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Relationships between climate, species composition, and species richness are of particular importance for understanding how boreal ecosystems will respond to ongoing climate change. This study aims to reconstruct changes in terrestrial vegetation composition and taxa richness during the glacial Late Pleistocene and the interglacial Holocene in the sparsely studied southeastern Yakutia (Siberia) by using pollen and sedimentary ancient DNA (sedaDNA) records. Pollen and sedaDNA metabarcoding data using the trnL g and h markers were obtained from a sediment core from Lake Bolshoe Toko. Both proxies were used to reconstruct the vegetation composition, while metabarcoding data were also used to investigate changes in plant taxa richness. The combination of pollen and sedaDNA approaches allows a robust estimation of regional and local past terrestrial vegetation composition around Bolshoe Toko during the last similar to 35,000 years. Both proxies suggest that during the Late Pleistocene, southeastern Siberia was covered by open steppe-tundra dominated by graminoids and forbs with patches of shrubs, confirming that steppe-tundra extended far south in Siberia. Both proxies show disturbance at the transition between the Late Pleistocene and the Holocene suggesting a period with scarce vegetation, changes in the hydrochemical conditions in the lake, and in sedimentation rates. Both proxies document drastic changes in vegetation composition in the early Holocene with an increased number of trees and shrubs and the appearance of new tree taxa in the lake's vicinity. The sedaDNA method suggests that the Late Pleistocene steppe-tundra vegetation supported a higher number of terrestrial plant taxa than the forested Holocene. This could be explained, for example, by the "keystone herbivore" hypothesis, which suggests that Late Pleistocene megaherbivores were able to maintain a high plant diversity. This is discussed in the light of the data with the broadly accepted species-area hypothesis as steppe-tundra covered such an extensive area during the Late Pleistocene.
Past and present biodiversity in northeastern Siberia inferred from sedimentary DNA metabarcoding
(2021)
The arctic-boreal treeline is a transition zone from taiga to tundra covering a vast area in Siberia. It often features large environmental gradients and reacts sensitively to changes in the environment. For example, the expansion of shrubs and a northward movement of the treeline are observable in Siberia as a response to the warming climate. The changes in vegetation across the treeline are known to influence the water chemistry in the lakes. This causes further alteration to the composition and diversity of sensitive aquatic organisms such as diatoms and macrophytes. Despite the rising awareness of the complex climate-feedback mechanisms of terrestrial plants, the understanding of their assembly rules and about responses of aquatic biomes in the surrounding treeline lakes is still limited. The goal of this thesis is to examine the previous and present biodiversity of terrestrial and freshwater biomes from the Siberian treeline ecotone, as well as their reactions to environmental changes. In particular, this thesis attempts to examine the performance of applying sedimentary DNA metabarcoding in terrestrial plants, aquatic macrophytes and diatoms, their spatial patterns along the environmental gradients and their temporal patterns throughout the climate transition from the late Pleistocene to Holocene. Sedimentary DNA metabarcoding combined with next-generation sequencing is applied as a primary tool to explore the composition and diversity of terrestrial plants, diatoms and aquatic macrophytes. The main study area is located in Chukotka of northeastern Siberia in the Arctic, a biodiversity hotspot due to its continental location and the diverse habitats of the glacial refugium. The modern diatom diversity was assessed with a specific diatom metabarcoding marker and morphological identification. Both approaches agree to a dominance of Fragilariaceae and Aulacoseiraceae, as well as on the environmental influential indicators of the diatom community. The high diversity of Fragilariaceae identified in the thermokarst lakes is found to follow the vegetation gradient along the treeline, suggesting that diatom metabarcoding can decipher relationships between diatom assemblage shifts and the relevant environmental changes. In particular, the metabarcoding approach detects diversification of fragilarioids in glacial lakes which is not visible using morphology. Sedimentary ancient DNA records indicate a vegetation mosaic of forb-dominated steppe-tundra during 28-19 ka, followed by a shift to dwarf-shrub tundra during 19-14 ka. During the most recent 14 thousand years, the vegetation consists of deciduous shrublands, then a change to boreal forest is observed. Investigations on the alpha diversity of the vegetation show that species richness is unexpectedly highest during pre-LGM, which is likely related to the extensive area that allows for more taxa. The optimum Holocene warming during 9-6 ka is not accompanied by a high richness as widely believed, but with an evenly distributed community by the fulfilment of erect shrubs. Furthermore, changes in taxonomic and phylogenetic diversity show complementary results in understanding community diversity. The composition and richness in the modern macrophytes community from Siberian Arctic and Chinese alpine are best co-influenced by July temperature and electrical conductivity.. Past macrophyte turnover during the late Pleistocene-Holocene is less noticeable in Siberia, whereas a pronounced community change from emergent to submerged plants is detected from Chinese alpine regions at about 14 ka due to increasing temperature and varying water conductivity. Finally, sedimentary DNA metabarcoding is a cost-effective and powerful proxy for ecological application, whereas completeness of the reference library, coverage and resolution of the metabarcoding marker are the major limitations of sedimentary DNA based diversity monitoring. The composition and richness in modern vegetation and macrophytes across broad spatial gradients is constrained by environmental variables, suggesting a potential usage for environmental monitoring. Diatom distributions are driven by different water variables along the treeline. Past records indicate that the shrub coverage has a noticeable influence on the assemblies of both terrestrial plants and aquatic macrophytes, though the shift in macrophyte community is relatively minor in the past 28 thousand years. In the long-term, the shrub expansion may eventually result in a genetically more diverse vegetation community but reduced species richness. When exceeding the optimal temperatures, further warming may lead to a decrease and putative loss of macrophytes and diatoms.
Woody plants are expanding into the Arctic in response to the warming climate. The impact on arctic plant communities is not well understood due to the limited knowledge about plant assembly rules.
Records of past plant diversity over long time series are rare. Here, we applied sedimentary ancient DNA metabarcoding targeting the P6 loop of the chloroplast trnL gene to a sediment record from Lake Ilirney (central Chukotka, Far Eastern Russia) covering the last 28 thousand years.
Our results show that forb-rich steppe-tundra and dwarf-shrub tundra dominated during the cold climate before 14 ka, while deciduous erect-shrub tundra was abundant during the warm period since 14 ka. Larix invasion during the late Holocene substantially lagged behind the likely warmest period between 10 and 6 ka, where the vegetation biomass could be highest.
We reveal highest richness during 28-23 ka and a second richness peak during 13-9 ka, with both periods being accompanied by low relative abundance of shrubs. During the cold period before 14 ka, rich plant assemblages were phylogenetically clustered, suggesting low genetic divergence in the assemblages despite the great number of species. This probably originates from environmental filtering along with niche differentiation due to limited resources under harsh environmental conditions. In contrast, during the warmer period after 14 ka, rich plant assemblages were phylogenetically overdispersed.
This results from a high number of species which were found to harbor high genetic divergence, likely originating from an erratic recruitment process in the course of warming. Some of our evidence may be of relevance for inferring future arctic plant assembly rules and diversity changes. By analogy to the past, we expect a lagged response of tree invasion. Plant richness might overshoot in the short term; in the long-term, however, the ongoing expansion of deciduous shrubs will eventually result in a phylogenetically more diverse community.