Abstract
In Amazonian non-flooded forests with a moderate dry season, many trees do not form anatomically definite annual rings. Alternative indicators of annual rings, such as the oxygen (δ(18)Owc) and carbon stable isotope ratios of wood cellulose (δ(13)Cwc), have been proposed; however, their applicability in Amazonian forests remains unclear. We examined seasonal variations in the δ(18)Owc and δ(13)Cwc of three common species (Eschweilera coriacea, Iryanthera coriacea, and Protium hebetatum) in Manaus, Brazil (Central Amazon). E. coriacea was also sampled in two other regions to determine the synchronicity of the isotopic signals among different regions. The annual cyclicity of δ(18)Owc variation was cross-checked by (14)C dating. The δ(18)Owc showed distinct seasonal variations that matched the amplitude observed in the δ(18)O of precipitation, whereas seasonal δ(13)Cwc variations were less distinct in most cases. The δ(18)Owc variation patterns were similar within and between some individual trees in Manaus. However, the δ(18)Owc patterns of E. coriacea differed by region. The ages of some samples estimated from the δ(18)Owc cycles were offset from the ages estimated by (14)C dating. In the case of E. coriacea, this phenomenon suggested that missing or wedging rings may occur frequently even in well-grown individuals. Successful cross-dating may be facilitated by establishing δ(18)Owc master chronologies at both seasonal and inter-annual scales for tree species with distinct annual rings in each region.
Highlights
Tropical trees growing in climates with weak seasonality generally do not form anatomically distinct or regular annual rings
The consistency between the seasonal patterns of δ18Oprec and δ18O of wood cellulose (δ18Owc) in the EC samples supports our hypothesis that the δ18Owc of the trees reflects δ18Oprec
The concurrent increase in δ18Owc and δ13C of wood cellulose (δ13Cwc) values in some individuals implies that the seasonal δ18Owc pattern can reflect physiological responses to water deficit in some cases
Summary
Tropical trees growing in climates with weak seasonality generally do not form anatomically distinct or regular annual rings. Our knowledge of forest dynamics in the region, including rates of individual tree growth, mortality, and lifespan, is limited (Chambers et al 2013), in part due to the absence of annual rings as a convenient dating tool. Such information is critical for understanding many processes related to forest dynamics and carbon cycling and for evaluating the impacts of future climate change on tropical ecosystems (e.g., Clark et al 2003; Clark 2007; Brienen et al 2010; Vlam et al 2014).
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