Growth ring formation of Dichrostachys cinerea and Senegalia mellifera in arid environments in Namibia

Dichrostachys cinerea and Senegalia mellifera trees are the most widespread invasive encroacher species in Namibia. Understanding their growth performance under arid conditions is key to understanding and managing the arid ecosystems of Namibia. To determine their utility in dendrochronology, we analyzed the formation of growth rings for two species from three sites in Namibia. We applied cross-dating techniques to 69 tree discs, the analysis of which confirmed the formation of annual growth rings. The rings are characterised by alternating layers of low and high vessel density and are separated by bands of marginal parenchyma. D. cinerea shows a well-defined pattern of growth rings, whereas the ring patterns of S. mellifera are more complex. Because aging trees tend to display narrower rings toward the outside of the stem, we applied both ring-width index and basal area increment methods in developing ring-width chronologies for the two species. Descriptive statistics for the chronologies indicate that the average ring chronologies are of good quality and that the trees are responding to common environmental signals. The basal area increment methodology produced better statistics than the ring-width index for these two species. Furthermore, statistical characteristics of the chronologies indicate that D. cinerea has a higher mean sensitivity (MS), expressed population signal (EPS), and signal-to-noise ratio (SNR) than S. mellifera. Thus, we find that both species are suitable for dendrochronological study, with D. cinerea being better than S. mellifera.