Evaluation of X-ray densitometry to identify tree-ring boundaries of two deciduous species from semi-arid forests in Brazil

Abstract The presence of visible annual rings in semi-arid tropical trees may allow the application of dendrochronological methods. However, variation in water availability may cause the formation of narrow, irregular ill- or non-defined annual rings hindering the correct dating of tree-ring series. We aimed to evaluate X-ray densitometry as a method to identify tree rings of two deciduous tree species from the Caatinga forest, a semi-arid region in the northeast of Brazil, and compare with two other methods commonly used in dendrochronology, the sliding-stage micrometer and image analysis. Xylem was observed macro- and microscopically and wood anatomical features were assessed in Aspidosperma pyrifolium and Poincianella pyramidalis trees. In both species, tree-ring boundaries were identified considering intra-annual density patterns and wood anatomical features. No significant differences in tree-ring widths were found among methods. X-ray densitometry measurements showed a positive correlation with the measurements obtained with image analysis and sliding-stage micrometer in A . pyrifolium and P . pyramidalis , revealing the high reliability of the methods used. However, inter-correlation of tree-ring width series showed differences in the accuracy of crossdating across measuring methods. The maximum, mean and minimum density values were species-dependent, with mean wood density of A . pyrifolium lower than P . pyramidalis . Our results highlight X-ray densitometry as an important and complementary tool to identify tree-rings boundaries in semi-arid tree species, especially in A . pyrifolium . Along with other measuring methods, it may provide higher accuracy in dendrochronological studies in semi-arid or subtropical environments.