Doctoral Defense in Biology - Ingvild Ryde

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Doctoral candidate:
Ingvild Ryde

Title of thesis:
Chemical warfare in a degraded land: The allelopathic capacity and allelochemical diversity of Empetrum nigrum in Icelandic tundra ecosystems

Opponents:
Dr. Nicole van Dam, Scientific director at the Leibniz-Institute for Horticultural Sciences (IGZ) and Professor at the Friedrich-Schiller University Jena, Germany
Dr. Mikael Ohlson, Professor emeritus at the Faculty of Environmental Sciences and Resource Management, Norwegian University of Life Sciences, Norway

Advisor:
Dr. Ingibjörg Svala Jónsdóttir, Professor emerita at the Faculty of Life and Environmental Sciences, University of Iceland

Other members of the doctoral committee:
Dr. Elizabeth HJ Neilson, Associate Professor at the Department of Plant and Environmental Sciences, University of Copenhagen, Denmark (co-supervisor)
Dr. Kari Anne Bråthen, Professor at the Department of Arctic and Marine Biology, Arctic University of Norway (co-supervisor)
Dr. Judith Trunschke, Professor at the Faculty of Life and Environmental Sciences, University of Iceland (administrative supervisor)

Chair of Ceremony:
Dr. Snæbjörn Pálsson, Professor and Head of the Faculty of Life and Environmental Sciences, University of Iceland

Abstract:
Iceland has major issues with land degradation due to centuries of unsustainable land use. Degraded Icelandic tundra ecosystems do not easily recover after removing one of the main drivers of degradation – livestock overgrazing – and they appear to be “trapped” in degraded states. It is important to identify the “trapping mechanisms” potentially interfering with restoration efforts to avoid further loss of plant and soil resources. One such mechanism may be “chemical warfare” through the release of allelochemicals by unpalatable plants that inhibit seed germination and seedling growth in neighboring species and modify ecosystem processes. The aim of this thesis was to investigate the role of the unpalatable and allelopathic evergreen dwarf shrub Empetrum nigrum in keeping degraded Icelandic tundra ecosystems trapped in degraded states, by assessing the allelopathic capacity and allelochemical diversity of Icelandic Empetrum. This thesis comprises three research papers based on bioassays and chemical analyses following targeted and untargeted liquid chromatography-mass spectrometry metabolomics approaches. The results show that volcanic soils alleviate Empetrum allelopathic capacities considerably when compared with non-volcanic soils, reducing negative soil legacy effects of Empetrum-associated soils. Nevertheless, the allelopathic capacity of Empetrum is likely high enough to retard seedling establishment in Icelandic tundra ecosystems, considering the high amount of Empetrum leaf biomass in the system. The synoecious Empetrum group hermaphroditum was found to have stronger allelopathic capacities than the monoecious nigrum group, partly explained by higher foliar concentrations of the previously designated key allelochemical Batatasin-III. Furthermore, male nigrum plants were found to have stronger allelopathic capacities than female nigrum plants. That means that the magnitude of Empetrum allelopathic capacities likely vary with the relative abundance of intra-specific groups and sexes. These allelopathically distinct groups and sexes were used as a biological system to identify more potential Empetrum allelochemicals. The untargeted chemical analysis revealed an Empetrum chemical profile characterized by putative phenolics and terpenoids as expected, but also naphthalene- and alkaloid-derivatives which have not been recorded in Empetrum before. Chemical profiles were clearly distinct between the Empetrum groups, whereas there were no clear differences between the nigrum sexes. Empetrum allelopathic capacities may be driven by terpenoids, naphthalenes, alkaloids, and various phenolics besides Batatasin-III. Thereby, this thesis broadens our understanding of how Empetrum may chemically interact with its environment and is the first step towards expanding our mechanistic understanding of Empetrum allelopathic effects. In conclusion, Icelandic Empetrum has high allelopathic capacities and produces a wide range of potential allelochemicals by which it may contribute to keeping Icelandic tundra ecosystems trapped in degraded states. Future studies should aim to assess Empetrum allelopathic capacities in natural Icelandic tundra ecosystems and how they may change with climate warming.

About the doctoral candidate:
As an undergraduate student, Ingvild quickly developed a strong interest in plant ecophysiology and Arctic ecology. She completed her bachelor’s degree at the University of Copenhagen, Denmark, in 2016, writing a thesis on the effects of in-field warming on carbon fluxes from different vegetation types in Greenland. Ingvild’s interest in plants led her to pursue a master’s degree in plant science at the University of Copenhagen, which she completed in 2020. Her master’s thesis project combined field manipulations conducted in Tromsø with metabolic analyses of volatile and non-volatile specialized metabolites. The results demonstrated that insect herbivory affects the ecophysiology of mountain birch, with consequences for biosphere-atmosphere interactions and ecosystem processes. The master’s thesis project ignited Ingvild’s interest in chemical interactions between plants and their surroundings, which she subsequently explored in her PhD thesis, “Chemical warfare in a degraded land,” investigating the allelopathic capacity and allelochemical diversity of crowberry (Empetrum nigrum) in degraded Icelandic tundra ecosystems.