PIXE and RBS elemental analyses of tree rings from Mexico
Basin forests as a record of pollution.
J. Miranda, G. Calva-Vásquez1, C. Solís, and L. Huerta
Instituto de Física, Universidad Nacional Autónoma de México
Apartado Postal 20-364, 01000 México, D.F., MEXICO
Abstract. Particle induced X-ray emission (PÏXE) and Rutherford backscattering (RBS) elemental analyses of tree rings
and soils from forests around the Mexico City Metropolitan Area (MCMA) were performed. The aim was to estimate the
impact of pollution on the forests. Cores from Pinus montezumae and Abies religiosa trees, in four forests around the
MCMA (Desierto de los Leones, Iztapopocatépetl, Villa del Carbón and Zoquiapan) and a reference site (El Chico).
Differences were observed in samples from the different forests, showing higher values in the areas closest to the
MCMA. A correlation of several elements with ring width was found using cluster analysis. Additionally, soil analyses
from different depths in the forests were carried out, trying to relate the elemental concentrations measured in the tree
rings with cation mobility. In this case, samples taken in 1993 and 1999 were analyzed, showing elemental mobility to
the various depths.
Basin are affected in different ways by the pollutants
emitted in the MCMA. The Environment Secretary
of the local Government (SMA-GDF), reported the
existence of 6×106 ha of conifer forests in the Basin,
including several National Parks [3]. The degradation
of these forested areas is apparent. Industrial
emissions alter the concentration in trees and other
plants of elements known as macronutrients (C, K,
Ca, Mg, P, S), and micronutrients (B, Fe, Cu, Zn,
Mo). Something similar occurs with other trace
elements, like Cd, Co, Cr, Pb, Hg, Ni, and V. The
accumulation of the elements in the trees may affect
their normal development, according to elemental
concentrations and toxicity [4]. The species Pinus
montezumae and Abies religiosa are present in
altitudes ranging between 2800 m and 3200 m above
sea level. The ages of the trees in the Mexico Basin
forests oscillate between 150 and 200 years, with
heights around 25 m. It is important to point out that
these species present well defined growth rings.
Thus, it was considered as an important task to
measure elemental concentrations, using particle
induced X-ray emission (PIXE), in rings extracted
from trees located in forests around the Mexico
Basin. This technique has proven to be a very
powerful tool for this kind of studies [5, 6]. The
INTRODUCTION
Atmospheric pollution in the Mexico Basin
started significantly around 1975, together with
official measurements and reports. However, the
effect on forest ecosystems has not been investigated
extensively [1]. Therefore, it is necessary to study
them using very sensitive and reliable methods. The
Metropolitan Environmental Commission (CAM) [2]
reported that total emissions in the Mexico City
Metropolitan Area (MCMA) reached 2.5×106 ton
during 1998, with 84% produced by mobile sources,
12% by area sources, 3% by point sources, and 1%
by soils and vegetation. Among the main indicators
of pollution are suspended particulate matter with
dimensions below 10 µm (PM10), SO2, and NOx.
The CAM predicts a general growth in the emissions
for the year 2010. Thus, PM10 would increase by
26%, SO2 by 48%, and NOx by 37%. According to
information provided by the Automatic Atmospheric
Monitoring Network (RAMA), the North and Central
regions of the Mexico Basin have larger deposits of
particulate matter, which are dispersed by the
dominant winds towards the SW-SWW area, while
the phenomenon is inverted during the dry season.
As a consequence, the forests around the Mexico
1
On leave from Facultad de Estudios Superiores Zaragoza, UNAM.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
444
material. The computer code GUPIX [8] was used to
obtain quantitative results, while the NIST tomato
leaf reference material was employed to check
analytical accuracy. Rutherford backscattering (RBS)
with a 0.7 MeV He beam helped to determine the
matrix composition for the soils and the wood in the
tree cores. Also, ring thicknesses were determined
with the WinDENDRO [9] computer code, from
digitized images of the tree cores.
purpose of this work was to identify the influence of
pollutants emitted in the MCMA on the development
of the trees, estimating differences among the studied
forests.
MATERIALS AND METHODS
Four forests around the Mexico Basin were
selected as study sites: Izta-Popo National Park,
Zoquiapan National Park, Desierto de los Leones
recreational area, and Villa del Carbón. Moreover,
the El Chico National Park was taken as a reference
site, as it is located north of the Basin, and is mostly
subjected to winds that disperse pollutants coming
from the Southern region. Fig. 1 shows the location
of the sites.
RESULTS AND DISCUSSION
First, elemental concentrations measured in tree
rings are considered. In general, elements S, Cl, K,
Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, and Zn were detected,
with eventual appearances of As, Br, Rb, Sr, Y, Zr,
and Pb. RBS provided information mainly on C, N,
and O. As an example, Fig. 2 shows the results for a
Pinus core from the Iztapopocatépetl site. Although
the uncertainties do not allow a definite conclusion, a
growing trend is observed in several elements, like
Cr, Mn, Fe, and Cu, which are normally related to
human activities. A similar phenomenon was
observed by Watmough and Hutchinson [10] in
sacred fir tree rings from the Desierto de los Leones
site. To estimate the reproducibility of the
measurements, Fig. 3 displays data for two different
cores from the Iztapopocatepétl National Park.
There, S concentrations are plotted as a function of
time. It can be seen that the data have a very similar
behavior. However, in this case there is not a trend
observed in the data, except for a few peaks in the
concentrations. It should be noted that the peak in
1992 is in agreement with the highest pollution
indices measured in Mexico City [1].
It is also of interest to identify some kind of
correlation among the different elements and the
width of the tree rings. To carry out this task, the
application of cluster analysis, with the 1-r Pearson
correlation and Ward’s method of amalgamation [11]
is useful to present the results in a graphical form.
Thus, Fig. 4 shows a dendogram for a Pinus core
from the Iztapopocatépetl forest. There, a linkage is
found among the ring width and the elements Cl, K,
Ca, and Mn. This is expected, as these elements are
usually associated to the growth of living organisms.
It should be pointed out, also, that a correction in the
rings width due to volume increase in the trees is
done by the WinDENDRO program. Additionally,
groups of elements possibly anthropogenic are
identified, including Ni, Ti, Cu, and S in one group,
and Fe, Cr, Zn, and V in another.
FIGURE 1. Location of sampling sites.
Five trees from every forest and each one of the
Pinus and Abies species were selected to extract 40
cm long cores, 5 mm in diameter, using a Pressler
drill. Additionally, samples of soils from every forest
were collected at depths ranging from 0 to 120 cm, in
15 cm steps, in 1993 and 1999. Tree cores were cut
to show a flat surface, using glass tools, while soil
samples were palletized. PIXE analyses were
performed with external beam setup at the 3 MV
Pelletron accelerator, Instituto de Física, UNAM [7].
A 3 MeV proton beam was the primary radiation. A
Canberra LEGe detector was used to measure
contents of heavy elements, while an Amptek Si
detector provided data about lighter elements.
Calibration of the detection systems was carried out
with pellets of IAEA Lake Sediment SL-1 reference
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elemental concentrations measured in the cores are
averaged along the thirty years of interest. The
results are shown in Figs. 5 and 6. There, the
elemental contents are higher in Desierto de los
Leones, as a result of human influence, polluting the
environment in this forest. This is supported by the
fact that winds in the Mexico Basin tend to
accumulate the pollutants in the Southwestern area,
so this forest receives a very strong influence of the
anthropogenic emissions.
Concentration (µg/g)
2
0
0
6
2
2.5
2.0
1.0
0.0
6
Cr
4
2
FIGURE 4. Dendogram based on data obtained in the
analysis of a core taken from Pinus montezumae at the
Iztapopocatépetl site.
0
1970
1975
1980
1985
1990
1995
Width
0.5
Cl
8
6
4
2
0
1.5
Mn
2000
K
1995
V
1990
Ca
1985
Zn
1980
Cr
1975
S
1970
X 100
Fe
7
6
5
4
3
2
1
0
Linkage Distance
Mn
Fe
0
Ti
Ni
4
Cu
Cu
2
Ni
Zn
4
2000
Year
7000
FIGURE 2.
Elemental concentrations in a Pinus
montezumae core from the Iztapopocatépetl site, as a
function of the year corresponding to each ring.
El Chico
Desierto de los Leones
6000
Concentration (µg/g)
5000
3500
Core 1
Concentration (µg/g)
3000
Core 2
2500
4000
3000
2000
1000
2000
0
S
1500
1000
K
Ca
Ti
Mn
Fe
FIGURE 5. Mean elemental concentration in tree rings
(corresponding to 30 years), from cores extracted at the
Desierto de los Leones and El Chico, for the most abundant
elements.
500
0
1975
Cl
1980
1985
1990
Year
1995
2000
Also, Ca concentrations measured in soil samples
extracted in 1993 and 1999 are given in Fig. 7. These
results correspond to the Iztapopocatépetl National
Park. The data show similar contents near the
surface, but they are very different in the deepest
regions. This is because there is a higher cation
mobility in the shallower layers, as was shown with
measurements of this variable. Therefore, an
accumulation of Ca was observed in the 1999
samples.
FIGURE 3. Concentration of S in tree rings from two
cores extracted at the Iztapopocatépetl site.
The effect of pollution can be observed when
comparing the Desierto de los Leones site, which is
very close to the MCMA, with the reference site, El
Chico. Both sites are chosen as they are extreme
situations: the former is expected to be the most
affected due to its proximity to the urban area. The
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10
4
10
3
10
2
10
1
10
0
J.C. Pineda for RBS experiments. Work supported in
part by DGAPA-UNAM (contract IN204999) and
CONACYT (contracts F036-9109 and G-0010-E).
Concentration (µg/g))
El Chico
Desierto de los Leones
REFERENCES
1.
V
Cr Ni Cu Zn Ga As Br Rb Sr
Y
Molina, L., Molina, M.J., Eds. Air Quality in the
Mexico Megacity. Kluwer Academic Publishers,
Amsterdam, 2002.
2. CAM. Program to Improve Air Quality in the Mexico
City Metropolitan Area 2002-2010. Comisión
Ambiental Metropolitana, Mexico City, 2002. (In
Spanish).
Zr Pb
FIGURE 6. Mean elemental concentration in tree rings
from cores extracted at the Desierto de los Leones and El
Chico, for trace elements.
3.
120
1993
1999
Secretaría del Medio Ambiente del Gobierno del
Distrito Federal (SMA-GDF). Annual Report of Air
Quality in the Mexico Valley, Evaluation of
Environemntal Performance in Air. Mexico City,
1997. (In Spanish).
Soil Depth (cm)
90
4. Ford, D.E. y Kiester, A.R. Modeling the efects of
pollutants on the processes of tree growth IN Dixon,
R.K.; Meldahl, R.S.; Remark, G.A.; Warren, W. (eds)
Process Modeling of forest growth responses to
environmental stress. Timber Press, Oregon, USA,
1990 . Chap. 29.
60
30
0
0
1
2
3
4
5
6
5. Legge, A.H.; Kaufmann, H.C.; Winchester J. W. Treering analysis by PIXE for a historical record of soil
chemistry response to acidic air pollution, Nucl. Instr.
and Meth. B3 (1984) 507-510.
7
Ca concentration (mg/g)
FIGURE 7. Depth distribution of Ca in soils from
Iztapopocatépetl, in samples taken in 1993 and 1999.
6. McClenahen, J.R.; Vimmerstedt J.P.; Scherzer A.J.
Elemental concentrations in tree rings by PIXE:
statistical variability, morbility, and effects of altered
soil chemistry, Can. J. For. Res. 19 (1989) 880-88.
CONCLUSIONS
7. Ruvalcaba, J.L., Monroy, M., Morales, J.G., Lopez, K.
The New External Set-up of the Pelletron Acelerator at
U.N.A.M., Mexico. Abstracts of the Ninth International
Conference on PIXE and its Analitical Applications.
Guelph, Canada, 2001, 62.
The results presented in this work represent only
examples of the advances obtained in the study of
tree rings and soils from the Mexico Basin forests.
The effects of pollution were observed in several
cases, although more analyses are required in order
to validate statistically the information obtained until
now. The consideration of official pollution records
is still pending, as well as the explanation of the role
of elemental mobility in soils in the tree rings.
8. Maxwell, J.A., Teesdale, W.J., Campbell, J.L., The
GUPIX software package. Nucl. Instr. and Meth. B 95
(1995) 407.
9. WinDENDRO 2001b. Regent Instruments Inc., Québec,
Canada, 2001.
10. Watmough, S.A.; Hutchinson, T.C. Change in the
dendrochemistry of sacred fir close to Mexico City
over the past 100 years. Environmental Pollution 104
(1999) 79-88.
ACKNOWLEDGMENTS
11. Statistica for Windows, V. 4.5. Statsoft., Tulsa, OK,
USA, 1992.
The authors acknowledge the technical assistance
of K. López and F.J. Jaimes for PIXE analyses and
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