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Available online at www.sciencedirect.com R Earth and Planetary Science Letters 218 (2004) 301^316 www.elsevier.com/locate/epsl Resolving seasonality in tropical trees: multi-decade, high-resolution oxygen and carbon isotope records from Indonesia and Thailand Pascale F. Poussart a;Ã, Michael N. Evans b, Daniel P. Schrag a a Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, USA b Laboratory of Tree-RingResearch, University of Arizona, Tucson, AZ 85719, USA Received 16 June 2003; received in revised form 27 October 2003; accepted 4 November 2003 Abstract Dendrochronological techniques have found limited applications in the tropics because of invisible or indistinct banding in wood. The seasonal cycles of rainfall and relative humidity in these regions, while not sufficient to produce distinct visible rings, may still generate seasonal signals in the oxygen isotopic composition of tree cellulose which can be used for climate reconstruction and chronology. We explore this approach using trees from Indonesia and Thailand, from three different families. Multi-decade N18O records from Javanese cross-dated teak rings and bomb radiocarbon-dated Suar wood lacking visible rings demonstrate the reproducibility of the signal between trees grown at the same locality as well as from wider geographical regions. These results confirm predictions that the trees oxygen isotopic signatures reflect an external climatic forcing. High-resolution N18O records reveal large seasonal cycles: up to 4x for Javanese Suar samples and up to 18x for a Thai Podocarpus sample. We show that the six N18O and N13C cycles measured on a Podocarpus match the number of growth years for the period spanning the time of wounding and cutting of the wedge section. This result demonstrates that the isotopic cycles found in this tree with indistinct annual rings are indeed seasonal and could be used for chronology. We present evidence that stable isotope chronologies of tropical trees also contain insights in tropical tree physiology and growth dynamics. ß 2003 Elsevier B.V. All rights reserved. Keywords: oxygen isotopes; carbon isotopes; cellulose; dendrochronology; dendroclimatology 1. Introduction a¡ected by anthropogenic climate change [1]. Others have attributed these patterns to natural Patterns of tropical climate variability observed variability on decadal time scales [2]. Because the over the past two decades have led some scientists tropics appear to play an important role in global to suggest that the climate of this region is being climate [3^5], it is critical that we reconstruct pat- terns of natural variability over the last several centuries. * Corresponding author. Tel.: +1-617-495-2664; One approach to reconstruction of natural cli- Fax: +1-617-496-4387. mate variability involves the generation and sta- E-mail address: [email protected] (P.F. Poussart). tistical analysis of multi-century climate records. 0012-821X / 03 / $ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00638-1 EPSL 6924 21-1-04 Cyaan Magenta Geel Zwart 302 P.F. Poussart et al. / Earth and Planetary Science Letters 218 (2004) 301^316 Unfortunately, only a very small number of loca- stein et al. [14] suggested that just as the isotopic tions around the world have instrumental weather composition of meteoric water varied with merid- records extending back more than a century and ional temperature changes, so should the hydro- most of them are con¢ned to Europe and eastern gen isotopic composition of biological systems North America [6]. Proxy records from the vary with climate. The idea has since been con- tropics, such as those derived from corals, ice ¢rmed in many studies using ND, N13C and N18O cores, varved sediments and speleothems have of tree cellulose. In this section, we review two yielded valuable information about the climate models which describe the physical and biochem- history of the tropical Paci¢c [7^10] (see ¢g. 1 of ical pathways taken by the oxygen and carbon [11]). However, the scarcity of sub-annually re- isotopes during their incorporation into biomass. solved climate proxies from tropical locales has Because the trees used for reconstruction were not impeded assessment of tropical climate evolution located close to meteorological stations, we do over the past few centuries, particularly in terres- not have access to su⁄cient environmental data trial environments. to quantitatively interpret our data using such Reliable chronologies derived from dendro- models. However, they are useful for understand- chronological methods have been few in the ing how the data we measure can be related to tropics. The general absence of anatomically dis- environmental variables. tinct annual growth rings and/or obstacles posed by discontinuous banding and presence of false 2.1. N18O model rings in many tropical tree species make accurate age-modeling di⁄cult [12]. In this paper, we re- Of the three possible sources of oxygen for pho- view the theoretical framework and demonstrate tosynthetically produced carbohydrates (H2O, an application of tropical isotope dendroclimatol- CO2 and O2), only the isotopic signature of ogy to samples from the forests of Indonesia and H2O is imprinted on the ¢nal cellulose product Thailand. This strategy exploits the pronounced [15^18]. An isotopic mass balance for N18Oof seasonality of tropical rainfall and humidity, ex- tree cellulose is described in Eq. 1 (e.g. [19]), 18 18 18 pressed in the oxygen isotopic composition of tree where N Ocx, N Owx and N Owl correspond to cellulose, to produce seasonally resolved climatic the xylem cellulose, xylem water and leaf water records from tropical woods. In addition, we use isotopic compositions, respectively, fo represents carbon isotope data, sample cross-dating, bomb the fraction of the carbon-bound oxygen that radiocarbon measurements, and known wound- gets exchanged with medium water (V0.42) and ing/cutting dates to test the hypothesized stable Oo is the biochemical fractionation factor (+27x) isotopic chronometer. From these multi-decadal [20]. modern records, we discuss the potentials and N18O ¼ f WðN18O þ O Þþð13f ÞWðN18O þ O Þ ð1Þ limitations of this method to yield insights about cx o wx o o wl o tropical plant ecophysiology and growth dynam- ics as well as help close paleo-observational gaps 2.1.1. E¡ect of source water in the forests of southeast Asia. At tropical latitudes, Dansgaard [21] observed 18 an inverse relationship between the N Orain and the amount of rain (low/high during the wet/dry 2. Controls on the stable isotopic composition of season, with as much as a 15x amplitude). The tropical wood model described in Eq. 1 implies that such season- 18 ality in the N Orain will get recorded in tropical The idea of using light stable isotopic signals trees which use soil water for their growth [11]. contained in tree cellulose as recorders of climate Here, the terms soil water and groundwater refer was ¢rst put forward by Urey [13] when he ob- to the water fractions located above and below served that the N13C of wood varied with environ- the water table, respectively. The amplitude of mental air temperature. Three decades later, Ep- this signal will be dampened for large trees EPSL 6924 21-1-04 Cyaan Magenta Geel Zwart P.F. Poussart et al. / Earth and Planetary Science Letters 218 (2004) 301^316 303 ( s 10 m) because they tend to rely more heavily plains why the discrepancy increases as transpira- on groundwaters which may have residence times tion rates increase. long enough to mask the seasonality in the signal Assuming that tree growth spans a period long 18 [22]. N Owx represents a mixture of soil water and enough to be a¡ected by the seasonal variations groundwater for which the proportions may vary in relative humidity (RH) and that intra-seasonal throughout the growing season. Soil water iso- carbohydrate transfer is negligible, high-resolu- topic composition should re£ect that of rainfall tion measurements of N18O of tropical tree cellu- but could be altered due to evaporative enrich- lose should re£ect the sub-annual pulsing of wet/ ment at the soil surface. There is generally no dry climate of the tropics [11]. In addition, be- fractionation associated with water uptake at the cause much of the climate variability that charac- roots [23]. terizes tropical latitudes occurs over interannual time scales (ENSO, southeast Asian monsoon), 2.1.2. E¡ect of leaf water enrichment these records should be sensitive to these events The model developed by Craig and Gordon [24] as they are themselves ampli¢cations of a season- describes two isotopic fractionation e¡ects associ- al-like cycle and should be independent of the ated with the process of evaporation over an open occurrence of annual growth rings. Given the va- water surface: an equilibrium e¡ect associated riety of tree physiologies, water uptake patterns with the phase change from liquid water to vapor and root structures found in tropical forests, we 18 (liquid phase gets enriched because the H2 O mol- expect that relating the precipitation amount and 16 ecule has a lower vapor pressure than H2 O RH to the xylem water and cellulose isotopic (Oe = 9.8x at 20‡C [25]) and a kinetic e¡ect composition may in some cases be masked by caused by the slower di¡usion rate of the heavier species-speci¢c e¡ects. 18 16 H2 O molecule compared to H2 O. This evapora- tive enrichment model has been modi¢ed by 2.2. N13C model Dongmann and Nu«rnberg [26] and Flanagan et al. [27] to account for evaporation taking place As opposed to the more recent development of at the leaf surface (Eq. 2). The extent of leaf water oxygen isotope models for plant biomass, the in- enrichment depends on the equilibrium (K*) and corporation of carbon isotopes into plants has kinetic fractionation (Kk (air) and Kkb (boundary been studied and modeled for over two decades layer)) e¡ects mentioned above, the ratio of the [30,31].

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