Historical droughts recorded in extended Juniperus procera ring-width chronologies from the Ethiopian Highlands

Abstract

In the Horn of Africa, little is known about temporal changes in hydroclimate owing to the influence of multiple weather systems, the complex terrain, and the sparse instrumental records. Absolutely dated tree-ring records offer the potential to extend our understanding of climate into the pre-instrumental era, but tree-ring studies in this region, and indeed all of tropical Africa, have been rare largely due to lack of an annual climate cycle that reliably produces annual tree-rings. In this study, 40 cores were obtained from 31 Juniperus procera trees growing in the grounds of Ethiopian Orthodox Tewahedo churches in the Gonder region of Ethiopia. The samples were cross-dated using a re-iterative process involving identifying anatomical features from high-resolution images. The tentative ring-width chronologies were revised after the determination of bomb-peak accelerator mass spectrometry radiocarbon dates. Individual series were significantly correlated to the respective master chronologies (r > 0.55; P < 0.05), and expressed population signal values ranged from 0.55 to 0.92. Historical drought years were successfully traced in the chronologies by pointer year analysis. This study confirms that Juniperus procera growing in areas of unimodal precipitation exhibits annual tree-rings and offers the potential as an indirect measure of past climate.

SabaTracheid 1.0: A Novel Program for Quantitative Analysis of Conifer Wood Anatomy — A Demonstration on African Juniper From the Blue Nile Basin

“…SabaTracheid ’s functions were demonstrated using tracheid LD and WT measurement data from 30 tree rings (AD 1901–1930) of an absolutely dated core sample of Juniperus procera from Gonder Qusquam church, Ethiopia, that was confirmed by radiocarbon dating ( Gebregeorgis et al, 2020 ). Firstly, measured values of tracheid LD and WT ( Figure 1 ) for five to eight radial rows per tree ring were entered into Excel spreadsheets named after each tree ring (e.g., 1905; Figure 2 ).…”

“…Globally, instrumental climate records are characterized by low temporal and spatial resolution which calls for climate proxies from natural archives, preferably tree rings. Moreover, in the Tropics, where the instrumental climate record is inadequate, dendroclimatology is limited to annual or seasonal resolution ( Worbes, 2002 ; Mokria et al, 2017 ; Gebregeorgis et al, 2020 ). However, finer temporal resolutions could be attained by extracting environmental information recorded in the dimensions of conifer wood cells, as it has been done for temperate and Mediterranean regions ( Panyushkina et al, 2003 ; Liang et al, 2013 ; Hetzer et al, 2014 ; Castagneri et al, 2018 ; Piermattei et al, 2020 ).…”

“…Existing methods are unable to consider the high intra-annual variation in tree ring width and frequent occurrence of micro rings. For instance, tree rings of J. procera are chracterised by 2–5 tracheids per radial row ( Gebregeorgis et al, 2020 ). This variation is a characteristic feature of most tropical and some Mediterranean conifers suitable for dendrochronology ( Gebregeorgis et al, 2020 ; Lange et al, 2020 ).…”

“…For instance, tree rings of J. procera are chracterised by 2–5 tracheids per radial row ( Gebregeorgis et al, 2020 ). This variation is a characteristic feature of most tropical and some Mediterranean conifers suitable for dendrochronology ( Gebregeorgis et al, 2020 ; Lange et al, 2020 ). Most dendroanatomical studies conducted in temperate or Mediterranean regions do sectoring prior to the measurement and standardization of tracheids ( Castagneri et al, 2017a ; Lange et al, 2020 ).…”

“…Moreover, the program provides fine temporal resolution of anatomical series by sectoring tracheidograms into multiple sectors per radial file. In this article, the program is demonstrated using tracheid measurement data from African Juniper tree rings from the Upper Blue Nile basin, Ethiopia ( Gebregeorgis et al, 2020 ).…”