Diversity, composition, and biogeography of trees in tropical mountain forests of Sulawesi, Indonesia
by Fabian Brambach
Date of Examination:2019-06-25
Date of issue:2019-10-28
Advisor:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Christoph Leuschner
Referee:PD Dr. Heike Culmsee
Referee:Prof. Dr. Erwin Bergmeier
Referee:Prof. Dr. Holger Kreft
Referee:Prof. Dr. Hermann Behling
Referee:Prof. Dr. Dirk Hölscher
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Abstract
English
Human activities are increasingly threatening the integrity of Earth’s ecosystems. For successful conservation measures, one necessary prerequisite is an adequate level of understanding about the ecosystems and their constituting organisms. Currently, however, there are considerable deficits in our knowledge about which and how many species exist (Linnean shortfall), where these species are distributed geographically (Wallacean shortfall) and ecologically (Hutchinsonian shortfall), how abundant they are (Prestonian shortfall), and their evolutionary histories (Darwinian shortfall), among others. Many methods are available to fill these existing knowledge gaps but one of the undoubtedly crucial approaches is work on the ground in tropical hinterland regions, where high levels of biodiversity, important knowledge gaps, and often imminent threats to natural habitats coincide. One example for such a hinterland region is the Indonesian island of Sulawesi in the centre of the Malesian archipelago, whose large portions of relatively undisturbed tropical mountain forests harbour a rich and biogeographically unique, yet poorly known flora. With his thesis, I aim to contribute to a better understanding of the ecology of Sulawesi’s tropical mountain forest and its tree flora in relation to environmental factors and evolutionary history. To do so, I conducted extensive plot-based field work in the Lore Lindu National Park in Central Sulawesi, putting special emphasis on rigorous species identification. In the different chapters, I address the specific knowledge gaps or shortfalls outlined above. In an attempt to deal with the Linnean shortfall, I describe seven new endemic tree species from Sulawesi (Chapters 1–3), including information of their distribution, ecology, and conservation status. The species descriptions furthermore provide the necessary foundation for analyses carried out in later chapters. Chapter 1 contains the description of Magnolia sulawesiana (Magnoliaceae), a new Magnolia species from upper montane forests of Sulawesi, as well as an identification key for all ten species of the genus native to the island. Another montane species, Elaeocarpus firdausii (Elaeocarpaceae) of the species-rich and predominantly Australasian genus Elaeocarpus, is described in Chapter 2. In the third chapter, I propose five new species of Syzygium (Myrtaceae), one of the largest genera of woody plants worldwide. Although the genus is widespread, diverse, and often dominant, the taxonomy of Syzygium in Sulawesi is largely unresolved and taxonomic work on the genus is urgently needed. The newly proposed species are Syzygium balgooyi, Syzygium contiguum, Syzygium devogelii, Syzygium eymae, and Syzygium galanthum. In Chapter 4, I investigate patterns of tree species diversity, endemism, and composition along gradients of elevation and soil properties. I show that species diversity in Sulawesi at the plot level is similar to other Malesian regions and that it monotonically declines with elevation. Over 25% of the recorded tree species are new occurrence-records for (Central-) Sulawesi or undescribed species, highlighting how underexplored the region is. Levels of endemism found in this study clearly exceed estimates from previous studies, especially in high-elevation forests. The forest communities are mostly determined by elevation and can be grouped into three floristically distinct elevational belts whereas soil characteristics play only a minor role. I take a closer look at understorey trees species, a guild that is often neglected in standard forest or biodiversity inventories and highlight the important contribution of these understorey tree species to local species diversity. Given Sulawesi’s geographical position and geological history, the island’s tree flora needs to be viewed in a wider spatial and temporal context that includes Asia and Australia. I therefore widen the scope in Chapter 5 to address the Darwinian shortfall, which is defined as the lack of knowledge about the evolutionary history of Malesian tree species. The central part of Malesia only formed in the last 23 My and in that timespan, plants have colonized Malesia from Asia and Australia in a process termed the Malesian Floristic Interchange. I used my dataset from Central Sulawesi and published studies from other Malesia subregions and inferred the geographical origin of the tree lineages in the dataset based on phylogenetic and fossil information. I then related the relative abundance of Asian versus Australian trees per plot to environmental parameters to show that the colonization success of tree lineages in Malesia is strongly determined by habitat. The results showed a higher proportion of Australian tree species and individuals at higher elevations, over ultramafic bedrock, and east of Wallace’s line. The trend is stronger for tree individuals than for species meaning that montane forests and those on poor soils in Malesia are mainly dominated by Australian tree-lineages. These patterns are a consequence of the climatic and biogeographical history of Malesia and indicate that environmental similarities between source and sink habitats have largely determined the colonization success during the Malesian floristic interchange. In conclusion, I show that the rain forests of Sulawesi harbour a species-rich and unique tree flora, characterized by high proportions of small understorey trees and endemic species, many of which remain to be discovered and formally described. The biogeographical history changes from sub-montane communities dominated by Asian tree lineages to upper montane forests composed of mainly Australian lineages and many endemics. Although less-diverse than lowland rain forests, these upper montane communities therefore have high conservation values. At the same time, they remain among the least-studied ecosystems in Malesia. While with this thesis I hope to have bridged some knowledge gaps regarding Sulawesi’s trees and forests, repeated biodiversity inventories including phylogenetic and trait-based approaches will be indispensable to improve our baseline knowledge about plant diversity patterns in Sulawesi.
Keywords: biodiversity; taxonomy; systematics; Malesia; Wallacea; shortfalls of biodiversity knowledge; tropical mountain forest; tropical rain forest; Sulawesi; Lore Lindu National Park; tree diversity