Elemental concentrations in tree rings by PIXE: statistical variability, mobility, and effects of altered soil chemistry

Abstract

Concentrations of up to 14 elements were determined from increment cores by proton-induced X-ray emission for dated series of annual rings of plantation Liriodendrontulipifera L. All trees were treated in 1965 with 841, 224, and 280 kg/ha of elemental N, P, K, respectively, and with either 0, 1683, or 3366 kg/ha of elemental Ca applied as pulverized agricultural limestone. Intraring elemental concentrations were not significantly different between earlywood and late-wood. Statistical variability of proton-induced X-ray emission elemental determinations was greatest for Al, Si, Cl, Cu, and Zn; concentrations of P, K, Ca, Mn, and Sr were least variable. Estimated standard errors among cores within trees averaged less than those among trees on a uniform site. Element mobility was tested in xylem core samples taken in 1965 and 1986. Liming significantly increased P, S, K, Ca, and Zn and decreased Mn in pre-1965 xylem. Retroactive liming effects were not detected for Mg, Al, Fe, Sr, or Ba; liming responses of Si, Cl, and Cu were unclear. Significant differences in elemental concentration independent of liming were detected between the 1965 and 1986 sampling times for P, K, Ca, Mn, Zn, Sr, and Ba. Of these, differences in P, K, and Mn may be due largely to heartwood formation between sampling times. Exchangeable Ca and Mg, followed by pH, were the most important soil variables in significant regressions with xylem elemental concentrations; only weak relationships were found with foliar elemental concentrations. Concentrations of Al, Si, Fe, and Cu in L. tulipifera xylem could be useful biomonitors of soil chemical changes by virtue of their apparent immobility; other elements, particularly S and Sr, may also be useful. However, expected time trends, especially as they pertain to heartwood formation, should be taken into account for each element.