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OCR for page 281
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
OCR for page 282
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.
OCR for page 283
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
OCR for page 284
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.
OCR for page 285
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.
OCR for page 286
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
10°h base saturation.
OCR for page 287
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.
OCR for page 288
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.
OCR for page 289
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.
OCR for page 290
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.
OCR for page 291
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).
OCR for page 292
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.
Representative terms from entire chapter:
national park
