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Abstract

The Macaronesian laurel forest is characterized by humidity-adapted, evergreen trees with glossy, entire and elongated leaves. Based on fossil data, this vegetation type has been regarded as a relict of Tertiary, European/Mediterranean forests since at least the middle of the 19th century. In contrast to that, more recent studies indicate that the Macaronesian laurel forest species may be much younger than previously thought, with the majority of the analyzed species dating to the Plio-/Pleistocene. Furthermore, they recovered a rather heterogeneous geographical origin, suggesting that the Mediterranean region, other Macaronesian vegetation zones as well as tropical areas have served as source areas for the corresponding species. Although previous analyses included quite characteristic taxa, e.g. all of the Macaronesian Lauraceae, only a small number (around 26%) of laurel forest genera has been studied to this day, most of them are woody.

In this dissertation, the biogeography of six typical and widespread Macaronesian laurel forest genera (Daucus, Geranium, Gesnouinia, Phyllis, Semele and Visnea) is studied, covering different life-forms and ecologies. Conducting molecular phylogenetic and dating analyses as well as ancestral area estimations, a) the timeframes for the colonization of Macaronesia and the laurel forest, b) the geographical origin of the colonizers and c) the timeframes for inter-archipelago and inter-island dispersal were studied. Furthermore, the usefulness of stem ages and crown ages for inferring the colonization times is tested. Additional analyses were conducted for Gesnouinia and Visnea. In Gesnouinia, the wood anatomy was studied as the genus was considered as potentially insular woody in previous studies, which would contradict a relict status. For Visnea, fossils of the extinct V. germanica from the Miocene to Pliocene of Germany and Italy were analyzed regarding their affinity to laurel forest V. mocanera using MicroCT scans.

The results obtained here provide further support for the heterogeneous origin of the Macaronesian laurel forest and indicate that stem ages should be preferred over crown ages for inferring the relict status. A relict origin of Visnea (Oligocene age) and the laurel forest taxa of Geranium (Miocene age) is very likely, whereas the situation is ambiguous in Semele and Daucus. The latter two are of Miocene age, but their phylogenetic position is poorly resolved. Laurel forest Gesnouinia and Phyllis originated within Macaronesia and are clearly no relicts from the Tertiary by their source area. Dispersal from or into the dry infra-Canarian vegetation is indicated for both genera, with the time frames differing. In Phyllis, dispersal falls into the Early Pliocene, whereas in Gesnouinia, an overlap with range-shifts associated with the Pleistocene glaciation cycles is recovered. The non-relictual trait of insular woodiness could not be unambiguously inferred for Gesnouinia. While woodiness in Gesnouinia probably is derived, it may have evolved prior to island colonization. Interarchipelago colonization between Madeira and the Canary Islands is inferred to be young in most taxa, overlapping with Pleistocene sea-level fluctuations and the timeframes recovered for species from other Macaronesian vegetation zones. The same is found for inter-island colonization within the archipelagos.

For the Macaronesian laurel forest as a whole, the newly generated data as well as literature data indicate that there is likely no obvious relationship between time of colonization and life-form or time of colonization and the extant ecological niche occupied within the forest. Instead, data points towards a link between time of colonization and the main source area of the colonizers. In the humid climate of the Late Miocene, habitat conservative dispersal from the Mediterranean/Europe to newly emerged islands and habitat space created by catastrophic events seems to have predominated. In the still humid Early Pliocene, the influx from the Mediterranean/Europe decreased and the majority of colonizers originated within Macaronesia. During the Late Pliocene climatic deterioration (cooler, drier and increasing seasonal), dispersal from the Mediterranean, probably non-habitat conservatively, was prevalent. In the course of the Pleistocene (Early and Middle), climatic changes and range shifts associated with the glaciation cycles possibly promoted the arrival of a large amount of Macaronesian taxa. Pleistocene establishment is also indicated for a number of Mediterranean/European taxa, but restricted to the Early Pleistocene. Colonization events from Asia, the New World and (Eastern) Africa seem to be rare and likely occurred prior to the Pleistocene. They may have been facilitated by the lack of e.g. climatic, tectonic or marine barriers during certain periods of time.