8. elemental variability in tree rings of pine as observed by pixe

10th International Conference on Particle Induced X-ray Emission and its Analytical Applications PIXE 2004, Portorož, Slovenia, June 4-8, 2004 http://pixe2004.ijs.si/

917.1

Elemental Variability in Tree Rings of Pine as Observed By PIXE G. Calva-Vásquez (1), J. Miranda(2), and C. Solís(2), L. Huerta-Arcos(3)

(1) Laboratorio de Contaminación Atmosférica, FES-Z, UNAM, Av. I. Zaragoza Esq. Batallón 5 Mayo, Iztapalapa, 09230, M éxico D.F., México

(2) Instituto de Física, UNAM, Apartado Postal 20-364, 01000 México, D.F., México (3) Instituto de Investigaciones en Materiales, UNAM. México D. F. 04510

ABSTRACT

A study of elemental concentrations in tree-rings of pine along the past ten years from the template forests Desierto de los Leones, Iztapopocatépetl, Villa del Carbon, and El Chico (reference or control site), in Central Mexico, was carried out to investigate the effect of atmospheric deposition on the trees. These sites are located within a 90 km radius from Mexico City. Elemental analyses were done with Particle Induced X-ray Emission (PIXE) spectrometry. The results showed the major concentration in tree-rings corresponding to summer for elements S, Ni, Cu, and Zn.

Keywords: Dendrochemistry, tree-ring, PIXE Corresponding author: Germán Calva-Vásquez, UNAM. [email protected]

1. INTRODUCTION

The massive death of the trees in the forest basin of Mexico has caused a decrease from 110,000 ha down to 96,000 ha of pine and sacred fir forest areas in a few years. In order to explain such effect, several hypotheses exist, from the chemical immobility of the elements, to the metal accumulation in the ring, which does not allow the proper function of the physiological processes in the tree. Additionally, particulate matter and atmospheric gases (such as SO2) may interact with the tree either through the bark and leaves, or through the deposit in soils. Regarding the latter process, results about the behavior of several of these metals in the soils of the forests under study has been discussed in detail elsewhere [1]. In dendrochemistry [2], the chemical makeup of the annual wood increment is considered as a basic phenomenon. The trees produce annual growth rings, distinguished as light-colored spring-time, and followed by dark-colored summer-time. These regions might be used as monitors of the environmental changes [3]. The goal of this work is to investigate about the elemental variations in the spring rings as contrasted to those found in summer rings. The changes in the elemental concentrations of the tree-rings are expected to be related with growth rate and the presence of the elements due to atmospheric pollution and subsequent deposition [4]. 2. MATERIALS AND METHODS 2.1 Study area Four forest areas in the Mexico Basin were chosen, namely, Villa del Carbón, Iztapopocatepetl, El Chico and Desierto de los Leones (Fig. 1). The forests are mainly composed by sacred fir (Abies religiosa), in the altitude 2,850 m to 3,200 m; and pine (Pinus hartwegii, P. montezumae, P. patula) distributed between 3,000 m and 3,800 m. The cores were obtained at breast height using a 5 mm diameter stainless steel increment corer Presser (35 cm). Prior to each insertion in the tree, the corer was rinsed with reagent grade acetone. After removal from the increment corer, the cores were placed in plastic tubes for transport back to the laboratory; the cores were dried at 100 °C for 48 hours for elimination of the excess of water in the wood.

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Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical Applications , Portorož, Slovenia, June 4-8, 2004

Although the cores were 35 cm long, for this study only ten rings were considered, five for spring and five for summer. No further preparation of the samples was done. Direct PIXE analyses of cores samples were carried out with an external 3 MeV proton beam, generated by the Pelletron accelerator at the Instituto de Física, UNAM. A 155 eV resolution Ge-Hyperpure detector (placed at 55° with respect to the beam direction and 7 cm away from the sample), detected the emitted X-rays. The efficiency of the X-ray detection system was measured using a pellet of the NIST SRM-2704 Buffalo River standard reference material. Also, a 25 µm Mylar filter was used in front of the detector together with a polyethylene funny filter. The PIXE spectra were analyzed using the GUPIX program [5]. The counts due to Ar were used as a relative measure of the ion charge reaching tree rings. Uncertainties in elemental concentrations were estimated in 8%.

Figure 1: Location of the sampling sites in the Mexican Basin.

4. RESULTS AND DISCUSSION

The PIXE analyses showed (Figures 2 and 3) that, generally, higher concentrations are found during the summer period. S increments its concentration in the tree-ring summer rings (200 µg/g to 3,500 µg/g), as compared to spring-time (0-200 µg/g). The order of magnitude of these results are in agreement with those given by Harju et al. [6], who reported that the concentration of S in inner bark of pine was around 1700 µg/g. Regarding the differences among sites, the highest level was found in the forests Villa del Carbón and Iztapopocatépetl. The concentrations of this element in tree-rings must be related to the atmospheric contamination in the Mexican Basin by strong emissions of SO2 gas and particula te matter transport of industrial and volcanic sources, as they have been found in Mexico City’s atmosphere in high concentrations [7]. The levels found for Ni are comparable to those measured in other work [8]; however, this is not the case for Cu and Zn. Summer is the rainy season in the Mexico Basin; thus, it is expected to have a stronger deposition along this season, as compared to spring. This makes the studied elements more readily available to the trees. The pathways along which S, Ni, Cu, and Zn may be incorporated into the pine xylem are two, mainly: uptake by roots via acidic soils (metal dissolution) [1] and through bark [8, 9]. Elemental concentrations in the atmosphere are higher during the spring, suggesting an uptake mainly via bark in this season;

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however, the highest elemental contents in the summer rings are in agreement with the availability for uptake through roots, besides the bark processes.

Figure 2: Comparison among S and Ni concentrations in tree-rings, spring-time against summer–time in the sampling sites (Villa del Carbon, Izta-popo, Desierto de los Leones, and El Chico). Ring number 1 is the most recent one.

Figure 3: Comparison among Cu and Zn concentrations in tree-rings, spring-time against summer–time in the sampling sites (Villa del Carbon, Izta-popo, Desierto de los Leones, and El Chico). Ring number 1 is the most recent one.

0 1 2 3 4 5 60.00.51.01.52.02.5

Tree-ring number

El Chico

[Cu] µg/g

0.00.51.01.52.02.5

Desierto de los Leones

0.00.51.01.52.0

Izta-popo

0.60.81.01.21.41.6

Villa del Carbon

SUMMER

0 1 2 3 4 5 6

1.0

1.2

1.41.6

Tree-ring number

El Chico1.01.5

2.02.5

3.0

Desierto de los Leones0.0

0.2

0.4

0.6

Izta-popo

0.81.01.21.41.61.8

Villa del Carbon[Cu] µg/g SPRING

0 1 2 3 4 5 60

12

3

4

Tree-ring number

El Chico

0

24

6

8


0.4

0.8

1.2

1.6

Izta-popo

SUMMER

0.40.60.81.01.21.4

Villa del Carbon

[Zn] µg/g

0 1 2 3 4 5 6

1.0

1.21.4

1.6

Tree-ring number

El Chico2468

1012


0.0

0.2

0.4

0.6

Izta-popo

[Zn] µg/g

0.81.01.21.41.61.82.0

Villa del Carbon

SPRING

0 1 2 3 4 5 6500

1000

1500

Tree-ring number

El Chico

[S] (µg/g)

0

1000

2000

3000

Desiertos de los Leones

0

1000

2000

3000

Izta-popo

1000

2000

3000

4000

Villa del Carbon

SUMMER

0 1 2 3 4 5 6

1000

1500

2000

Tree-ring number

El Chico

0

500

1000

1500


0500

100015002000

Izta-popo

500

1000

1500

2000

Villa del Carbon[S] (mg/gr) SPRING

0 1 2 3 4 5 60.00.51.01.52.02.5

Tree-ring number

El

Chic

o

0.00.51.01.52.02.53.0


0.0

0.5

1.0

1.5

Izta-popo

0.00

0.25

0.50

0.75

Villa del Carbon[Ni] (µg/g)

SUMMER

0 1 2 3 4 5 6

0.2

0.4

0.6

0.8

Tree-ring number

El Chico

0

1

2

3 Desierto de los Leones

0

Izta-popo

below detection level

0.20

0.25

0.30

0.35

0.40

Villa del Carbon[Ni] (µg/g)SPRING

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5. CONCLUSIONS

• There were significant differences in elemental concentrations measured in spring tree rings, as compared to summer rings.

• The higher elemental contents measured for summer rings are explained by a double uptake process: via bark and roots, due to a more important deposition along this season.

ACKNOWLEDGMENTS

The authors thank the technical assistance of K. López and F.J. Jaimes for Pelletron accelerator operation. Work partially supported by PAPIIT, contract IN216903.

REFERENCES

1. Calva-Vásquez, G., Miranda J., Huerta-Arcos, L., Solis, C. and Cetina, V. M., Rev. Int. Contam. Ambient., 2004. submitted.

2. Nabais, C., Freitas, H., and Hagemeyer, Sci. Total Environ. 232, 33-37 (1999). 3. Prohaska, T., Stadlbauer, C, Wimmer, R., Stingeder, G., Latoczy, Ch., Hoffmann, E., Stephanowitz, H., Sci.

Total Environ. 219, 29-33 (1998). 4. Miranda, J., Calva-Vásquez, G. Solís, C., and Huerta, L., AIP Conf. Proc. 680, 444-447 (2003). 5. Maxwell, J. A., Teesdale, W. J., and Campbell, J. L., Nucl. Instr. and Meth. B 95 (1995) 407. 6. Harju, L., Saarela, K-E., Rajander, J., Lill, J-O., Lindroos, A., and Heselius, S-J., Nucl. Instr. and Meth B 189,

163-167 (2002). 7. Miranda, J., Barrera, V.A., Espinosa, A.A., Galindo, O.S., Núñez-Orosco, A., Montesinos, R.C., Leal-Castro,

A. and Meinguer, J. Nucl. Instr. and Meth B 219-220, 157-160 (2004). 8. Watmough, S. A. and Hutchison, T. C., Environ. Pollut. 104, 79-88 (1999). 9. Cutter, B. E., Guyette, R. P. J. Environ. Quality 22, 611-619 (1993).

8. elemental variability in tree rings of pine as observed by pixe

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