Peter van der Sleen, Pieter A. Zuidema, Thijs L. Pons
Tree-ring research is a well-established field in temperate and other strongly seasonal climates. But during recent decades, the formation of annual growth rings has also been shown for a large and growing number of tropical trees, thus allowing tree-ring research in tropical climates. Even more recently, researchers have started to analyse the stable isotope composition in tropical tree rings. These measurements provide opportunities to reconstruct environmental conditions over multiple centuries, to evaluate the drivers of tree growth, and to assess effects of climate change. Here, we review published studies on stable isotopes of carbon (13C), oxygen (18O) and nitrogen (15N) in tropical tree rings and discuss their findings.
Stable isotopes have a larger number of neutrons compared to the common form of the element and as a consequence exhibit slightly different rates of transformation and transport processes, compared to the common, lighter, forms of the atom. This results in small differences in abundance of the isotopes between the product, wood in this case, and the source from which it originates. The sources of interest are CO2 for C, H2O for O and nitrate and/or ammonium for N. These elements are incorporated in wood constituents, such as cellulose (C and O) and protein (N).
Variation in abundances of isotopes in tree rings is thus representative, in the first place, of changes in their sources over time. When changes in the source are known, the magnitude of the difference in isotope abundances between product and source can indicate how a tree responded to its environment. We present theoretical frameworks for interpreting variation in isotope abundances and discuss findings of studies conducted so far. In short, 13C was successfully used for deriving light and drought conditions, 18O has been related to regional climate variability such as precipitation amount and El Niño events, and the few studies where 15N was measured suggest that it might record changes in atmospheric N deposition. We also discuss several methodological issues pertaining to tree ring studies in general and isotope studies of tropical trees in particular. Finally we formulate recommendations for future studies.
Image: Tropical trees stem discs. Image provided by authors