Marcus Hall defended his doctoral thesis in ecology, Causes and consequences of genetic variation in oak, at Linnaeus University this Friday 12 June 2026. The defence was held in the Azur lecture hall and was open to the public.
Professor Ben Sheldon of the University of Oxford was the opponent. The examination committee was Dr Bastien Castagneyrol of INRAE’s BIOGECO unit in Cestas, France, Associate Professor Jenny Hagenblad of Linköping University and Senior Lecturer Rosario Garcia Gil of the Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences. Professor Jonas Waldenström of Linnaeus University chaired the defence.
The thesis poses two questions about pedunculate oak, Quercus robur, at the northern edge of its range in Sweden: what shapes genetic diversity within its populations, and what that diversity goes on to do. Throughout his 4-year doctoral studies, Markus has conducted four studies to eludate these questions.
Markus' studies suggest that neutral genetic variation declined towards the northern margin, as long-standing theory predicts, but adaptive variation did not, and no signs of selection varying from stand to stand depending on timing of frost and interactions with other species. Diversity did not, however, make a stand grow better: it was unrelated to growth rate, to asynchrony in growth trends between neighbouring trees, and to how stable growth was over time. Where it did register was in the community living on and around the tree. Genetically diverse oak populations carried less oak powdery mildew, and that in turn shaped where leaf-mining and gall-forming insects occurred. Across six oak-associated groups (specialist and generalist arthropods, epiphytic lichens, mosses and vascular plants) greater genetic diversity went with higher species richness and abundance in some groups but not all, and where the effect appeared it was comparable to or stronger than that of oak biomass and latitude.
Markus' thesis conclusion is mixed : Oak’s genetic diversity is strong enough to show up in real forests rather than only in experimental plots, but it acts on the community around the oak rather than on the oak’s own growth, and it is specific enough to particular taxa to resist any sweeping rule. His recommendation for management is to conserve stand-specific adaptation and genetic diversity but also to treat genomic offset estimates with caution as a guide to assisted gene flow at this scale, since projected climate vulnerability varied widely between stands with no clear spatial pattern.
The examination committee approved the thesis, and Marcus Hall was awarded the degree of Doctor of Philosophy in ecology.
One of the four studies is already published: doi.org/10.1098/rsbl.2025.0180