Figure - available from: Frontiers in Earth Science
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Biogeochemical profiles of the core EN-20001 over time (calibrated thousand years before present (cal ka BP). (A) Sedimentation rates. (B) XRF-derived Zr. (C) Total organic carbon (TOC), shown as weight mass (%). (D) XRF-derived Br. (E) Total atomic nitrogen (TOC/TNatomic). (F) XRF-derived Mn/Fe, additive log ratio (alr)-transformed. (G) Lithology of core EN-20001. All XRF-derived elemental compositions are centred log ratio (clr)-transformed. XRF and SR are continuous records. Dots in TOC and TOC/TNatomic records indicate fragmentary samples. Horizontal grey-dotted line marks the reservoir effect depth. Grey shading indicates pre-lacustrine (shown as pre-lake) environment. See Figure 2 for details of the lithological units (LUs) of the core.
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Only a few palaeo-records extend beyond the Holocene in Yakutia, eastern Siberia, since most of the lakes in the region are of Holocene thermokarst origin. Thus, we have a poor understanding of the long-term interactions between terrestrial and aquatic ecosystems and their response to climate change. The Lake Khamra region in southwestern Yakutia i...
Citations
... The highly resolved charcoal record at Lake Khamra, an intermontane basin lake (Baisheva et al., 2024), permits the identification of a mean fire return interval of 43 years throughout the last 2 millennia (Fig. 1). Trends in the charcoal accumulation rate are compared to information on climate, vegetation, and the history of human activity. ...
[Thesis abstract]
Recent intense wildfire seasons in the circumpolar boreal forests raise questions regarding drivers and impacts of past fire regime changes. Charcoal in lake sediments is commonly used to reconstruct past trends of biomass burning. However, eastern Siberia is poorly covered by such data. Considering the unique larch-dominated forests and their climate-sensitive relationships with permafrost and fire, both affected by ongoing climatic warming, this lack of data presents a key research gap.
By applying both paleo-ecological and modeling methods, this dissertation evaluates (I) wildfire dynamics of the past ca. 20 000 years, (II) relationships between fire regime changes and larch forest structure, and (III) the potential human dimension of past fire regime changes in the Republic of Sakha (Yakutia). [...]
... Grass and herb taxa show higher values around the Last Glacial Maximum, approximately 25,000 to 19,000 years ago, in the northern high latitude records suggesting an intensified contribution of biomass from widespread Northern Hemisphere glacial steppes 40 to marine carbon burial. Enriched families in the glacial marine sedimentary records may be related to high percentages of certain genera in lake plant metabarcoding records from eastern Siberia and Chukotka from that period [41][42][43] . This includes Papaveraceae (Papaver sp., Papaver paucistamimum), Rosaceae (Dryas sp.), Poaceae (Puccinellia sp., Festuca sp.), Cyperaceae (Carex, Kobresia), Boraginaceae (Eritrichium). ...
Terrigenous organic matter in marine sediments is considered a significant long-term carbon sink, yet our knowledge regarding its source taxa is severely limited. Here, we leverage land-plant ancient DNA from six globally distributed marine sediment cores covering the Last Glacial–Holocene transition as a proxy for the share, burial rate, preservation, and composition of terrigenous organic matter. We show that the spatial and temporal plant composition as revealed by sedimentary ancient DNA records reflects mainly the vegetation dynamics of nearby continents as revealed by comparison with pollen from land archives. However, we also find indications of a global north-to-south translocation of sedimentary ancient DNA. We also find that plant sedimentary ancient DNA has a higher burial rate in samples from the Late Glacial, which is characterized by high runoff and mineral load. This study provides an approach to understanding the global linkages between the terrestrial and marine carbon cycle, highlighting the need for further research to quantify the processes of DNA preservation and dispersal in marine sediments.
... The Lake Khamra region lies nowadays within a part of the summergreen-evergreen forest transition zone where evergreen is mixed with summergreen forest [48]. According to paleoclimate lake sediment records, evergreen needleleaf tree taxa expanded in this region into the summergreen dominated forests already since the Mid-Holocene along a south-west to north-east transect [48]. ...
... The Lake Khamra region lies nowadays within a part of the summergreen-evergreen forest transition zone where evergreen is mixed with summergreen forest [48]. According to paleoclimate lake sediment records, evergreen needleleaf tree taxa expanded in this region into the summergreen dominated forests already since the Mid-Holocene along a south-west to north-east transect [48]. We see in our case study that for the current state of the summergreen-evergreen forest transition zone, S-2 satellite-based mapping can provide forest type distribution at a fine landscape scale. ...
Circumboreal forests covering about 30% of global forested areas are undergoing significant changes. In Siberia, global warming may reduce the dominance of summergreen larch forest inducing shifts towards evergreen forest types, specifically in the Eastern Siberian summergreen-evergreen forest transition zone. We create a Remote Sensing training dataset for summergreen and evergreen forest types from the SiDroForest Sentinel-2 image dataset. This new training dataset informed by expert field knowledge includes nearly two million Sentinel-2 pixels across the early summer, peak summer, and late summer phenophases. We create the equivalent seasonal SiDroTest dataset linked to in-situ forest plots for benchmarking the seasonal training dataset. To optimize satellite-based monitoring, we train a Random Forest classifier on the train dataset to map summergreen and evergreen forest resulting in accuracies of 63% for early summer, 89% for peak summer, and 99% for late summer, with an average accuracy of 82% across all seasons. Feature importance analysis highlights the Sentinel-2 shortwave infrared as crucial for distinguishing forest types in all seasons. Additional key features include the normalized difference vegetation index (NDVI) and the red wavelength region for early summer, shortwave infrared and the visible wavelength region for peak summer, and shortwave infrared, near-infrared and NDVI for late summer. This study provides a benchmarked training dataset for mapping boreal forest types in the Siberian summergreen-evergreen transition zone. The Random Forest classifier performs best in late summer, leveraging distinct spectral differences between evergreen forests' greenness and the seasonal coloring of summergreen larch forests.
... Uncertainties arise regarding whether one forest type's niches can be transferred to another, and whether the environmental space could support different species. For instance, evergreen forests have expanded into larch refugia [26,27], yet the availability of potential evergreen niches in eastern Siberia remains unsure. Such knowledge is crucial with progressing climate change, where species range and population size could shift under changing environmental conditions [28,29]. ...
... Additional environmental factors, such as fire history at a site can affect the recovery and trajectories of boreal forests [97,98], especially at Lake Khamra in the southern part of our study where wildfire history has been documented over the last two millennia [99]. Evergreen tree taxa, over the past millennia to recent years, have progressively transgressed in eastern Siberia from SW to the NE summergreen larch refugia [26,27]. Our results predict that these taxa have potential climatic niches towards the East (figure 7(c)). ...
The Siberian boreal forest is the largest continuous forest region on Earth and plays a crucial role in regulating global climate. However, the distribution and environmental processes behind this ecosystem are still not well understood. Here, we first develop Sentinel-2-based classified maps to show forest-type distribution in five regions along a southwest-northeast transect in eastern Siberia. Then, we constrain the environmental factors of the forest-type distribution based on a multivariate analysis of bioclimatic variables, topography, and ground-surface temperatur at the local and regional scales. Furthermore, we identify potential versus realized forest-type niches and their applicability to other sites. Our results show that mean annual temperature and mean summer and winter temperatures are the most influential predictors of forest-type distribution. Furthermore, we show that topography, specifically slope, provides an additional but smaller impact at the local scale. We find that the filling of climatic environmental niches by forest types decreases with geographic distance, but that the filling of topographic niches varies from one site to another. Our findings suggest that boreal forests in eastern Siberia are driven by current climate and topographical factors, but that there remains a portion of the variability that cannot be fully accounted for by these factors alone. While we hypothesize that this unexplained variance may be linked to legacies of the Late Glacial, further evidence is needed to substantiate this claim. Such results are crucial to understanding and predicting the response of boreal forests to ongoing climate change and rising temperatures.
Severe wildfire seasons in the Republic of Sakha (Yakutia) raise questions regarding long-term fire dynamics and their drivers. However, data on long-term fire history remains scarce across eastern Siberia. We present the first composite of reconstructed wildfire dynamics in Yakutia throughout the Holocene, based on eight newly contributed records of macroscopic charcoal in lake sediments in combination with published data. Increased biomass burning occurred in the Early Holocene, c. 10,000 years BP, before shifting to lower levels at c. 6000 years BP. Independent simulations of climate-driven burned area in an individual-based forest model reproduce this reconstructed Holocene trend (rs = 0.85), but the correlation on centennial timescales turns negative in the Late Holocene (rs = -0.70). This mismatch suggests that climate alone cannot explain Late Holocene wildfire dynamics. We propose that a human dimension needs to be considered. By example of the settlement of the pastoralist Sakha people c. 800 years BP, we show that implementing reduced fuel availability from Indigenous land management in the forest model leads to improved centennial-scale correlations (rs = 0.96). This study highlights the need for a better understanding of the poorly reported human dimension of past fire dynamics in eastern Siberia.
We studied a 2200-year-old sediment core from Lake Zapovednoye, a small, deep, freshwater lake near the site of the 1908 Tunguska impact event. Analysis of the sediment core for geochemistry, pollen, chironomids, Cladocera, and diatoms revealed traces of climatic fluctuations during the investigated time period during which a cool climate before 1000 CE was replaced by the Medieval Climatic Optimum, the Little Ice Age, and finally the modern warming. An increased content of terrigenous elements was identified at the depth corresponding to ca. 1908 CE. This layer presumably resulted from erosion of the soil cover after the tree fall caused by the Tunguska impact event (the largest recorded in history). For the first time, the reaction of lake biota to an impact event has been detected. Our study has demonstrated that the taxonomic diversity of hydrobionts (chironomids and cladocerans) significantly declined after the catastrophe, probably due to increased turbidity, and recovered in 6–10 years. The pollen and diatom assemblages, however, demonstrated weaker compositional shifts.
Wildfires constitute a key ecological disturbance in the world’s boreal forests. Driven by conditions of the atmosphere and vegetation, wildfires are also inherently connected to recent global change. Unusually intense fire seasons in Siberia, Canada, or Alaska in recent years are making headlines around the world. With their location in the high latitudes, boreal forests experience above-average climatic warming, and continued climate change is expected to further intensify boreal fire regimes. Remote sensing data and paleo-ecological methods are commonly used to evaluate relationships between fire regimes, climate, vegetation, and human activity, on various temporal and spatial scales. However, satellite data remains limited to only few decades of observations, preventing a direct assessment of long-term wildfire dynamics. Studies utilizing paleo-ecological approaches, on the other hand, including the well-established analysis of charcoal particles in lake sediments as an indicator of past wildfires, remain scarce in Siberia. Compared to other regions of the boreal zone, wildfire activity in boreal Siberia and its drivers and impacts remain poorly understood, especially on long timescales. Eastern Siberia is particularly under-represented in the global distribution of paleo-ecological reconstructions of long-term wildfire activity. Despite the high ecological significance of eastern Siberia’s unique deciduous larch forests, growing on deep permafrost in one of the coldest regions on Earth, this pronounced lack of data means that little is known about past trends of wildfire activity or long-term relationships of fire to its environment and human livelihoods.
This thesis uncovers long-term fire regime changes in the Republic of Sakha (Yakutia), eastern Siberia, throughout the past c. 20,000 years by applying a combination of paleo-ecological and modeling approaches. Eleven new records of wildfire activity throughout the Holocene are obtained, based on macroscopic charcoal particles in lake sediments from south-west Yakutia, Central Yakutia, the southern Verkhoyansk Mountains, and the Oymyakon Highlands. The new data, covering periods of the last c. 700 to 10,800 years, enable the creation of the first composite of Holocene charcoal accumulation for the region, representing trends of biomass burning. A high-resolution record of wildfire activity for the first time allows for a determination of fire return intervals throughout the past two millennia. Reconstructed wildfire activity is compared to reconstructions of past vegetation cover and human land use from palynological analyses and sedimentary ancient DNA, as well as climate data. The paleo-ecological approach is complemented by simulations in the individual-based, spatially explicit forest model LAVESI (Larix Vegetation Simulator). The model is expanded by a new fire module and applied to simulate long-term impacts of climate-driven fire regime changes on fine-scale forest dynamics since the Last Glacial Maximum. Findings show that open woodlands and a warm climate coincided with severe wildfires in the Early Holocene, c. 10,000 years ago, from which a potential positive feedback between thinning forests and intensifying wildfires is inferred. Simulations suggest medium-intensity wildfires at return intervals of 50 years or more are benefitting the dominance of fire-resisting larches, whereas stand-replacing fires facilitate the establishment of evergreen conifers. Over the last two millennia, the role of climatic trends was increasingly overruled by human interference as key driver of fire regime changes. A combination of both paleo-ecological and modeling approaches enables a preliminary identification of indigenous land use 800 years ago and its ability to decrease wildfire severity around settlements. Considering that many indigenous land use practices today are less often conducted, or, in the case of the traditional, controlled use of fire in the landscape, were prohibited, these findings have implications for present-day policies in a region where fire regimes are expected to continue intensifying. This thesis for the first time uncovers regional wildfire activity in Yakutia throughout the Holocene by applying a novel combination of paleo-ecological and modeling approaches, unravelling natural and human drivers, and discussing findings and their implications for present and future wildfire activity in a unique region already faced with rapid environmental changes.
