Abstract
Isotopes in tropical trees rings can improve our understanding of tree responses to climate. We assessed how climate and growing conditions affect tree-ring oxygen and carbon isotopes (δ18OTR and δ13CTR) in four Amazon trees. We analysed within-ring isotope variation for two terra firme (non-flooded) and two floodplain trees growing at sites with varying seasonality. We find distinct intra-annual patterns of δ18OTR and δ13CTR driven mostly by seasonal variation in weather and source water δ18O. Seasonal variation in isotopes was lowest for the tree growing under the wettest conditions. Tree ring cellulose isotope models based on existing theory reproduced well observed within-ring variation with possible contributions of both stomatal and mesophyll conductance to variation in δ13CTR. Climate analysis reveal that terra firme δ18OTR signals were related to basin-wide precipitation, indicating a source water δ18O influence, while floodplain trees recorded leaf enrichment effects related to local climate. Thus, intrinsically different processes (source water vs leaf enrichment) affect δ18OTR in the two different species analysed. These differences are likely a result of both species-specific traits and of the contrasting growing conditions in the floodplains and terra firme environments. Simultaneous analysis of δ13CTR and δ18OTR supports this interpretation as it shows strongly similar intra-annual patterns for both isotopes in the floodplain trees arising from a common control by leaf stomatal conductance, while terra firme trees showed less covariation between the two isotopes. Our results are interesting from a plant physiological perspective and have implications for climate reconstructions as trees record intrinsically different processes.
Highlights
For δ18O variation in tree ring cellulose (δ18OTR) most of the predicted variation comes from δ18Osw, but leaf water enrichment caused by changes in vapour pressure difference (VPD) and gsW responses contribute significantly to predicted δ18OTR for the two trees growing at the drier sites
The observed average amplitude for δ13CTR is similar to predictions (~1‰). δ13CTR in the initial section of individual rings was frequently higher than predicted in the M. acaciifolium from the wet floodplain and in some years for the C. odorata from moist terra firme, both of which show pronounced δ13CTR increases of up to 2‰ across ring boundaries (Figure 4b and c—red lines)
We investigated δ13C and δ18O in cellulose of two Amazon terra firma and two floodplain trees located along a precipitation gradient
Summary
Intra-annual, high-resolution oxygen and carbon isotopes are increasingly being used for a multitude of applications, including climate reconstructions (Barbour et al 2002, Ohashi et al 2009, Roden et al 2009, Fichtler et al 2010, Managave et al 2011), age and growth rate determinations in ringless tropical trees (Poussart et al 2004, Poussart and Schrag 2005, Pons and Helle 2011), and for studying seasonality in growth and use of carbohydrate reserves (Helle and Schleser 2004, Ohashi et al 2009, Fichtler et al 2010, Gulbranson and Ryberg 2013). Isotope studies of tropical trees remain scarce, despite their great potential to improve our understanding of tree functioning and for climate reconstructions (van der Sleen et al 2017) and the importance of these vast forests for the global carbon cycle (Phillips et al 2009, Beer et al 2010, Brienen et al 2015, Pan et al 2015). There is little information about what processes dominate variation of tree ring oxygen and carbon isotopes (δ18OTR and δ13CTR) in tropical environments, and how this varies between different tropical tree species. The contribution of leaf water enrichment to the final δ18OTR may vary between species and environments, due to variation in leaf transpiration arising from specific differences in leaf traits (e.g., varying pathlengths, Kahmen et al 2008, Holloway-Phillips et al 2016) and/or site humidity levels (Barbour and Farquhar 2000, Barbour et al 2002, Kahmen et al 2011)
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