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Arctic Borderlands Ecological Knowledge Co-op

Indicator Assessment: Tree Ring Analysis for Metals Levels

Introduction

Some researchers have used tree ring samples to try and identify the presence of specific metal contaminants that have been taken up by the tree and laid down in their growth rings. In this way trees may be used as environmental indicators to answer two questions. First, by sampling trees over a certain area one might learn the spatial extent of contamination (e.g. downwind from a smelter). Secondly, if the metals are absorbed by trees and then deposited in annual growth rings, then a careful, ring by ring analysis, might reveal the timing or chronology of the contamination event. Overall these studies tend to be sophisticated, require complicated chemical analysis, and there are mixed opinions about the usefulness of this technique as a way of monitoring change in the environment.

I have not uncovered any published work that has been done in the Yukon using this technique. I have come across two recent references to works in Canada; one study that examined mercury in black spruce in Quebec (Li and Planas, 1995) and the second, a Masters thesis from McGill although I have been unable to contact the author (Reeves, 1995).

How the technique works

A report by Yanosky and Vroblesky (1995) used element analysis of tree rings to examine the spread of a variety of contaminants carried by ground water from an old munitions and chemical warfare dump. They used proton-induced X-ray emission spectroscopy (PIXE) to irradiate individual growth rings and detect numerous elements. They reported that nickel and iron levels in tree rings (of a known date) agreed with the timing of the original contamination event (i.e. the establishment of the munitions dump in the 1930s), as well as the down slope spread of ground water (i.e. trees closest to the dump developed high metal levels before trees further down slope).

Their study also documented one of the serious drawbacks of element analysis of tree rings. The technique assumes that elements are laid down in the growth ring of the year when the tree absorbs the element. Yanosky and Vroblesky (1995) determined that potassium was moved (translocated) laterally between sapwood (containing live cells) and heartwood (where cells are dead) and upset this assumption. Forget and Zayed (1995) assessed the problem of "radial translocation" and the challenge it presents to accurate interpretation of results. They cited over a dozen studies drawn between 1966 and 1981 that had examined the problem of translocation. They draw two conclusions. First translocation may explain some of the discrepancy in element distribution patterns reported by different authors. Secondly lateral translocation may be less of a problem in conifers than angiosperms.

Evaluation of the proposed indicator

There are three drawbacks to the technique of element analysis of tree rings:

  • Although research has been carried out on the methods for years I could find no emerging standardized protocol. This may reflect the diversity of tree species and situations that have been investigated. For example, Forget and Zayed (1995) cite five different methods of analyzing wood samples: atomic absorption spectroscopy, neutron activation analysis, proton-induced-X-ray emission spectrometry (PIXE), inductively coupled plasma optical emission spectroscopy and X-ray fluorescence spectroscopy.
  • What is the relationship between the ambient levels of the element in the environment and that recorded in trees used as bioindicators? The munitions dump study by Yanosky and Vroblesky (1995) described above showed good agreement for nickel and iron. Unfortunately the uptake pathway that trees use to incorporate elements from the environment are varied, e.g. uptake from the soil via roots as well as dry and wet deposition on leaves and bark. Therefore it seems possible that unique situations may undermine the generality of this technique.
  • As noted above, the problem of translocation, may occur although this is said to be less of a problem with conifers which might the tree of choice in the northern Yukon.

References

Forget, E. and Zayed , J. 1995. Tree-ring analysis for monitoring pollution by metals. In: Tree Rings as Indicators of Ecosystem Health, ed. Timothy E. Lewis, CRC Press, p. 157-176.

Li, Z., Jun-long, Q., Planas, D. 1995. Mercury concentrations in tree rings of black spruce (Picea mariana Mill. B.S.P.) in boreal Quebec, Canada. Water, Air and Soil Pollution 81: 163-173.

Reeves, A.I. 1995. Contaminant tracking through dendro-chemical analysis of tree-radii. M.S. thesis, McGill University, Canada. 103 pp.

Yanosky, T.M. and Vroblesky, D.A. 1995. Element analysis of tree rings in ground-water contamination studies. In: Tree Rings as Indicators of Ecosystem Health, ed. Timothy E. Lewis, CRC Press, p.177-205.

Written by S. Gilbert, March 1997.