- Email: [email protected]
Investigation of element variability in tree rings of young Norway spruce by laser-ablation-ICPMS
The Science of the Total Environment 219 Ž1998. 29]39
Investigation of element variability in tree rings of young Norway spruce by laser-ablation-ICPMS T. Prohaskaa,U , C. Stadlbauer a , R. Wimmer b , G. Stingeder a , Ch. Latkoczy a , E. Hoffmann c , H. Stephanowitz c a
b
Institute of Chemistry, Uni¨ ersity of Agricultural Sciences, Muthgasse 18, A-1190 Wien, Austria Center for En¨ ironmental Studies and Nature Conser¨ ation, Uni¨ ersity of Agricultural Sciences, Peter Jordan Straße 76, A-1190 Wien, Austria c Institut fur ¨ Spektrochemie und angewandte Spektroskopie, Rudower Chaussee 5, D-12489 Berlin, Germany Received 10 April 1998; accepted 1 June 1998
Abstract Laser ablation inductively coupled plasma mass spectrometry ŽLA-ICPMS. has been used to investigate the elemental variation for tree rings of 20-year-old clonal Norway spruce trees that grew nearby an aluminum smelter. Four years prior to analysis this source of fluoride pollution was shut down and the subsequent variation of the concentration of elements in the tree ring according to their mobility in the tree stem was investigated by means of a LA-ICPMS. The elements Al, Fe, Ca, Ba and Sr showed a significant increase in values after the pollution shutdown. Elements with similar chemical behavior and mobility show similar patterns of distribution. Circular investigations of tree rings show a variation of up to 60% of the elemental concentration depending on the geographical direction following the changing properties Že.g density. of wood. The circular pattern correlate again for elements with similar properties and function. Furthermore, wood which develops in response to unbalanced mechanical pressure, such as wind for example Žso called compression wood. shows significant differences in element concentration compared with unstressed wood. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Dendrochemistry; Xylem; LA-ICPMS; Tree ring; Picea abies; Spruce; Elements
1. Introduction Past experience has indicated a strong link between plant tissue chemistry and soil- and bed-
U
Corresponding author. Tel.: q43 1 360066092; fax: q43 1 360066059; e-mail: [email protected]
rock-chemistry, the elements abundant of the chemical in tree rings may be used to assess differences in pollution levels and other environmental changes among forest sites ŽSymeonides, 1979; Cutter and Guyette, 1993; Halbwachs et al., 1993; DeWalle et al., 1995; Forget and Zayed, 1995.. Laser ablation inductively coupled plasma mass
0048-9697r98r$ - see front matter Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00224-1
30
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
spectrometry ŽLA-ICPMS. is a well established method for direct elemental trace and ultratrace analysis on solid samples which allows contamination free ablation Žno pretreatment necessary . and high spatial resolution Ž1]100 m m.. Therefore LA-ICPMS is currently the only well established technique which allows direct access to the elemental data stored in incrementally grown layers of biological structures ŽOutridge et al., 1995.. Thus LA-ICPMS is also widely used in dendrochemical studies to record pollution fluctuations by monitoring trace elements in single tree rings ŽHoffmann et al., 1994, 1996; Watmough et al., 1997; Garbe-Schonberg et al., 1997. ¨ and is highly advantageous to wet digestion procedures ŽWatmough and Hutchinson, 1996.. Kuhn et al. Ž1995. monitored the uptake of Mg, Ca and K by stable isotope labeling and analysis of root material by a laser mocroprobe mass analyser. The content of the chemical elements in annually formed tree rings are controlled by factors such as the cation binding capacity of woody tissue, radial growth rates, transformation of sapwood into heartwood and processes of radial translocation of elements in the tree stem. Atmospheric pollutants, secondary changes in soil chemistry or any other factors stimulating or reducing tree growth, can influence xylem chemistry, reflecting the actual condition of the tree ŽWimmer and McLaughlin, 1996.. The main focus of the current study was to demonstrate sources of element variability within tree rings of young spruce trees as caused by internal and external factors. 2. Experimental
and Kronberger, 1979; Halbwachs et al., 1993.. The fluoride pollution source was shut down in 1992 and in autumn 1995, after 3 years of growth in a pollution free environment, three stem disks from each of the four selected clones were prepared. The trees analyzed in this study provided an excellent basis to study element variability since all ramets of a clone share the identical genetic constitution. Trees were harvested and an approx. 2-cm thick stem disk was removed in approx. 1 m above the base of the tree. Details about the trees used in this study Žstem diameter and number of rings. are given in Table 1. For further analysis the disks were cut into sectors by means of a platinum mortise chisel to be suited for mounting in the sample cell of the laser ablation apparatus. Investigations about contamination levels by means of the Pt tool showed no significantly elevated platinum level in the resultant digested solutions. 2.2. Instrumentation A Perkin Elmer SCIEX Laser Sampler Model 320 was used for sample ablation. The system uses a Nd:YAG pulsed laser Žfundamental wavelength 1064 nm. pumped by a xenon flashlamp, which can be operated in either Q-switch or free running mode and may be used in a single pulse mode or a 10-Hz repetition rate resulting in a continuous signal. The sample was placed in an air tight sample cell on a motor driven sample stage and coupled to a Perkin Elmer ELAN 5000 mass spectrometer with a 40-MHz free running RF generator producing the high temperature Ar-plasma as ionization source.
2.1. Samples In 1971, a clonal test experiment was initiated using 400 Norway spruce w Picea abies ŽL.. Karst.x trees obtained by vegetative propagation. Trees were planted on a site close to an aluminum smelter in Ranshofen, upper Austria and were exposed to gaseous hydrogen fluoride as described by Halbwachs and co-workers ŽHalbwachs
Table 1 Origin
Examined stem diameter Žcm.
Number or rings
TogingrGermany ¨ RuhrgebietrGermany Poland RanshofenrAustria
6.5]8.5 6.8]7.5 4.5]6 9.5]13.5
15 15 12 14
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
2.3. Optimization of the laser parameters Due to the inhomogeneous sample composition and ablation, internal standardization is necessary for accurate quantitative measurements. The 13 C isotope was chosen for internal standardization since its signal intensity in the mass spectrum has the same order of magnitude as the signal intensities of the investigated isotopes what is a prerequisite for correct internal standardization in ICPMS analysis ŽHoffmann et al., 1994.. Test measurements were carried out in both Q-switch and free running mode using energies between 130 and 350 mJ, showing the Q-switch mode to be most appropriate. The Q-switch mode generated smooth, deep and narrow craters whereas under free running mode conditions the woody tissue was burnt around the edges and which led to smeared craters.
31
Depending on the energy of the laser the material volatilized during one LA-analysis amounts between 1 and 2 mg leading to a crater between 0.5 and 2.5 mm in diameter on the cross-sectional surface of the sample. The distance between the single craters was approx. 2 mm. In each growth ring 14 adjacent craters were ablated as far as the rings had enough area to fit them. Ablation was performed mostly in the spring built parts of the tree ring, the so called earlywood, beginning from the outermost ring adjacent to the bark towards the pith. The geographical direction of the trunk was not taken into account. To remove contamination preablation was performed prior to the actual analysis. The intensive Q-switch pulses are penetrating the tissue much faster and more efficiently than in the free running mode. Fig. 1 shows that the signal intensity reaches a constant level after approx. 20 s indicat-
Fig. 1. Preablation of the contaminated sample surface on the example of Mg and Ca by laser ablation. The laser is running in Q-switch mode at 190 mJ. 13 C was used as a reference.
32
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
ing that the total surface contamination is ablated. The data obtained were accumulated on an ELAN host-computer and then transferred to an external computer for further handling. The parameters optimized for laser sampling of spruce wood are given in Table 2. The verification of the LA data was performed by digestion of selected samples and subsequent analysis of the solution by ICPMS using a Meinhard nebulizer as introduction system. Digestion was performed in a high pressure asher ŽHPA.. An amount of sample Ž0.3 g. was weighed into 30 ml quartz cups, 3 ml of sub-boiled nitric acid Ž65%. doped with 10 ppb scandium for internal standardization was added and the vessel was sealed and stored overnight. After digestion, the solution, which was free of wood residues, was diluted to 10 ml with sub-boiled water. 2.4. Calibration Wood standards for calibration are not available for LA-ICPMS. Cellulose, as the major constituent of wood, has a composition similar to most types of wood. For calibration six pellets of increasing multielement concentration were prepared. Cellulose powder Ž0.6 g. ŽAldrich, diameter approx. 20 m m. was doped with multielement standard solutions and pressed into pellets under a pressure of 110 MPa for 1 min. The pellets were introduced into the laser sample chamber and analyzed by the same procedure as the wood samples using 13 C as a reference. A relative S.D. - 10% could be found. The calibration graphs are linear within the concentration range of interest for all measured elements. The accuracy of Table 2 Laser type
Nd-YAG-Laser, 1064 nm
Operating mode Energy Frequency Preablation Analysis time Replicates Number of craters
Q-switch 170 mJ 10 Hz 20 s 45 s 15 14
the calibration by LA-ICPMS was verified by digestion of the pellets in the same way as the wood samples and subsequent measurement by ICPMS. The results agree within the confidence limits Ž95%.. The limits of detection found for LAICPMS of tree rings are specified in more detail by Hoffmann et al. Ž1994.. 3. Results and discussion 3.1. Radial distribution of elements Eight consecutive rings starting with the most recently formed ring in 1995 were measured towards the pith. Hence, the data comprise of laser sampling values for the period 1988]1995. The elemental concentrations for each year were averaged across the ramets of a clone. Because of the young age Ž12]15 years. of the stem disks the analyzed rings are considered to be mostly juvenile. Therefore only a small portion of the crosssection could be attributed to heartwood. It was found with a 18-year-old spruce tree, that between 10 and 12 tree rings are involved in water transportation ŽCutter and Guyette, 1993.. The conducting system is most efficient in the outermost rings, but it is likely that a higher proportion of tree rings are more or less involved in conducting water. As a consequence, translocation of elements in the xylem tissue may smear the trend signals to be examined and the analysis of an individual ring does not pinpoint the specific year of environmental change ŽSymeonides, 1979; Cutter and Guyette, 1993.. The elements investigated were grouped according to their horizontal mobility in the tree stem. According to Cutter and Guyette Ž1993., element mobility is based on a number of factors such as ion solubility, chargerionic radius ratio and essential nature. Sap pH, bonding in the xylem matrix as well as heartwood]sapwood equilibrium concentration are additional factors present in tree stem. For example the elemental distributions of Ca and Al and Fe are shown in Fig. 2a]c. 3.2. Elements of high mobility: Mg Magnesium is a macronutrient with high mobil-
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
33
Fig. 2. Ža. Comparison of elemental concentrations in growth rings of four different clones: concentration distribution of Ca. Žb. Comparison of elemental concentrations in growth rings of the four different clones: concentration distribution of Al. Žc. Comparison of elemental concentrations in growth rings of the four different clones: concentration distribution of Fe.
34
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
ity in the xylem and its concentration is influenced by soil pH and Al content ŽKuhn et al., 1995.. The mean curve of the clone that originates from Ranshofen shows an overall trend which is different from the other three clones. The concentration levels towards the most recent formed tree rings increases, whereas the other clones reveal no pronounced radial trends. It can be concluded that the clone from Ranshofen responds more rapidly to changed environmental conditions. 3.3. Elements of moderate mobility: Ca, Sr, Ba, Zn, Co, Cr and Mn In general, the radial trends of the elements such as Ca, Mg, Sr, Mn and Zn are negative which means that the values decline from pith to bark ŽTendel and Wolf, 1988.. This is also true for most of our curves and no obvious change is observed after the shutdown of the pollution source. The only exception is for the mean curve of the Ranshofen clone, which originates from the experimental site. For the elements Ca, Sr and Ba, an upward trend can be seen from pith to bark starting in 1992. It can be inferred that these trees were most adapted to the local environmental conditions. The terminated fluoride deposition has started to alter the soil condition, which has effected as a consequence the Ca concentration in the xylem. ŽDeWalle et al., 1991.. Furthermore, physiologically active Ca Žand possibly other divalent cations. can form stable complexes, e.g. CaF2 ŽScheffer, 1984.. The conclusion is that the reduced fluoride deposition has made additional Ca Žand other divalent cations. available for the plants ŽCronan and David, 1995.. Kuhn et al. Ž1995. observed that the absolute Ca content in the cell walls of the roots of Norway spruce was moderately reduced by low pH values and strongly reduced in the presence of Al. Additional research is required to shed light behind these complex plant]soil interactions. It could be evident that the other clones, which did not show responses within the 3-years after shutdown, might show such responses years later. Other evidence comes from the radial growth
rate of the tree rings formed after the shutdown. The radial growth rates accelerated significantly after the pollution has stopped. Goransson and ¨ Eldhuset Ž1991. reported on the effects of aluminum on growth and nutrient uptake and observed reductions in growth rate and a reduced Ca uptake when adding Al. The trends for the elements Ba, Sr, and Ca are very similar. This is especially true for Sr and Ca, which are incorporated into the woody tissue in a similar way ŽMomoshima et al., 1995; Wimmer and McLaughlin, 1996.. The radial distribution of Mn, Co and Cr is stable across the clones, without any particular trends. The Toging clone trees ¨ exhibited the lowest element concentrations among all clone trees. 3.4. Elements of low mobility: Al, Pb, Fe, Cd Aluminium and Fe show a similar pattern whereas Pb reacts differently. The recent increase of Al and Fe indicates an enhanced Al uptake from the soil, although it is not clear if this is related to soil acidification ŽArp and Manasc, 1988.. The onset of this increase can be dated to the years 1992 and 1993. It is feasible that this change could be related to the shutdown of the pollution source with the Ranshofen clone responding most significantly. It was found that Al and water soluble fluorine was significantly correlated Ž r s 0.93. due to its strong complexation ŽWenzel and Blum, 1992.. The drastic reduction Žto zero. of fluoride deposition released Al in the soil from fluorine complexes and possibly more Al was made available to the trees. A high Al influx can be related to toxic effects on the fine root system with increased blockage of Ca uptake ŽSchlegel et al., 1992; Forget and Zayed, 1995.. A combination of pH 4.5 and increased free Al reduced Ca binding by 83% ŽSchroder et al., ¨ 1988.. The radial element concentrations of Cd and Pb in the tree stems were lowest in the recently built rings whereas maximum values were close to the pith. This observed decrease from pith to bark is in accordance with other findings by Hagemeyerand co-workers ŽHagemeyer et al., 1992; Hagemeyer and Lohrie, 1995; Hagemeyer and Weinand, 1996..
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
35
Fig. 3. Ža. Circular distribution of the concentrations of Ca and Sr in the growth ring of the year 1993 of one Ruhrgebiet clone. Both distributions show similar pattern. Žb. Circular distribution of the concentrations of Al and Fe in the growth ring of the year 1993 of one Ruhrgebiet clone. Žc. Circular distribution of the concentration of Cr in the growth ring of the year 1993 of one Ruhrgebiet clone.
36
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
The shutdown of the pollution source has altered the biological activity of the humus layer ŽWenzel, 1990. which recuperated to some extent and consequently effected the trees. The changed conditions are seen best with the Ranshofen clone. 3.5. Circular scanning Within the growth ring 1993 of one Ruhrgebiet ramet, the element concentrations were analyzed in the tangential direction, following the circular path of the tree ring. Selected results are displayed as contour lines in Fig. 3a]c and considerable fluctuations are visible along this tree ring. As similar inconsistencies can be assumed to be valid for all the trees, a tree side dependency of the elemental concentration within a single tree ring with a variation up to 60% was observed. This variation can be explained by the changing proportion of compression wood at the various sides of the tree stem. Compression wood formation is mainly linked to the frequency and the direction of wind since it is formed on the stressed side of the tree. The effects of compression wood are explained in more detail in the following paragraph. The results of the circular scanning show clearly that the success of a dendrochemical study is highly governed by a very careful and consistent sampling strategy. The circular scanning demonstrates again the correlation between elements with similar properties and functions in the wooden tissue. The earth alkaline elements Ca, Sr and Ba were observed to behave similarly Žsee Fig. 3a.: Ca and Sr as divalent cations have a similar ionic radius. Therefore these elements substitute each other in various biological reactions. Al and Fe, considered to exhibit low mobility, are displayed in Fig. 3b and show very unstable concentration pattern along the circular path within the investigated tree ring. Elements, such as Cr display very stable distribution around the circular line ŽFig. 3c.. 3.6. Compression wood This specialized type of woody tissue is formed only in gymnosperms. Compression wood typically develops in response to unbalanced mechanical
Fig. 4. Scanned trunk with compression wood and its opposite wood.
pressure, such as wind, snow or slope, or can be formed due to the close proximity of another tree. The wood on the stressed side of the tree is observed to be compressed ŽFig. 4.. The growth rings are wider with higher density and appear darker due to the enhanced lignin and reduced cellulose content. An increased concentration of elements is expected in compression wood due to these differences in wood chemistry. To some extent, compression wood behaves similar to juvenile wood ŽTimell, 1986.. The unstressed wood has more cellulose than the normal wood. As 60]70% of the cations bind to hydroxyl groups, the lignin rich compression wood provide more binding sites than the opposite wood. This phenomenon was visualized by LA-ICPMS measurements with a specimen of the Toging ¨
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
37
Fig. 5. Ža. Concentration distribution of Ca in compression wood and opposite wood of a Toging clone. Žb. Concentration ¨ distribution of Al in compression wood and opposite wood of a Toging clone. Žc. Concentration distribution of Cr in compression ¨ wood and opposite wood of a Toging clone. ¨
38
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
clones with pronounced compression wood. The results for Ca, Al and Cr are shown in Fig. 5. The compression wood shows significantly higher concentrations Žby a factor of 4]6. for all investigated elements Žwith the exception of Mn.. 4. Conclusions The analytical potential of LA-ICPMS was used to determine the elemental variation of individual growth rings, to establish radial and circular element trends in tree rings. We were able to highlight the importance of taking into account the stem sides as source of element variation. In addition, elements with similar chemical behavior and mobility show similar patterns in single growth rings. On one disk we have also demonstrated the dramatic difference between compression and opposite wood. Three years after the shutdown of the aluminum smelter, changes in element uptake from the soil were found. An important question is why elements show specific radial trends while others do not. The sap in a tree refers to the liquid that flows in the apoplast, primarily in the conducting elements of the wood and most elements do not follow simple diffusion or mass-flow kinetics from the soil to the stem ŽSmith et al., 1996.. Very high concentrations of metals such as Pb in the soil may not be detected in the tree rings of fast-growing trees, as showed with Populus marilandica ŽLukaszewski et al., 1993.. Most trees show a decline in cation concentration with increasing tree age which is linked more to the binding exchange properties in the woody tissue than to the sap]soil chemistry. Another general problem is the question if the analysis of individual rings can pinpoint a particular year of environmental change. If this is the case, a ring by ring solution is provided with the LA-ICPMS technique. However, some metals in the wood are easily translocated in the apoplast Žcell walls. and also symplast Žcytoplasma. within the tree rings involved in water conduction Žsapwood.. Therefore the presence of an element in a specific tree ring to a precise year does not necessarily mean that the element was there in the tree environment of that year. This is espe-
cially true for very young trees with no distinct heartwood formation. This conclusion was also drawn by workers who have analyzed 5-year-old spruce trees ŽDeWalle et al., 1995.. The radial distribution of Pb did not reflect an applied change in soil Pb synchronically. As the age of the investigated stem disks are between 12 and 15, heartwood formation might have just started a few years ago. This could have resulted in significant element trends as seen for Al, Fe, Sr, Ba and Ca in some of the sample trees. These trees might have just started to form heartwood while the others have not. The higher concentrations of elements of the alkaline earth cations as well as Al and Fe in the more recently formed tree rings could indicate that these rings are conducting more efficiently than in the previous years. In dendrochemical studies of tree species with many tree rings present in the sapwood, the ringby-ring analysis may not be a cost-effective way of analysis. In this case, pooling samples over decades or even bi-decadal periods may be more realistic ŽSmith et al., 1996.. However, for tree species with very quick heartwood formation, such as Juniperus, Thuja, Robinia, Pseudotsuga or Larix, for intra-ring variability, for wound-initiated discoloration and other pathological wood formations, a high spatial resolution detection as achieved by LA-ICPMS is a very efficient and useful technique in dendrochemistry. Acknowledgements We would like to acknowledge laboratory assistance by A. Pederzolli, Ch. Ludke and M. ¨ Grabner. Thanks are due to Drs H. Steinkellner and J. Glossl ¨ for testing the genetical identity of our clones. Financial support was provided by the GOS ŽGesellschaft zur Forderung angewandter ¨ Optik, Optoelektronik, Quantenelektronik und Spektroskopie.. References Arp PA, Manasc J. Red spruce stands downwind from a coal-burning power generator: tree ring analysis. Can J For Res 1988;18:251]264. Cutter B, Guyette R. Anatomical, chemical and ecological
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39 factors affecting tree species choice in dendrochemistry studies. J Environ Qual 1993;22:611]619. Cronan CS, David FG. Use of calciumraluminum ratios as indicators of stress in forest ecosystems. J Environ Qual 1995;24:209]226. DeWalle DR, Swistock BR, Sayre RG, Sharpe WE. Spatial variations of sapwood chemistry with soil acidity in appalachian forests. J Environ Qual 1991;20:486]491. DeWalle D, Sharpeand W, Swistock B. Dendrochemistry and the soil chemistry environment. In: Lewis TE, editor. Tree rings as indicators of ecosystem health. Boca Raton, Florida: CRC Press, 1995:157]176. Forget E, Zayed J. Tree ring analysis for monitoring pollution by metals. In: Lewis TE, editor. Tree rings as indicators of ecosystem health. Boca Raton, Florida: CRC Press, 1995:157]176. Garbe-Schonberg CD, Reimann C, Pavlov VA. Laser ablation ¨ ICP-MS analyses of tree-ring profiles in pine and birch from N Norway and NW Russia } a reliable record of the pollution history of the area? Environ Geol 1997;32r 1:9]16. Goransson A, Eldhuset TD. Effects of aluminium on growth ¨ and nutrient uptake of small Picea abies and Pinus syl¨ estris plants. Trees 1991;5:136]142. Hagemeyer J, Lulfsmann A, Perk M, Breckle SW. Are there ¨ seasonal variations of trace element concentrations ŽCd, Pb, Zn. in wood of Fagus trees in Germany? Vegetatio 1992;101:55]63. Hagemeyer J, Lohrie K. Distribution of Cd and Zn in annual xylem rings of young spruce Ž Picea abies L. wKarstx. grown in contaminated soil. Trees 1995;9:195]199. Hagemeyer J, Weinand T. Radial distribution of Pb in stems of young Norway spruce trees grown in Pb-contaminated soil. Tree Physiol 1996;16:591]594. Halbwachs G, Kronberger W. Das Verhalten F- und SO 2 -resistenter Fichtenklone verschiedener Herkunft in einem Rauchschadensgebiet. Zb ŽMitteilungen. Int Verband forstlicher Versuchsanstalten 1979;1r390:259]268. Halbwachs G, Bellusova M, Nemetz C, Novak H, Wimmer R, Zehetner H. Schriftenreihe der Sektion I d. Bundesminist Umwelt, Jugend Familie 1993;3r1:211]215. Hoffmann E, Ludke C, Scholze H, Stephanowitz H. Analytical ¨ investigations of tree rings by laser ablation ICP-MS. Fres J Anal Chem 1994;350:253]259. Hoffmann E, Stephanowitz H, Skole J. Investigations of the migration of elements in tree rings by Laser-ICP-MS. Fres J Anal Chem 1996;355:690]693. Kuhn AJ, Bauch J, Schroder WH. Monitoring uptake and ¨ contents of Mg, Ca and K in Norway spruce as influenced by pH and Al, using microprobe analysis and stable isotope labelling. Plant Soil 1995;168r169:135]150. Lukaszewski Z, Siwecki R, Opydo J, Zembrzuski W. The
39
effect of industrial pollution on copper, lead, zinc and cadmium concentration in xylem rings of resistant Ž Populus marandica. and sensitive Ž Populus balsamifera. species of poplar. Trees 1993;7:169]174. Outridge PM, Veinott G, Evans RD. Laser ablation ICP-MS analysis of incremental biological structures: archives of trace-element accumulation. Environ Rev 1995;3:160]170. Momoshima N, Eto I, Kofuji H, et al. Distribution and chemical characteristics of cations in annual rings of Japanese cedar. J Environ Qual 1995;24:1141]1149. Scheffer F, Lehrbuch der Bodenkunde, Enke, Stuttgart, 1984. Schlegel H, Amundson RG, Huttermann A. Element distribu¨ tion in red spruce Ž Picea rubens. fine roots; evidence for aluminium toxicity at Whiteface Mountain. Can J For Res 1992;22:1132]1138. Schroder WH, Bauch J, Endeward R. Microbeam analysis of ¨ Ca exchange and uptake in the fine roots of spruce: influence of pH and aluminium. Trees 1988;2:96]103. Smith KT, Shortle WC. Tree biology and dendrochemistry. In: Dean JS, Meko DM, Swetnam TW Žeditors., Tree rings, environment and humanity. Tuscon: Radiocarbon, 1996: 629]635. Symeonides C. Tree ring analysis for tracing the history of pollution: application to a study in northern Sweden. J Environ Qual 1979;8r4:482]486. Tendel J, Wolf K. Distribution of nutrients and trace elements in annual rings of pine trees as an indicator of environmental changes. Experientia 1988;44:975]980. Timell TE. Compression wood in gymnosperms, 3 vols. Berlin, Heidelberg, New York: Springer, 1986. Watmough SA, Hutchinson TC. Analysis of tree rings using inductively coupled plasma mass spectrometry to record fluctuations in a metal pollution episode. Environ Pollut 1996;93:93]102. Watmough SA, Hutchinson TC, Evans D. Application of laser ablation inductively coupled plasma mass spectrometry in dendrochemical analysis. Environ Sci Technol 1997;31: 114]118. Wenzel WW. Bodenbelastungen durch Fluor und Schwermetalle im Immissionsbereich der Aluminiumhutte ¨ Ranshofen und ihre Auswirkungen auf Bodenzustand und Bodenfunktion. Vienna: PhD thesis submitted at the University of Agricultural Sciences, 1990. Wenzel WW, Blum WEH. Effects of fluorine deposition on the chemistry of acid Luvisols. Int J Environ Anal Chem 1992;46:223]231. Wimmer R, McLaughlin SB. Possible relationships between chemistry and mechanical properties in the microstructure of red spruce xylem. In: Dean JS, Meko DM, Swetnam TW, editors. Tree rings, environment and humanity. Tuscon: Radiocarbon, 1996:659]668.
Investigation of element variability in tree rings of young Norway spruce by laser-ablation-ICPMS T. Prohaskaa,U , C. Stadlbauer a , R. Wimmer b , G. Stingeder a , Ch. Latkoczy a , E. Hoffmann c , H. Stephanowitz c a
b
Institute of Chemistry, Uni¨ ersity of Agricultural Sciences, Muthgasse 18, A-1190 Wien, Austria Center for En¨ ironmental Studies and Nature Conser¨ ation, Uni¨ ersity of Agricultural Sciences, Peter Jordan Straße 76, A-1190 Wien, Austria c Institut fur ¨ Spektrochemie und angewandte Spektroskopie, Rudower Chaussee 5, D-12489 Berlin, Germany Received 10 April 1998; accepted 1 June 1998
Abstract Laser ablation inductively coupled plasma mass spectrometry ŽLA-ICPMS. has been used to investigate the elemental variation for tree rings of 20-year-old clonal Norway spruce trees that grew nearby an aluminum smelter. Four years prior to analysis this source of fluoride pollution was shut down and the subsequent variation of the concentration of elements in the tree ring according to their mobility in the tree stem was investigated by means of a LA-ICPMS. The elements Al, Fe, Ca, Ba and Sr showed a significant increase in values after the pollution shutdown. Elements with similar chemical behavior and mobility show similar patterns of distribution. Circular investigations of tree rings show a variation of up to 60% of the elemental concentration depending on the geographical direction following the changing properties Že.g density. of wood. The circular pattern correlate again for elements with similar properties and function. Furthermore, wood which develops in response to unbalanced mechanical pressure, such as wind for example Žso called compression wood. shows significant differences in element concentration compared with unstressed wood. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Dendrochemistry; Xylem; LA-ICPMS; Tree ring; Picea abies; Spruce; Elements
1. Introduction Past experience has indicated a strong link between plant tissue chemistry and soil- and bed-
U
Corresponding author. Tel.: q43 1 360066092; fax: q43 1 360066059; e-mail: [email protected]
rock-chemistry, the elements abundant of the chemical in tree rings may be used to assess differences in pollution levels and other environmental changes among forest sites ŽSymeonides, 1979; Cutter and Guyette, 1993; Halbwachs et al., 1993; DeWalle et al., 1995; Forget and Zayed, 1995.. Laser ablation inductively coupled plasma mass
0048-9697r98r$ - see front matter Q 1998 Elsevier Science B.V. All rights reserved. PII S0048-9697Ž98.00224-1
30
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
spectrometry ŽLA-ICPMS. is a well established method for direct elemental trace and ultratrace analysis on solid samples which allows contamination free ablation Žno pretreatment necessary . and high spatial resolution Ž1]100 m m.. Therefore LA-ICPMS is currently the only well established technique which allows direct access to the elemental data stored in incrementally grown layers of biological structures ŽOutridge et al., 1995.. Thus LA-ICPMS is also widely used in dendrochemical studies to record pollution fluctuations by monitoring trace elements in single tree rings ŽHoffmann et al., 1994, 1996; Watmough et al., 1997; Garbe-Schonberg et al., 1997. ¨ and is highly advantageous to wet digestion procedures ŽWatmough and Hutchinson, 1996.. Kuhn et al. Ž1995. monitored the uptake of Mg, Ca and K by stable isotope labeling and analysis of root material by a laser mocroprobe mass analyser. The content of the chemical elements in annually formed tree rings are controlled by factors such as the cation binding capacity of woody tissue, radial growth rates, transformation of sapwood into heartwood and processes of radial translocation of elements in the tree stem. Atmospheric pollutants, secondary changes in soil chemistry or any other factors stimulating or reducing tree growth, can influence xylem chemistry, reflecting the actual condition of the tree ŽWimmer and McLaughlin, 1996.. The main focus of the current study was to demonstrate sources of element variability within tree rings of young spruce trees as caused by internal and external factors. 2. Experimental
and Kronberger, 1979; Halbwachs et al., 1993.. The fluoride pollution source was shut down in 1992 and in autumn 1995, after 3 years of growth in a pollution free environment, three stem disks from each of the four selected clones were prepared. The trees analyzed in this study provided an excellent basis to study element variability since all ramets of a clone share the identical genetic constitution. Trees were harvested and an approx. 2-cm thick stem disk was removed in approx. 1 m above the base of the tree. Details about the trees used in this study Žstem diameter and number of rings. are given in Table 1. For further analysis the disks were cut into sectors by means of a platinum mortise chisel to be suited for mounting in the sample cell of the laser ablation apparatus. Investigations about contamination levels by means of the Pt tool showed no significantly elevated platinum level in the resultant digested solutions. 2.2. Instrumentation A Perkin Elmer SCIEX Laser Sampler Model 320 was used for sample ablation. The system uses a Nd:YAG pulsed laser Žfundamental wavelength 1064 nm. pumped by a xenon flashlamp, which can be operated in either Q-switch or free running mode and may be used in a single pulse mode or a 10-Hz repetition rate resulting in a continuous signal. The sample was placed in an air tight sample cell on a motor driven sample stage and coupled to a Perkin Elmer ELAN 5000 mass spectrometer with a 40-MHz free running RF generator producing the high temperature Ar-plasma as ionization source.
2.1. Samples In 1971, a clonal test experiment was initiated using 400 Norway spruce w Picea abies ŽL.. Karst.x trees obtained by vegetative propagation. Trees were planted on a site close to an aluminum smelter in Ranshofen, upper Austria and were exposed to gaseous hydrogen fluoride as described by Halbwachs and co-workers ŽHalbwachs
Table 1 Origin
Examined stem diameter Žcm.
Number or rings
TogingrGermany ¨ RuhrgebietrGermany Poland RanshofenrAustria
6.5]8.5 6.8]7.5 4.5]6 9.5]13.5
15 15 12 14
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
2.3. Optimization of the laser parameters Due to the inhomogeneous sample composition and ablation, internal standardization is necessary for accurate quantitative measurements. The 13 C isotope was chosen for internal standardization since its signal intensity in the mass spectrum has the same order of magnitude as the signal intensities of the investigated isotopes what is a prerequisite for correct internal standardization in ICPMS analysis ŽHoffmann et al., 1994.. Test measurements were carried out in both Q-switch and free running mode using energies between 130 and 350 mJ, showing the Q-switch mode to be most appropriate. The Q-switch mode generated smooth, deep and narrow craters whereas under free running mode conditions the woody tissue was burnt around the edges and which led to smeared craters.
31
Depending on the energy of the laser the material volatilized during one LA-analysis amounts between 1 and 2 mg leading to a crater between 0.5 and 2.5 mm in diameter on the cross-sectional surface of the sample. The distance between the single craters was approx. 2 mm. In each growth ring 14 adjacent craters were ablated as far as the rings had enough area to fit them. Ablation was performed mostly in the spring built parts of the tree ring, the so called earlywood, beginning from the outermost ring adjacent to the bark towards the pith. The geographical direction of the trunk was not taken into account. To remove contamination preablation was performed prior to the actual analysis. The intensive Q-switch pulses are penetrating the tissue much faster and more efficiently than in the free running mode. Fig. 1 shows that the signal intensity reaches a constant level after approx. 20 s indicat-
Fig. 1. Preablation of the contaminated sample surface on the example of Mg and Ca by laser ablation. The laser is running in Q-switch mode at 190 mJ. 13 C was used as a reference.
32
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
ing that the total surface contamination is ablated. The data obtained were accumulated on an ELAN host-computer and then transferred to an external computer for further handling. The parameters optimized for laser sampling of spruce wood are given in Table 2. The verification of the LA data was performed by digestion of selected samples and subsequent analysis of the solution by ICPMS using a Meinhard nebulizer as introduction system. Digestion was performed in a high pressure asher ŽHPA.. An amount of sample Ž0.3 g. was weighed into 30 ml quartz cups, 3 ml of sub-boiled nitric acid Ž65%. doped with 10 ppb scandium for internal standardization was added and the vessel was sealed and stored overnight. After digestion, the solution, which was free of wood residues, was diluted to 10 ml with sub-boiled water. 2.4. Calibration Wood standards for calibration are not available for LA-ICPMS. Cellulose, as the major constituent of wood, has a composition similar to most types of wood. For calibration six pellets of increasing multielement concentration were prepared. Cellulose powder Ž0.6 g. ŽAldrich, diameter approx. 20 m m. was doped with multielement standard solutions and pressed into pellets under a pressure of 110 MPa for 1 min. The pellets were introduced into the laser sample chamber and analyzed by the same procedure as the wood samples using 13 C as a reference. A relative S.D. - 10% could be found. The calibration graphs are linear within the concentration range of interest for all measured elements. The accuracy of Table 2 Laser type
Nd-YAG-Laser, 1064 nm
Operating mode Energy Frequency Preablation Analysis time Replicates Number of craters
Q-switch 170 mJ 10 Hz 20 s 45 s 15 14
the calibration by LA-ICPMS was verified by digestion of the pellets in the same way as the wood samples and subsequent measurement by ICPMS. The results agree within the confidence limits Ž95%.. The limits of detection found for LAICPMS of tree rings are specified in more detail by Hoffmann et al. Ž1994.. 3. Results and discussion 3.1. Radial distribution of elements Eight consecutive rings starting with the most recently formed ring in 1995 were measured towards the pith. Hence, the data comprise of laser sampling values for the period 1988]1995. The elemental concentrations for each year were averaged across the ramets of a clone. Because of the young age Ž12]15 years. of the stem disks the analyzed rings are considered to be mostly juvenile. Therefore only a small portion of the crosssection could be attributed to heartwood. It was found with a 18-year-old spruce tree, that between 10 and 12 tree rings are involved in water transportation ŽCutter and Guyette, 1993.. The conducting system is most efficient in the outermost rings, but it is likely that a higher proportion of tree rings are more or less involved in conducting water. As a consequence, translocation of elements in the xylem tissue may smear the trend signals to be examined and the analysis of an individual ring does not pinpoint the specific year of environmental change ŽSymeonides, 1979; Cutter and Guyette, 1993.. The elements investigated were grouped according to their horizontal mobility in the tree stem. According to Cutter and Guyette Ž1993., element mobility is based on a number of factors such as ion solubility, chargerionic radius ratio and essential nature. Sap pH, bonding in the xylem matrix as well as heartwood]sapwood equilibrium concentration are additional factors present in tree stem. For example the elemental distributions of Ca and Al and Fe are shown in Fig. 2a]c. 3.2. Elements of high mobility: Mg Magnesium is a macronutrient with high mobil-
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
33
Fig. 2. Ža. Comparison of elemental concentrations in growth rings of four different clones: concentration distribution of Ca. Žb. Comparison of elemental concentrations in growth rings of the four different clones: concentration distribution of Al. Žc. Comparison of elemental concentrations in growth rings of the four different clones: concentration distribution of Fe.
34
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
ity in the xylem and its concentration is influenced by soil pH and Al content ŽKuhn et al., 1995.. The mean curve of the clone that originates from Ranshofen shows an overall trend which is different from the other three clones. The concentration levels towards the most recent formed tree rings increases, whereas the other clones reveal no pronounced radial trends. It can be concluded that the clone from Ranshofen responds more rapidly to changed environmental conditions. 3.3. Elements of moderate mobility: Ca, Sr, Ba, Zn, Co, Cr and Mn In general, the radial trends of the elements such as Ca, Mg, Sr, Mn and Zn are negative which means that the values decline from pith to bark ŽTendel and Wolf, 1988.. This is also true for most of our curves and no obvious change is observed after the shutdown of the pollution source. The only exception is for the mean curve of the Ranshofen clone, which originates from the experimental site. For the elements Ca, Sr and Ba, an upward trend can be seen from pith to bark starting in 1992. It can be inferred that these trees were most adapted to the local environmental conditions. The terminated fluoride deposition has started to alter the soil condition, which has effected as a consequence the Ca concentration in the xylem. ŽDeWalle et al., 1991.. Furthermore, physiologically active Ca Žand possibly other divalent cations. can form stable complexes, e.g. CaF2 ŽScheffer, 1984.. The conclusion is that the reduced fluoride deposition has made additional Ca Žand other divalent cations. available for the plants ŽCronan and David, 1995.. Kuhn et al. Ž1995. observed that the absolute Ca content in the cell walls of the roots of Norway spruce was moderately reduced by low pH values and strongly reduced in the presence of Al. Additional research is required to shed light behind these complex plant]soil interactions. It could be evident that the other clones, which did not show responses within the 3-years after shutdown, might show such responses years later. Other evidence comes from the radial growth
rate of the tree rings formed after the shutdown. The radial growth rates accelerated significantly after the pollution has stopped. Goransson and ¨ Eldhuset Ž1991. reported on the effects of aluminum on growth and nutrient uptake and observed reductions in growth rate and a reduced Ca uptake when adding Al. The trends for the elements Ba, Sr, and Ca are very similar. This is especially true for Sr and Ca, which are incorporated into the woody tissue in a similar way ŽMomoshima et al., 1995; Wimmer and McLaughlin, 1996.. The radial distribution of Mn, Co and Cr is stable across the clones, without any particular trends. The Toging clone trees ¨ exhibited the lowest element concentrations among all clone trees. 3.4. Elements of low mobility: Al, Pb, Fe, Cd Aluminium and Fe show a similar pattern whereas Pb reacts differently. The recent increase of Al and Fe indicates an enhanced Al uptake from the soil, although it is not clear if this is related to soil acidification ŽArp and Manasc, 1988.. The onset of this increase can be dated to the years 1992 and 1993. It is feasible that this change could be related to the shutdown of the pollution source with the Ranshofen clone responding most significantly. It was found that Al and water soluble fluorine was significantly correlated Ž r s 0.93. due to its strong complexation ŽWenzel and Blum, 1992.. The drastic reduction Žto zero. of fluoride deposition released Al in the soil from fluorine complexes and possibly more Al was made available to the trees. A high Al influx can be related to toxic effects on the fine root system with increased blockage of Ca uptake ŽSchlegel et al., 1992; Forget and Zayed, 1995.. A combination of pH 4.5 and increased free Al reduced Ca binding by 83% ŽSchroder et al., ¨ 1988.. The radial element concentrations of Cd and Pb in the tree stems were lowest in the recently built rings whereas maximum values were close to the pith. This observed decrease from pith to bark is in accordance with other findings by Hagemeyerand co-workers ŽHagemeyer et al., 1992; Hagemeyer and Lohrie, 1995; Hagemeyer and Weinand, 1996..
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
35
Fig. 3. Ža. Circular distribution of the concentrations of Ca and Sr in the growth ring of the year 1993 of one Ruhrgebiet clone. Both distributions show similar pattern. Žb. Circular distribution of the concentrations of Al and Fe in the growth ring of the year 1993 of one Ruhrgebiet clone. Žc. Circular distribution of the concentration of Cr in the growth ring of the year 1993 of one Ruhrgebiet clone.
36
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
The shutdown of the pollution source has altered the biological activity of the humus layer ŽWenzel, 1990. which recuperated to some extent and consequently effected the trees. The changed conditions are seen best with the Ranshofen clone. 3.5. Circular scanning Within the growth ring 1993 of one Ruhrgebiet ramet, the element concentrations were analyzed in the tangential direction, following the circular path of the tree ring. Selected results are displayed as contour lines in Fig. 3a]c and considerable fluctuations are visible along this tree ring. As similar inconsistencies can be assumed to be valid for all the trees, a tree side dependency of the elemental concentration within a single tree ring with a variation up to 60% was observed. This variation can be explained by the changing proportion of compression wood at the various sides of the tree stem. Compression wood formation is mainly linked to the frequency and the direction of wind since it is formed on the stressed side of the tree. The effects of compression wood are explained in more detail in the following paragraph. The results of the circular scanning show clearly that the success of a dendrochemical study is highly governed by a very careful and consistent sampling strategy. The circular scanning demonstrates again the correlation between elements with similar properties and functions in the wooden tissue. The earth alkaline elements Ca, Sr and Ba were observed to behave similarly Žsee Fig. 3a.: Ca and Sr as divalent cations have a similar ionic radius. Therefore these elements substitute each other in various biological reactions. Al and Fe, considered to exhibit low mobility, are displayed in Fig. 3b and show very unstable concentration pattern along the circular path within the investigated tree ring. Elements, such as Cr display very stable distribution around the circular line ŽFig. 3c.. 3.6. Compression wood This specialized type of woody tissue is formed only in gymnosperms. Compression wood typically develops in response to unbalanced mechanical
Fig. 4. Scanned trunk with compression wood and its opposite wood.
pressure, such as wind, snow or slope, or can be formed due to the close proximity of another tree. The wood on the stressed side of the tree is observed to be compressed ŽFig. 4.. The growth rings are wider with higher density and appear darker due to the enhanced lignin and reduced cellulose content. An increased concentration of elements is expected in compression wood due to these differences in wood chemistry. To some extent, compression wood behaves similar to juvenile wood ŽTimell, 1986.. The unstressed wood has more cellulose than the normal wood. As 60]70% of the cations bind to hydroxyl groups, the lignin rich compression wood provide more binding sites than the opposite wood. This phenomenon was visualized by LA-ICPMS measurements with a specimen of the Toging ¨
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
37
Fig. 5. Ža. Concentration distribution of Ca in compression wood and opposite wood of a Toging clone. Žb. Concentration ¨ distribution of Al in compression wood and opposite wood of a Toging clone. Žc. Concentration distribution of Cr in compression ¨ wood and opposite wood of a Toging clone. ¨
38
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39
clones with pronounced compression wood. The results for Ca, Al and Cr are shown in Fig. 5. The compression wood shows significantly higher concentrations Žby a factor of 4]6. for all investigated elements Žwith the exception of Mn.. 4. Conclusions The analytical potential of LA-ICPMS was used to determine the elemental variation of individual growth rings, to establish radial and circular element trends in tree rings. We were able to highlight the importance of taking into account the stem sides as source of element variation. In addition, elements with similar chemical behavior and mobility show similar patterns in single growth rings. On one disk we have also demonstrated the dramatic difference between compression and opposite wood. Three years after the shutdown of the aluminum smelter, changes in element uptake from the soil were found. An important question is why elements show specific radial trends while others do not. The sap in a tree refers to the liquid that flows in the apoplast, primarily in the conducting elements of the wood and most elements do not follow simple diffusion or mass-flow kinetics from the soil to the stem ŽSmith et al., 1996.. Very high concentrations of metals such as Pb in the soil may not be detected in the tree rings of fast-growing trees, as showed with Populus marilandica ŽLukaszewski et al., 1993.. Most trees show a decline in cation concentration with increasing tree age which is linked more to the binding exchange properties in the woody tissue than to the sap]soil chemistry. Another general problem is the question if the analysis of individual rings can pinpoint a particular year of environmental change. If this is the case, a ring by ring solution is provided with the LA-ICPMS technique. However, some metals in the wood are easily translocated in the apoplast Žcell walls. and also symplast Žcytoplasma. within the tree rings involved in water conduction Žsapwood.. Therefore the presence of an element in a specific tree ring to a precise year does not necessarily mean that the element was there in the tree environment of that year. This is espe-
cially true for very young trees with no distinct heartwood formation. This conclusion was also drawn by workers who have analyzed 5-year-old spruce trees ŽDeWalle et al., 1995.. The radial distribution of Pb did not reflect an applied change in soil Pb synchronically. As the age of the investigated stem disks are between 12 and 15, heartwood formation might have just started a few years ago. This could have resulted in significant element trends as seen for Al, Fe, Sr, Ba and Ca in some of the sample trees. These trees might have just started to form heartwood while the others have not. The higher concentrations of elements of the alkaline earth cations as well as Al and Fe in the more recently formed tree rings could indicate that these rings are conducting more efficiently than in the previous years. In dendrochemical studies of tree species with many tree rings present in the sapwood, the ringby-ring analysis may not be a cost-effective way of analysis. In this case, pooling samples over decades or even bi-decadal periods may be more realistic ŽSmith et al., 1996.. However, for tree species with very quick heartwood formation, such as Juniperus, Thuja, Robinia, Pseudotsuga or Larix, for intra-ring variability, for wound-initiated discoloration and other pathological wood formations, a high spatial resolution detection as achieved by LA-ICPMS is a very efficient and useful technique in dendrochemistry. Acknowledgements We would like to acknowledge laboratory assistance by A. Pederzolli, Ch. Ludke and M. ¨ Grabner. Thanks are due to Drs H. Steinkellner and J. Glossl ¨ for testing the genetical identity of our clones. Financial support was provided by the GOS ŽGesellschaft zur Forderung angewandter ¨ Optik, Optoelektronik, Quantenelektronik und Spektroskopie.. References Arp PA, Manasc J. Red spruce stands downwind from a coal-burning power generator: tree ring analysis. Can J For Res 1988;18:251]264. Cutter B, Guyette R. Anatomical, chemical and ecological
T. Prohaska et al r The Science of the Total En¨ ironment 219 (1998) 29]39 factors affecting tree species choice in dendrochemistry studies. J Environ Qual 1993;22:611]619. Cronan CS, David FG. Use of calciumraluminum ratios as indicators of stress in forest ecosystems. J Environ Qual 1995;24:209]226. DeWalle DR, Swistock BR, Sayre RG, Sharpe WE. Spatial variations of sapwood chemistry with soil acidity in appalachian forests. J Environ Qual 1991;20:486]491. DeWalle D, Sharpeand W, Swistock B. Dendrochemistry and the soil chemistry environment. In: Lewis TE, editor. Tree rings as indicators of ecosystem health. Boca Raton, Florida: CRC Press, 1995:157]176. Forget E, Zayed J. Tree ring analysis for monitoring pollution by metals. In: Lewis TE, editor. Tree rings as indicators of ecosystem health. Boca Raton, Florida: CRC Press, 1995:157]176. Garbe-Schonberg CD, Reimann C, Pavlov VA. Laser ablation ¨ ICP-MS analyses of tree-ring profiles in pine and birch from N Norway and NW Russia } a reliable record of the pollution history of the area? Environ Geol 1997;32r 1:9]16. Goransson A, Eldhuset TD. Effects of aluminium on growth ¨ and nutrient uptake of small Picea abies and Pinus syl¨ estris plants. Trees 1991;5:136]142. Hagemeyer J, Lulfsmann A, Perk M, Breckle SW. Are there ¨ seasonal variations of trace element concentrations ŽCd, Pb, Zn. in wood of Fagus trees in Germany? Vegetatio 1992;101:55]63. Hagemeyer J, Lohrie K. Distribution of Cd and Zn in annual xylem rings of young spruce Ž Picea abies L. wKarstx. grown in contaminated soil. Trees 1995;9:195]199. Hagemeyer J, Weinand T. Radial distribution of Pb in stems of young Norway spruce trees grown in Pb-contaminated soil. Tree Physiol 1996;16:591]594. Halbwachs G, Kronberger W. Das Verhalten F- und SO 2 -resistenter Fichtenklone verschiedener Herkunft in einem Rauchschadensgebiet. Zb ŽMitteilungen. Int Verband forstlicher Versuchsanstalten 1979;1r390:259]268. Halbwachs G, Bellusova M, Nemetz C, Novak H, Wimmer R, Zehetner H. Schriftenreihe der Sektion I d. Bundesminist Umwelt, Jugend Familie 1993;3r1:211]215. Hoffmann E, Ludke C, Scholze H, Stephanowitz H. Analytical ¨ investigations of tree rings by laser ablation ICP-MS. Fres J Anal Chem 1994;350:253]259. Hoffmann E, Stephanowitz H, Skole J. Investigations of the migration of elements in tree rings by Laser-ICP-MS. Fres J Anal Chem 1996;355:690]693. Kuhn AJ, Bauch J, Schroder WH. Monitoring uptake and ¨ contents of Mg, Ca and K in Norway spruce as influenced by pH and Al, using microprobe analysis and stable isotope labelling. Plant Soil 1995;168r169:135]150. Lukaszewski Z, Siwecki R, Opydo J, Zembrzuski W. The
39
effect of industrial pollution on copper, lead, zinc and cadmium concentration in xylem rings of resistant Ž Populus marandica. and sensitive Ž Populus balsamifera. species of poplar. Trees 1993;7:169]174. Outridge PM, Veinott G, Evans RD. Laser ablation ICP-MS analysis of incremental biological structures: archives of trace-element accumulation. Environ Rev 1995;3:160]170. Momoshima N, Eto I, Kofuji H, et al. Distribution and chemical characteristics of cations in annual rings of Japanese cedar. J Environ Qual 1995;24:1141]1149. Scheffer F, Lehrbuch der Bodenkunde, Enke, Stuttgart, 1984. Schlegel H, Amundson RG, Huttermann A. Element distribu¨ tion in red spruce Ž Picea rubens. fine roots; evidence for aluminium toxicity at Whiteface Mountain. Can J For Res 1992;22:1132]1138. Schroder WH, Bauch J, Endeward R. Microbeam analysis of ¨ Ca exchange and uptake in the fine roots of spruce: influence of pH and aluminium. Trees 1988;2:96]103. Smith KT, Shortle WC. Tree biology and dendrochemistry. In: Dean JS, Meko DM, Swetnam TW Žeditors., Tree rings, environment and humanity. Tuscon: Radiocarbon, 1996: 629]635. Symeonides C. Tree ring analysis for tracing the history of pollution: application to a study in northern Sweden. J Environ Qual 1979;8r4:482]486. Tendel J, Wolf K. Distribution of nutrients and trace elements in annual rings of pine trees as an indicator of environmental changes. Experientia 1988;44:975]980. Timell TE. Compression wood in gymnosperms, 3 vols. Berlin, Heidelberg, New York: Springer, 1986. Watmough SA, Hutchinson TC. Analysis of tree rings using inductively coupled plasma mass spectrometry to record fluctuations in a metal pollution episode. Environ Pollut 1996;93:93]102. Watmough SA, Hutchinson TC, Evans D. Application of laser ablation inductively coupled plasma mass spectrometry in dendrochemical analysis. Environ Sci Technol 1997;31: 114]118. Wenzel WW. Bodenbelastungen durch Fluor und Schwermetalle im Immissionsbereich der Aluminiumhutte ¨ Ranshofen und ihre Auswirkungen auf Bodenzustand und Bodenfunktion. Vienna: PhD thesis submitted at the University of Agricultural Sciences, 1990. Wenzel WW, Blum WEH. Effects of fluorine deposition on the chemistry of acid Luvisols. Int J Environ Anal Chem 1992;46:223]231. Wimmer R, McLaughlin SB. Possible relationships between chemistry and mechanical properties in the microstructure of red spruce xylem. In: Dean JS, Meko DM, Swetnam TW, editors. Tree rings, environment and humanity. Tuscon: Radiocarbon, 1996:659]668.