Annual Growth Rings in a Sample of Hymenaea Courbaril

IAWA Journal, Vol. 27 (2), 2006: 193–197

ANNUAL GROWTH RINGS IN A SAMPLE OF HYMENAEA COURBARIL

Jessica A. Westbrook1,*, Thomas P. Guilderson2 and Paul A. Colinvaux3

SUMMARY Several tropical tree species form regular growth rings that may be annual and thus be useful in paleoclimate reconstructions. Regular growth rings were observed in one stem disk of the tropical legume Hymenaea courbaril harvested near the city of David, Panama. By comparison to annual reference Δ14C values from temperate tree wood and air, ∆14C values from the growth rings in this H. courbaril sample indicated the rings were annual. During this study, H. courbaril was also observed to translocate recently produced photosynthate into older growth rings as sapwood is converted to heartwood. This process alters the overall ∆14C values of these transitional growth rings as organic carbon with a higher ∆14C content is translocated into growth rings with relatively lower ∆14C content. Proper chemical pretreatment can recover the original cellulose ∆14C content. Key words: Growth rings, Hymenaea courbaril, ∆14C.

INTRODUCTION Hymenaea courbaril L. (Fabaceae) is a large tropical hardwood tree. This species is widespread in Central and South America; there are related species in Madagascar and Africa (Lewis 1987). Hymenaea courbaril wood is diffuse porous with distinct growth rings demarked by marginal parenchyma. These distinct rings suggest that H. courbaril may have potential for dendrochronology (Enquist & Leffler 2001). The Panama area experiences a yearly dry season that may induce annual growth rings in H. courbaril. One stem disk from a H. courbaril tree harvested from Panama in 1997 was observed to form regular growth rings (Fig. 1). In order to determine the annual nature of these rings, radiocarbon (14C) analyses were performed on each growth ring of this stem disk and compared to atmospheric 14C data to determine if each ring represented one year of growth. If the bands in this tree species are confirmed to be annual, it may be possible to use it along with other species of Hymenaea in multi-proxy climate reconstructions.

1) Department of Plant Biology, University of California, 1 Shields Road, Davis, CA 95616, U.S.A. 2) Center for Accelerator Mass Spectrometry, UC/LLNL, 7000 East Avenue, Livermore, CA 94550 & Department of Ocean Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, U.S.A. 3) Marine Biological Laboratories, Woods Hole, MA 02543, U.S.A. *) Present address: Department of Horticulture, 134A Plant Science Bldg, Cornell University, Ithaca, NY 14853, U.S.A. Associate Editor: Thomas Yanosky

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Fig. 1. Cross section of a Hymenaea courbaril specimen felled in the fall of 1997 near David, Panama (43.2 × 35.6 cm nominal elliptical axis dimensions). The visible (density) variations are consistent with the bands being delimited by parenchyma tissue (the band is approx. 6 mm). Ring widths average 5.7± 2.9 mm and range from 2 to 15 mm.

MATERIALS AND METHODS A single stem disk of Hymenaea courbaril felled in September 1997 was obtained near the city of David, Panama (8° 26' N, 82° 26' W). The disk (~3" thick) was sanded to a smooth fine finish. Fifty-one distinct bands were observed. Bands that were continuous around the disk circumference were assumed to be annual. Samples of each ring were taken along one radius and treated with a modified DeVries treatment of successive acid and base treatments of 1 N HCl, 1 N NaOH, and 1 N HCl at 90 °C, followed by repeated rinsing with deionized (Milli-Q) water to remove wood lignins (Hoper et al. 1998). For all 51 samples, approximately 2.5 mg of pretreated wood was combusted with CuO and Ag to form CO2 gas in an evacuated and sealed quartz test tube. CO2 was reduced to elemental carbon in the form of graphite with H2 gas in the presence of Fe catalyst (Vogel et al. 1987). Δ14C values were determined for each graphite target at the Center for AMS, LLNL (Davis et al. 1990) and were reported in accordance with Stuiver and Polach (1977). The Δ14C reported values include a background subtraction determined from 14C free coal and a δ13C correction. Analytical error on average is ± 5‰. To present a more representative comparison with atmospheric Δ14C data, the tree ring data are presented as age corrected as described in Stuiver and Polach (1977). In order to assess chemical pretreatments, a small subset of 6 samples during the early rise in atmospheric Δ14C was processed to holocellulose with an additional acid-bleach pretreatment prior to graphitization and was then compared to Δ14C data of the same rings receiving only the standard acid and base pretreatment.

RESULTS Δ14C values were determined for each growth ring of Hymenaea courbaril, with each ring assumed to represent the growth during one year (Fig. 2). The Δ14C values form

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1000

Wellington Schaunisland / Vermont 800 Olympic Washington Firs H. courbaril, Panama, this study

600 C (‰)

14 400 ∆

200

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Date Fig. 2. ∆14C data of Hymenaea courbaril increment zones as compared to the reference ∆14C data of wood and air samples of known time, with a ± 5‰ error. Closed circles represent data from H. courbaril material pretreated with the DeVries method, and open circles represent data from H. courbaril material pretreated with an additional acid-bleach treatment to holocellulose. Air flask data are from Europe (Levin et al. 1994) and New Zealand (Manning & Melhuish 1994). Pre-bomb tree-ring data are from Stuiver and Quay (1981).

a distinct curve with an amplitude of ~550‰. The maximum Δ14C value is ~650‰ at the growth ring purportedly representing the 1967 growing season. The minimum Δ14C value is 106 ‰ which occurred in the last growth ring representing the 1997 growing season. The H. courbaril wood sampled for the 1957 growing season pretreated to holocellulose represents a minimum Δ14C value of approximately 95‰. A small and distinct peak in the Δ14C values occurs in the increment zones that represent growth in years in the late 1950s.

DISCUSSION The Δ14C values of Hymenaea courbaril were compared to Δ14C values of atmospheric CO2 samples from Wellington, New Zealand (Manning & Melhuish 1994), Vermunt/ Schaunisland, Germany (Levin et al. 1994), and tree-ring based measurements from Olympic Peninsula, USA firs (Stuiver & Quay 1981). Above-ground nuclear bomb test- ing in the Northern Hemisphere in the late 1950s and early 1960s created a spike in 14 14 atmospheric C. C in the atmosphere in the form of CO2 is taken up and incorpo- rated into wood cellulose in plants through photosynthesis. The elevated Δ14C values observed in growth rings were a result of the incorporation of elevated 14C levels in atmospheric CO2 during this time. It is expected that this tropical tree-ring based record will bear more of a resemblance to the Southern Hemisphere atmospheric record due to

Downloaded from Brill.com09/25/2021 06:24:27AM via free access 196 IAWA Journal, Vol. 27 (2), 2006 Westbrook, Guilderson & Colinvaux — Growth rings in Hymenaea 197 the migration of the intertropical convergence zone (ITCZ) and the intense recycling of air masses associated with deep convection in the tropics. In comparing wood samples of H. courbaril to the reference data, the similarity of Δ14C values of the two data sets suggest that H. courbaril forms annual growth rings. Hymenaea courbaril Δ14C values on DeVries treated samples form a small and distinct elevated spike in the late 1950s that does not correspond to the Δ14C values of some of the reference data. This variation is most likely due to the translocation of photosynthate from younger wood having a higher Δ14C content into older growth rings as sapwood is converted to heartwood. A similar observation was made by Worbes and Junk (1989) regarding the Δ14C values of wood from trees in Costa Rica and Venezuela. Many trees translocate photosynthate as secondary metabolites from the cambial region to older growth where it is stored as resinous compounds. Incomplete removal of these resinous compounds during chemical pretreatment can lead to a compromised carbon isotopic record when there is a large Δ14C difference between the respective years. This trend has been extensively observed in δ13C studies of oak (Quercus) and cedar (Libocedrus) (Hoper et al. 1998), and by Worbes and Junk (1989). Chemical pretreatments can be used to remove these resinous compounds, resulting in a reliable tree-ring α-cellulose based 14C record comparable to representative air flask samples (Stuiver & Quay 1981). In this study, samples from the late 1950s that were converted to holocellulose had lower Δ14C values that were more representative of the atmosphere reference records.

CONCLUSION The comparison of the Δ14C values in growth rings in this Hymenaea courbaril sample with Δ14C atmospheric values suggest the rings are annual, likely associated with the annual dry season in the Panama area. Only one stem disk of H. courbaril was exam- ined in this study. In order to confirm that growth rings are consistently annual within this tropical hardwood species, similar analyses of additional H. courbaril specimens must be completed. If the annual nature of H. courbaril growth rings is confirmed, its Δ14C record may be cross referenced with similar H. courbaril wood samples from different tropical regions in order to investigate whether Δ14C value differences are induced by environmental differences.

ACKNOWLEDGEMENTS

Funding for this research was provided by the University of Californiaʼs Directed Research and Development Program. Radiocarbon analyses were performed under the auspices of the U.S. Depart- ment of Energy by the University of California Lawrence Livermore National Laboratory (contract W-7405-Eng-48). We thank Eduarado Montenegro for field assistance.

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