dated core at the same site and used to cross-date trees among sites prior to dissection for chemical analysis.
For isotope analysis, cores from each position sampled around the bole were grouped and dissected into annual growth rings. The visibly distinct earlywood and latewood were separated in each growth ring and a sub-sample of the latewood from every even year was taken for carbon isotope analysis. α-Cellulose was prepared using a modification of the technique described by Loader et al. (1997) based on Green (1963). The carbon isotopic composition (δ13C value) of the α-cellulose fraction was measured using an Elemental Analyzer (EA) coupled to an Isotope Ratio Mass Spectrometer (IRMS) operating in Continuous Flow (CF) mode. EA-CF-IRMS analyses were performed using either a Carlo Erba NCS 2500 EA coupled to a Finnigan MAT 252 IRMS or a Costech ECS 4010 coupled to a Finnigan MAT Delta Plus XP in the Queen's Facility for Isotope Research. The δ13C values are reported in units of permil (‰) (Faure, 1986) relative to V-PDB. Using these techniques, NIST-19 returned a δ13C value of 1.95‰ and NIST-21 gave a δ13C value of − 28.1 ± 0.2‰. Replicate analyses of a laboratory α-cellulose standard (δ13C = − 24.3 ± 0.3‰) and selected samples indicate an uncertainty of 0.3‰ (2σ).