Stable isotopes in tropical tree rings: theory, methods and applications

The notion that many tropical tree species form annual growth rings has triggered research on their growth and its environmental drivers over long periods of time. Even more recently, a large number of studies have also analysed the natural abundance of stable isotopes in tropical tree rings. The rapid developments in this young field call for a review. Here, we focus on stable isotopes of carbon (13C), oxygen (18O) and nitrogen (15N). We start by explaining how environmental and physiological effects define the isotopic composition of wood in tropical trees. Abundance of 13C is mainly driven by water, light and nutrient availability. Here 18O values are chiefly determined by those of rainwater and additionally by rooting depth and factors determining leaf water evaporation. The 15N levels are determined by the 15N signature of nitrogen uptake, which in turn depends in complex ways on various processes in the nitrogen cycle. We then discuss methodological aspects of isotopes studies in tropical tree rings. An important requirement is that rings are reliably dated. Furthermore, a key methodological concern is that temporal changes in isotopic values can be confounded by tree-size driven changes, which can be avoided by sampling from a fixed diameter range or accounted for statistically. Next, 50 studies are reviewed that measured stable isotopes of C, O, and/or N in tree rings of a total of 85 tropical tree species. Temporal variation in both δ13C and δ18O was correlated with precipitation and El Niño Southern Oscillation variability. Seasonality in δ13C and δ18O was successfully used for delimiting visually non-distinct annual rings. Tropical tree responses to increasing atmospheric [CO2] were effectively quantified, using δ13C as a measure of intrinsic water use efficiency. And finally, anthropogenic changes in the nitrogen cycle in tropical forests have been inferred from δ15N. We conclude with methodological and ecophysiological recommendations for isotope studies in tropical tree rings. Future perspectives include the analysis of intramolecular isotopic distributions of isotopes in glucose that can advance our understanding of environmental effects on tropical tree physiology. Finally, we recommend that tropical tree ring isotope data are deposited in open access databases. A lay summary is available for this article