Changes in tree ring chemistry in sugar maple ( Acer saccharum) along an urban–rural gradient in southern Ontario

Abstract

Changes in xylem chemistry in sugar maple ( Acer saccharum Marsh.), sampled from four woodland sites along an urban–rural gradient were examined. Two sites were located 4 km apart within metropolitan Toronto, a third was adjacent to a major highway 24 km north-east of Toronto and a rural site was chosen 150 km to the north-east of Toronto along the prevailing wind direction. Concentrations of Pb and Cu in surface soil were elevated at the two city centre sites and the highway site compared to the rural woodland, but only the city site (which was located close to major roads) and the highway site had elevated Zn, Cr and V. Pb concentrations in tree rings were highest at the two city centre sites, but a steady decline from around 1 mg kg −1 dry wt in wood formed in the early 1920s to present levels of 0.5 mg kg −1 dry wt was recorded. In contrast, Pb levels were low at the highway site in wood formed prior to the 1940s, but increased dramatically to peak around 1.5 mg kg −1 dry wt in the 1950s and then declined during the 1960s to levels comparable to the city sites. Pb concentrations at the rural woodland were consistently less than 0.2 mg kg −1 dry wt. Concentrations of all other trace elements in xylem were approaching detection limits by inductively coupled plasma mass spectrometry (ICP–MS). No difference in Ca, Fe, K, Mg, Mn, P or Sr concentrations existed between sites; however, principal component analysis indicated that the distribution of Fe and P was similar in sugar maple xylem, increasing sharply in the outer rings. Ca and Sr were related, declining steadily from the pith to the cambium, whilst the distribution of Mg, K and Mn was similar by having no radial trend in sugar maple xylem.