Sequential Extractions of Elements in Tree Rings of Balsam Fir and White Spruce

Abstract

Many attempts have been made to reconstruct past soil chemistry from tree rings' total element concentrations. However, a few recent studies have shown that some elements are highly mobile within some tree species' sapwood, which may complicate the interpretation of temporal trends. To investigate element mobility in xylem of balsam fir (BF) and white spruce (WS), a dendrogeochemical method was utilized, which consists in sequentially extracting tree ring samples with water and diluted hydrochloric acid (HCl; 0.05 M) followed by a complete digestion in nitric acid (residual). The results show that, within the sapwood of BF and WS, potassium (K) and phosphorus (P) are found mostly in the water extract whereas divalent cations [calcium (Ca), magnesium (Mg), strontium (Sr), manganese (Mn), zinc (Zn), and barium (Ba)] are mainly present as soluble or exchangeable forms. Total xylem concentration generally decreased in the following order for both tree species: Ca > Mg > Mn > Zn, Ba > Sr. At the opposite, the vast majority of aluminum (Al) (>99%) and iron (Fe) (>95%) is found in the residual fraction, suggesting that these elements are not affected by radial reequilibration during circulation of the sap. Because Al soil availability is known to increase with decreasing pH, this element can potentially be used for past reconstruction of soil pH.

Keywords:

• Dendrogeochemical
• tree rings
• xylem