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TECHNICAL REPORTS

Plant and Environment Interactions

Extractability of Elements in Sugar Maple Xylem along a Gradient of Soil Acidity

^a Environment Canada, 105 McGill St., Montreal, QC, H2Y 2E7, Canada
^b Dép. des Sciences Biologiques, Faculté des sciences, Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montreal, QC, H3C 3P8, Canada
^c Direction de la Recherche Forestière, Forêt Québec, Ministère des Ressources Naturelles et de la Faune du Québec, 2700 rue Einstein, Sainte-Foy, QC, G1P 3W8, Canada
^d Dep. of Natural Resource Sciences, Macdonald Campus of McGill Univ., 21,111 Lakeshore Rd., Ste.-Anne-de-Bellevue, QC, H9X 3V9, Canada

^* Corresponding author (bilodeau-gauthier.simon{at}courrier.uqam.ca).

Received for publication April 23, 2007. Dendrochemistry has been used for the historical dating of pollution. Its reliability is questionable due primarily to the radial mobility of elements in sapwood. In the present study, the extractability of seven elements was characterized to assess their suitability for the monitoring of environmental conditions. Nine mature sugar maple trees (Acer saccharum Marsh.), a wide-ranging species in eastern North America that has suffered decline in past decades, were sampled in three Quebec watersheds along a soil acidity gradient. Five-year groups of annual tree rings were treated by sequential chemical extractions using extractants of varying strength (deionized H₂O, 0.05 M HCl, and concentrated HNO₃) to selectively solubilize the elements into three fractions (water-soluble, acid-soluble, and residual). Monovalent K; divalent Ba, Ca, Cd, Mg, Mn; and trivalent Al cations were found mostly in the water-soluble, acid-soluble, and residual fractions, respectively. Forms more likely to be mobile within the tree (water-soluble and acid-soluble) do not seem to be suitable for temporal monitoring because of potential lateral redistribution in sapwood rings. However, certain elements (Cd, Mn) were responsive to current soil acidity and could be used in spatial variation monitoring. Extractability of Al varied according to soil acidity; at less acidic sites, up to 90% of Al was contained in the residual form, whereas on very acidic soils, as much as 45% was found in the water-soluble and acid-soluble fractions. Sequential extractions can be useful for determining specific forms of metals as key indicators of soil acidification.