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THE POTENTIAL OF TREES TO RECORD ALUMINUM MOBILIZATION AND CHANGES IN ALKALINE EARTH AVAILABILITY E. A. Bondietti C. F. Baes III S. B. McLaughlin Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6038 ABSTRACT The mobilization of exchangeable soil cations by atmospheric deposition of mineral acid anions and the distribution of polyvalent cations in the xylem are discussed to provide the basis for interpreting both radial concentration and concentration ratio patterns of polyvalent cations in annual growth rings of trees. There is strong circumstantial evidence that the increases in Al:Ca ratios in the annual rings of red spruce and eastern hemlock in the Great Smoky Mountains National Park have occurred in recent decades and are related to aluminum mobilization. Decreases in radial growth rates as well as changes in the storage patterns of alkaline earth elements may also be linked to aluminum mobilization. Thus, the same mechanism responsible for surface decline water acidification may be important in the growth phenomenon associated with trees growing on very acidic soils. Conclusions as to the current utility of radial concentration patterns in evaluating air pollution effects on trees are discussed and further research needs are outlined. BACKGROUND One of the consequences of increasing the deposition of sulfate and nitrate to forests is the mobilization of exchangeable soil cations to maintain the charge balance in the soil solution Depending on the base saturation or acidity of the soil, increased leaching of Al , base cations, or both can occur [ 1~. Mineral cycling rates may also increase [13. An important question related to this workshop is whether annual growth rings contain a record of the impact of atmospheric deposition on the availability of exchangeable soil cations, particularly in soils low in exchangeable bases. The movement and retention of cations in the transpiration stream are influenced by adsorption to charged sites in the cell walls of the xylem. Cation follows well established charge/ionic radius relationships so that thought of as an ion exchange column r2,31 with most of the binding in the cell wall the stem xylem can be sites occupied by four elements--K, Na, Ca, and Mg 4--as well as lesser amounts of Mn, Sr, Al, etc. Each successive annual ring, therefore, can be thought of as a newly formed ion exchange column, and the sapwood can be considered a moving zone of cation exchange columns which conduct water and solutes at varying efficiencies because of age, blockages, and other factors. Radiotracer studies indicate that calcium taken up from soil concentrates near the phloem [5~; thus, newly formed cell walls are incorporating recently assimilated calcium. Although radial concentrations of nitrogen, phosphorus, and even monovalent cations 281

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282 often change greatly at the heartwood-sapwood boundary, divalent cations generally do not, exhibiting a radial concentration decline from the pith to the cambium [6,7,G,9, 101. However, discontinuities in cation concentration trends in tree rings can occur in response to wounding. The potential importance of the radial migration of polyvalent cations has been recognized ~ 11 ~ but appears to be a minor factor affecting radial distributions [9l, at least when compared to the uncertainty in how many annual rings are conducting solutes at any given time. The Stem as a Recorder of Changes in Cation Availability The exchangeable cation composition of soils or the composition of nutrient solutions affects the bulk composition of woody stems and automobile lead and smelter emission histories are recorded in tree rings [1 11. However, there are very little empirical data relating changes in soil solution chemistry It a given site to changes in the radial concentration patterns of Ca, Mg, Al, Mn, and other cations in xylem from that site. Differences in cation availability between field sites is easier to demonstrate. To compare trees of the same species but of different ages, it is necessary to normalize for the pith-to-cambium decline in concentrations of divalent cations. Normalizing to calcium, an essential cell wall constituent, allows comparisons based on relative differences rather than on absolute concentrations. When the Al:Ca ratios in the xylem from three sets of five red spruce (Picea rubens Sarg.) trees cored at 550 m, 790 m and 975 m elevation on Camels Hump Mountain in Vermont [10] are compared, the Al:Ca ratios increase with elevation (Fig. 1~. This finding is consistent with the increase in soil acidity which occurs on Camels Hump with increasing elevation [121. The Mn:Ca ratios also increase with elevation [101. For comparison, the average Al:Ca ratio (0.0035) in increments from 22 red spruce cores from the Great Smoky Mountains National Park (GSMNP) [9,10] is also plotted as a solid line in Figure 1. Because the calcium concentrations in the growth increments of red spruce of comparable age from Vermont and from GSMNP are similar [9,10], Figure 1 illustrates not only that differences in soil pH are apparently reflected in xylem but more importantly, that these differences between elevations existed in the past. For example, soils at 975 m on Camels Hump have apparently been more acid than soils at 550 m or soils in GSMNP for a very long time. Several examples illustrate that changes in cation availability during the life of a tree are reflected in radial concentrations in wood. When yellow-poplar trees were cored 20 years after a dolomite soil application, elevated calcium levels remained in the xylem formed immediately after the treatment. The authors concluded that tree rings could be used to understand historical changes in the chemical environment of trees [ 131. A slightly different and more universal perspective on how accurately differed tree species record changes in cation availability can be obtained by using fallout Sr, a calcium analog With a known input history. For example, Figure 2a illustrates the distribution of Sr in GSMNP red spruce radial growth increments Treasured by the senior author. Also plotted is the deposition history (in New York). Although Sr does appear in wood formed prior to the fallout period, reflecting either conduction in active sapwood or radial migration, the amountgOand number of years affected is relatively small. This and other measurements of Sr in different species indicate that 5- or 1 O-year increments are satisfactory for determining changes in the availability of calcium.

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283 0.01 o ._ co Con . of: 0~001 0.0001 0.1 ~ 1 1 1 1 1 1 1 1 1 ~ 0 560 m i 790 m 975 m . ~ ~ AA &- ~ I-- .~e ~ GSMNP Red Spruce Avg. 0 ~ ~ 8 o ~ so . - - o - 1 , 1 1 1 1 1 1 1 1 1820 1840 1860 1880 1900 1920 1940 1960 1980 Year Wood Formed Figure 1. Ratios of Al:Ca in red spruce increment cores sampled at three elevations on Camels Hump Mountain in Vermont. The line is the average Al:Ca ratio in 22 red spruce cores sampled in the Great Smoky Mountains National Park. Aluminum behavior in xylem is illustrated by the Ca, Mg, and Al concentration trends in an increment core from a red spruce sampled by C. F. Baes III at a site in the GSMNP where a fire occurred in the early 1920s (Fig. 2b). The radial concentration

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900 800 - c~ 700- bOO- 500- o ._ 400 - 300 - 200 - 100 - A 1934 1939 1944 1949 1954 1959 1964 1969 1974 1979 1984 5 4 3 2 1 o 800 B Pi re 1860 1880 1900 1920 1940 1960 1980 Year Wood Formed 700 600 500 Ad' 400 ~' 300 u 200 100 O Figure 920 Examples of historical changes in cation availability: (A) the concentrations of fallout Sr in increments of a red spruce stem section from thegOGreat Smoky Mountains National Park (solid line) compared to the deposition history of Sr in New York (bars); (B) the concentration trends of Al, Ca, and Mg in a red spruce core sampled in 1982 at Double Springs Gap, Great Smoky Mountains National Park, where a fire occurred in the 1920s.

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285 trends in this core show rather pronounced A1, Ca, and Mg maxima in wood formed shortly after the fire. While this pattern is probably a consequence of the fire (NO~-- induced cation mobilization?), it also illustrates that fluctuations in polyvalent cation concentrations are preserved. The aluminum and calcium trends after 1940 will be discussed below. Aluminum:Calcium Ratios in Red Spruce and Eastern Hemlock in the Great Smoky Mountains National Park In 1983, Baes and McLaughlin sampled coniferous trees at various locations in GSMNP and measured a number of elements in 1 2-mm-diameter increment cores using inductively coupled plasma-optical emission spectroscopy [9~. They reported that aluminum concentrations in xylem began increasing in all species in the mid- 1 900s even though the concentrations of the alkaline earths, particularly Ca, were often declining during this same period. When Al:Ca ratios were calculated [10], the 1940s stood out as the period when xylem aluminum began increasing at various high-elevation locations in the Park. Our hypothesis is that aluminum mobilization in the soil probably accounts for these increases because regional SOX and NOX and emissions also increased substantially in the 1940s. - ~~ Soils at high elevations in the GSMNP are very acid, with much less than 10% of the exchange complex ~uraie+d with base cations [14]. Figure 3a illustrates that under these conditions the A1 :Ca ratio in the soil solution will increase as mineral acid anion concentrations increase [11. The ratio also increases as base saturation decreases [exchangeable acidity increases). This generic example was derived using the soil chemistry model described in the appendix of Reference [11. Fig 3b shows estimated regional SOX and NOX emissions upwind from GSMNP [15], illustrating the timing of increasing anion inputs into these soils. Figures 3c and ad illustrate the trends in Al:Ca earth ratios in three GSMNP red spruce cores and an eastern hemlock (Tsuga canad~ensis L.) core representative of the data set [9,101. In 18 of 22 red spruce and 14 of 17 hemlock cores, the average Al:Ca ratio was greater in wood formed after 1938 (a common date where a split in the growth increments was made [91) than before. A transitory increase in the Al:Ca ratio was frequently found in wood formed around the turn of the century [103. This increase is contemporaneous with large SO2 emissions from open pit roasting of sulfide ore at Ducktown, Tennessee, 90 km upwind from the GSMNP [16]. These increases are consistent with the idea that this ecosystem contains soils which are very sensitive to A1 mobilization by mineral acid anion inputs. A comparable example may exist in red cedar measurements made in the lead-mining region of Missouri, where increases in wood aluminum coincide with PbS ore production histories [171. Calcium concentrations in the spruce core illustrated in Fig. 2b begin increasing in the 1940s, possibly reflecting the mobilization of a Ca pool originally released by the fire (Fig. 2b). Aluminum begins increasing at a later date than calcium. No comparable example exists in the rest of the GSMNP data or in the two highest elevation Vermont data where such a sustained increase in calcium concentrations occurs. Similar calcium trends do occur in four out of five of the Vermont cores from 550 m elevation [101.

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286 25 20 a ._ cat .. - 15 a > 10 - - o cc o A : - / / / / / 2.5X / _~ _ 0 100 200 300 Strong Acid Anion (,u~q L-1) Figure 3a. Potential interrelationships between atmospheric deposition and aluminum availability to trees: a generic model of the effect of increasing soil solution concentrations of mineral acid anions on the solution A13+:Ca2+ ratios in soils with 2.5 to 10h base saturation.

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287 20 1 5 10 c o 5 1.5 1.0 or o.5 - x a - ac ~ . 5 < 1.0 0.5 0.0 B soy and NOy Emissions C~ LC/S-5 - Red Spruce O ST-2 - Red Spr uce 1 1 ~ 1 1 1 1 1 1 ~ ' ~ 'I 1 1 1 1 1 1 1 ~ | D ~ LC/WA-2 - Red Spruce ~ l LC/Eh\-8 - Hem 1 0 c Ic 74 ~ ~ ~frN 1830 1850 1870 _ 1 , ,1 , 1 , 1 1890 1910 1930 1950 1970 1990 Year Wood Formed Figure 3b,c,d. Potential interrelationships between atmospheric deposition and aluminum availability to trees: (B) historical emission trends of SOx and NOx upwind from the Great Smokey Mountains National Park; the trends in Al:Ca ratios in (C) individual red spruce sampled at Mt. LeConte summit (LC/S) and at Mt. Sterling (ST), in the Great Smokey Mountains National Park, and (D) in a red spruce sampled at Rainbow Falls (LC/WM) and an eastern hemlock sampled at Trillium Gap (LC/EM), also in the Great Smokey Mountains National Park.

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288 Figure 4 presents both Al:Ca and Al:Mg ratios (4a) as well as concentration trends (4b) in a spruce core from Mt. LeConte in GSMNP. It illustrates that aluminum increases are responsible for most of the increases in aluminum ratios, and that calcium and magnesium concentrations generally decline regardless of how aluminum concentrations are changing. Another feature of the data set in Figure 4 is that radial growth declines frequently coincide with increases in aluminum or aluminum ratios. Using the Spearman Rank Order Correlation test [18], we have found that after the 1938 increment, over 50% of the spruce and hemlock cores have a significant (9SYo confidence level) inverse relationship between radial growth and the Al:Ca ratio. No such strong relationship was found before 1938. Increasing aluminum availability either coincides with or is linked to radial growth declines. Another perspective on both the increase in Al:Ca ratios and the decreases in radial growth can be obtained by plotting Al:Ca ratios vs. radial growth for two sets of cores sampled at sites only 37 km apart and at similar elevations [9~. Figure 5 shows the four cores from Indian Gap, where the Al:Ca ratio increased over three-fold after 1938 (Fig. Sa), and the four cores from Mt. Sterling, where the ratio did not even double (Fig. fib). The four cores from Indian Gap indicate that slower radial growth rates coincide with the high Al:Ca ratios. Another difference between the two sets of cores illustrated in Figure 5 is that at Indian Gap an abrupt decrease in the amount of calcium stored in wood in proportion to magnesium (the Ca Mg ratio) occurred in the 1940s at the same time that the Al:Ca ratio increased (Sa). The Mt. Sterling cores (not illustrated) showed smaller changes in the historical Ca:Mg ratio trend in only two cores. The coincidence between the aluminum increases, the CaMg decreases, and the radial growth declines at Indian Gap are probably not fortuitous. It has long been known that even adding neutral salts like CaSO4 or KC1 to very acid soils mobilizes aluminum and reduces plant growth [19,20] and that the ratio of aluminum or other cations to calcium is critical in the calcium nutrition of trees [e.g., reference 213. CONCLUSIONS AND RECOMMENDATIONS Current and Future Utility of Using Al:Ca and Other Ratios in Wood as Markers of Air Pollution Effects on Forests. 1. The trend in Al:Ca ratios in a tree growing on a relatively undisturbed site is a sensitive method of evaluating when aluminum mobilization by atmospheric deposition occurred. The measurement of Al:Ca ratios is a survey methodology now being used to compare sites where red spruce is in various stages of growth decline. 2. The use of Al:Ca ratios and radial growth rate correlations is a promising approach by which the linkage between aluminum mobilization (due to atmospheric deposition or any other cause) and growth rate declines can be evaluated. An expansion of its use is recommended. Similarly, the use of Al:Ca and Ca:Mg ratios appears to be a sensitive method for detecting altered calcium and magnesium availability during growth. An increase in aluminum availability or changes in alkaline earth behavior are not necessarily evidence of an adverse physiological effect, however.

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289 2.0 t.6 1.0 0.E 0.0 3.0 2.0 1 . 0 S ~ lo A. 3 - 2 1 o ~ 'I I ~ ~ .! I a. _ , . _ O AI:Ce ~ A1:~g r A! !2 ' = ~ J: - - ~ A1 _ O c. -9X3 _ L I ~ ; ~ ~ o,o 1, 1 1 ; 1 1 1 ' 1 V _ it= 1 1 1 1 1 1 1 1 1875 1890 1905 1920 1935 1950 Year Wood Formed 1965 1980 600 BOO c. 400 ce 300 200 _ 100 O Figure 4. The trends in Al:Ca and Al:Mg ratios (A), radial growth (B) and concentrations in a red spruce sampled at Mt. LeConte summit, Great Smoky Mountains National Park.

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290 4- _ 3 2 1 x x x ^_ v- . I . I ~ 1939-1983 x x before 1939 ,;5 `~ ~. .e ~ - e - ' F. Y ~2 0 0.5 1 1.5 0 0.5 1 1.5 Relative Al Ca Ratio in Red Soruce Cores c a ._ ~ 7.5 U 5.5 3.5 - 5 1 ~ ~ ~ ~ ~ ' I ' ~ ' ~ 1' ' I ' I - ' I ' I ' 1 ' 1765 1785 1805 1825 1845 1865 1885 1905 1925 1945 1965 1985 O ._ ~' 7.5- a 55~ U . 3.5 - 1 X IG-3 ~ ' 1 ~ I ' I ' I ' ~ ' I ~ ~l. ~ ' I ~ . . 765 178S 1805 t825 1845 !865 1885 1905 1925 1945 1965 1985 GSN(NP Red Spruce Wood Year ~ IG-2 ~ Figure 5. A comparison of Great Smoky Mountain National Park red spruce cores from locations 37 km apart but at similar elevations: (A) radial growth vs. Al:Ca ratios (x 100) for four trees sampled at Indian Gap ( 1682 m elevation), and (B) for four trees sampled at Mt. Sterling ( 1781 m elevation). Subplots (C) and (D) show that an abrupt decrease in CaMg ratios occurred in the four Indian Gap (IG) cores in the decade of the 1940s.

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291 3. Although not discussed here, Mn:Ca ratios in wood generally do not change when Al:Ca ratios change, strongly suggesting that aluminum mobilization, not net base cation loss, is the cause of the increased aluminum availability to trees. This comparison has utility in evaluating watershed-scale effects of acidic deposition on forest soils. 4. To capitalize on the large inventory of existing increment cores and associated evaluation data, we suggest that research on analytical applications of (for example) laser ionization-mass spectrometry be supported. This is a promising technology which uses a laser to vaporize solids (in this case wood in mounted increment cores). The volatile elements are then fed into a mass spectrometer for analysis. With such a system, the Al:Ca trends in existing increment core sets could be evaluated without the necessity of additional field sampling. ACKNOWLEDGMENTS Publication No. 3126, Environmental Sciences Division. Research sponsored by the Oak Ridge National Laboratory and the Electric Power Research Institute (Integrated Forest Study on Effects of Atmospheric Deposition). Oak Ridge National Laboratory is operated by Martin Marietta Energy Systems, Inc., under Contract No. DE-AC05-840R21400 with the U. S. Department of Energy. REFERENCES 1. J. O. Reuss and D. W. Johnson. 1986. Acid Deposition and the Acidification of Soils and Waters (Springer-Verlag, New York). 2. I. B. Ferguson and E. G. Bollard. 1976. The movement of calcium in woody stems. Ann. Bot., 40:1057- 1065. 3. J. F. McGrath and A. De Robson. 1984. The movement of zinc through excised stems of seedlings of Pinus radiata D. Don. Ann. Bot., 54:231-242. 4. S. Saka and D. A. I. Goring. 1983. The distribution of inorganic constituents in black spruce wood as determined by TEM-EDXA. Mokuzai Gakkaishi, 29:648-656. 5. H. Riekerk. 1967. The movement of phosphorus, potassium, and calcium in a Douglas-fir forest ecosystem. Ph.D. dissertation, University of Washington, Seattle. 6. H. R. Orman and G. M. Will. 1960. The nutrient content of Pinus radiata trees, New Zealand J. Sci., 3:510-22. 7. C. V. McMillan. 1970. Mineral content of loblolly pine wood as related to specific activity, growth rate, and distance from pith. Holzforschung, 24:152-157. 8. R. Tout, W. Gilboy, and N. Spyrou. 1977. Neutron activation studies of trace elements in tree rings. J. Radioanal. Chem., 37:705-709. 9. C. F. Baes III and S. B. McLaughlin. 1986. Multielemental Analysis of Tree Rings: A Survey of Coniferous Trees in the Great Smoky Mountains National Park. Report ORNL-6155 (National Technical Information Service, Springfield, Virginia).

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292 11. 10. E. A. Bondietti and C. F. Baes III. 1988. Multielemental Analysis of Tree Cores from the Great Smoky Mountains National Park, a Supplement. Report ORNL-6 1 55/S, Oak Ridge National Laboratory. (in prep.) M. A. S. Burton. 1985. Tree Rings. Pp. 175-202 in Historical Monitoring, MARC Report Number 31. Monitoring and Assessment Research Centre, University of London, U.K. 12. T. G. Siccama. 1974. Vegetation, soil and climate on the Green Mountains of Vermont. Ecol. Monographs, 44:325-349. 13. J. R. McClenahen, I. P. Vimmerstedt. and R. C. Lathron. 1987. Historv of the chemical environment from elemental analysis of tree rings. Report CONF-8608144, Technical Information Center, U.S. Department of Energy, Oak Ridge, in Proceedings, International Symposium on Ecological Aspects of Tree-Ring Analysis, pp. 690-698 (August 17-21, 1986, Tarrytown, New York). 14. R. J. McCracken, R. E. Shanks, and E. E. C. Clebsch. 1962. Soil Morphology and Genesis at higher elevations of the Great Smoky Mountains. Soil Sci. Soc. Am. Proc., 26:384-388. 15. S. B. McLaughlin, D. J. Downing, T. J. Blasing, B. L. Jackson, D. J. Pack, L. K. Mann, and T. W. Doyle. FORAST Data Base Documentation. U.S. Environmental Protection Agency Report (in press). 16. C. F. Baes III and S. B. McLaughlin. 1984. Science, 224:494-497. 17. R. Guyette and E. A. McGinnes, Jr. 1987. Potential in using elemental concentrations in radial increments of old growth eastern red cedar to examine the chemical history of the environment. Report CONF-8608144, Technical Information Center, U.S. Department of Energy, Oak Ridge, Tennessee, in Proceedings, International Symposium on Ecological Aspects of Tree-Ring Analysis, pp. 671-680 (August 17-21, 1986, Tarrytown, New York). 18. SAS Users Guide. 1986. SAS Institute, Inc., Cary, North Carolina. 19. Fried, M. and M. Peech. 1946. The comparative effects of lime and gypsum upon plants grown on acid soils. J. Amer. Soc. Agron., 38:614-633. 20. Ragland, J. L., and N. T. Coleman. 1959. The effect of soil solution aluminum and calcium on root growth. Soil Sci. Soc. Am. Proc., 23:355-357. 21. Lyle, E. S. Jr., and F. Adams. 1971. Effect of available soil calcium on taproot elongation of loblolly pine (Pinus taeda L.) seedlings. Soil Sci. Soc. Am. Proc., 35:800-805.