The applicability of radiocarbon dating to understanding lifespan and mortality patterns in the quiver tree, Aloidendron dichotomum (Masson) Klopper & Gideon F.Sm.

Thesis (MSc)--Stellenbosch University, 2024.

ENGLISH ABSTRACT: This thesis investigates the use of accelerator mass spectrometry (AMS) radiocarbon dating as a means of estimating the maximum longevity and time-of-death across the range of the quiver tree (Aloidendron dichotomum (Masson) Klopper & Gideon F.Sm.). The quiver tree was identified as an indicator species for climate change impacts due to measured increases in population mortality and decreased juvenile recruitment in regions of its range that have experienced warming and drying over the past century. Dead quiver trees persist in situ due to slow decay rates meaning a metric of mortality rate can be estimated as a proportion of the number of live to dead individuals in the population. The use of this metric was subsequently challenged due to doubt over the true time-of-death of trees in situ. The attribution of mortality to climate change was disputed citing instead centennial and millennial-scale population extinction debt as the cause for increased mortality and adult-heavy population structure. Discerning historical and contemporary causes of population mortality is, thus, of great importance for understanding and mapping climate change impacts on the quiver tree and co-occurring species. Doing so requires population demographic modelling which depends on metrics for the age of maximum longevity of the tree and verified time-of-death of the mortality class. In this thesis I aimed to develop radiocarbon dating methods for accurate estimation of these two metrics. After a first chapter reviewing the literature relevant to the quiver tree as an indicator species, my second chapter estimated the maximum age for four trees from the arid north of the species’ range in Namibia by analysing the radiocarbon ages of samples collected across the radius of the tree base. The oldest sample originated between 1277 - 1452 (95% CI) placing the maximum age of this tree at more than double that of the previous maximum estimates (350 years old). I discovered that the core tissue of the trees was consistently contemporaneous with the date-of-death, and that other samples along the radius deviated from the expected linear model of growth. This suggests material turnover and potential mixing of old and new carbon - a process which needs further investigation to understand how this impacts interpretation of results. As the core material is not present, Bayesian age-depth modelling should be used to estimate the true maximum age of the trees, and this can be supported by increasing the number of samples analysed. The third chapter of my thesis aimed to test whether trees in the mortality class died before or after 1950 using the presence or absence of a specific radiocarbon signal known as “bomb carbon”. This would test a hypothesis that the mortality is a relict of an early 1900’s drought. The majority of samples from the core and all of the samples from the edge of the tree post-date 1950, but five trees analysed only at the core pre-date 1950. I have deemed these results inconclusive considering material turnover at the core, and thus the inference which can be drawn from this chapter is incomplete. Despite this, radiocarbon dating shows promise as a forensic method for estimating an accurate year of death of the quiver tree if sampling is expanded to two proximal samples from the edge of the tree.

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Masters