Cation binding in wood: applications to understanding historical changes in divalent cation availability to red spruce

Abstract

The ion exchange characteristics of red spruce (Picearubens Sarg.) bolewood were investigated to understand factors that control divalent cation and H+ concentrations in tracheid cell walls. Vertical concentration measurements of selected cations along the stem demonstrated that the alkaline earths showed the expected chromatographic fractionation, while other cations showed variable patterns relative to calcium. The concentration of cation binding sites (pectates) decreased in a predictable manner from the pith towards the cambium as a function of radius, not chronological age, explaining why in red spruce calcium concentrations are usually highest in the oldest wood. Red spruce wood from Maine and Tennessee had divalent base saturations of about 60 to 65% before the mid-1900s; however, divalent base saturations increased to about 75 to 80% in the mid-1900s, after which they decreased. Wood pH values were highest when divalent base saturation was highest. Calcium and potassium equilibration experiments were conducted, and the results were used to parameterize the Donnan model of cation exchange to understand why base saturation in woody tissues changes in different time periods. Increased divalent cation concentrations in red spruce wood formed during the 1940s to the 1970s are explainable by small increases in both sap pH and divalent cation concentrations and may be the signal of cation mobilization following increasing mineral acid anion deposition from the atmosphere.