| Summary: | Parasitic plants are found in nearly every major terrestrial ecosystem and can be divided into two groups, holoparasites (lacking chlorophyll) and hemiparasites (chlorophyllous). The host-parasite connection is either situated above (stem parasites) or below ground (root parasites). In this thesis I examine the host-parasite interactions, host range and selection, and population dynamics of four root hemiparasitic species in a subarctic habitat in northernmost Sweden. Two species were annuals (Euphrasia frigida and Rhinanthus minor) and two were clonal perennials (Bartsia alpina and Pedicularis lapponica). Resources (e.g., nitrogen and carbon) are usually thought to be transported only from the host plant to the hemiparasite, thus promoting the growth of the parasite. Interestingly, however, in a study on B. alpina I found that there is probably transport in the opposite direction as well. Furthermore, nitrogen-rich hosts have often been shown to result in stronger parasite growth, but in the study on B. alpina I found that a species with a lower content of nitrogen might effect the higher growth stimulation, perhaps due to its mycorrhizal connections. A surprisingly high number of species was found to be hosts for B. alpina and P. lapponica using two methods: visually examining haustoria and monitoring carbon flow from potential hosts. As potential hosts differ in suitability, I experimentally tried to find out whether E. frigida and R. minor could selectively choose their hosts. No clear answer emerged; the results differed depending on whether the actual number of haustorial connections or the number of haustoria per g host root was analysed. The positive effect of a suitable host, however, seems to last only one generation in the annual E. frigida since I found no carry-over effects from the maternal host. All four species fluctuated in both density and flowering frequency during a five-year study period, particularly the two annual species. Both weather variables, coexisting species, and the presence of ...
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