Environmental and climate dynamics in northeastern Siberia according to diatom oxygen isotopes

The sedimentary sequence from Lake Emanda (65°17′N; 135°45′E; 675 m a.s.l), one large freshwater body (33.1 km2) in the continuous permafrost of the Verkhoyansk Mountains, has been investigated within the German-Russian ‘Paleolimnological Transect’ (PLOT) project. It provided important insight into the environmental and climate dynamics in northeastern Siberia

Advances in the Derivation of Northeast Siberian Forest Metrics Using High-Resolution UAV-Based Photogrammetric Point Clouds

Forest structure is a crucial component in the assessment of whether a forest is likely to act as a carbon sink under changing climate. Detailed 3D structural information about the tundra–taiga ecotone of Siberia is mostly missing and still underrepresented in current research due to the remoteness and restricted accessibility. Field based, high-resolution remote sensing can provide important knowledge for the understanding of vegetation properties and dynamics. In this study, we test the applicability of consumer-grade Unmanned Aerial Vehicles (UAVs) for rapid calculation of stand metrics in treeline forests. We reconstructed high-resolution photogrammetric point clouds and derived canopy height models for 10 study sites from NE Chukotka and SW Yakutia. Subsequently, we detected individual tree tops using a variable-window size local maximum filter and applied a marker-controlled watershed segmentation for the delineation of tree crowns. With this, we successfully detected 67.1% of the validation individuals. Simple linear regressions of observed and detected metrics show a better correlation (R2) and lower relative root mean square percentage error (RMSE%) for tree heights (mean R2 = 0.77, mean RMSE% = 18.46%) than for crown diameters (mean R2 = 0.46, mean RMSE% = 24.9%). The comparison between detected and observed tree height distributions revealed that our tree detection method was unable to representatively identify trees 15–20 m to capture homogeneous and representative forest stands. Additionally, we identify sources of omission and commission errors and give recommendations for their mitigation. In summary, the efficiency of the used method depends on the complexity of the forest’s stand structure

Late Holocene fire history documented at Lake Khamra, SW Yakutia (Eastern Siberia)

Recent large-scale fire events in Siberia have drawn increased attention to boreal forest fire history. Boreal forests contain about 25% of all global biomass and act as an enormous carbon storage. Fire events are important ecological disturbances connected to the overarching environmental changes that face the Arctic and Subarctic, like vegetation dynamics, permafrost degradation, changes in soil nutrient cycling and global warming, and act as the dominant driver behind boreal forest’s landscape carbon balance. By looking into past fire regimes we can learn about fire frequency and potential linkages to other environmental factors, e.g. fuel types, reconstructed temperature/humidity or geomorphologic landscape dynamics. Unfortunately, fire history data is still very sparse in large parts of Siberia, a region strongly influenced by climate change. The Global Charcoal Database (www.paleofire.org) lists only a handful of continuous charcoal records for all of Siberia, with only three of those featuring published data from macroscopic charcoal as opposed to microscopic charcoal from pollen slides. We aim to reconstruct the late Holocene fire history using lacustrine sediments of Lake Khamra (SW Yakutia at N 59.99°, E 112.98°). It covers an area of c. 4.6 km² with about 22 m maximum water depth, located within the zone of transition from summer-green and larch-dominated to evergreen boreal forest. We present the first continuous, high-resolution (c. 10 years/sample) macroscopic charcoal record (> 150 μm) including information on particle size and morphology for the past c. 2200 years. We compare this to complementary information from microscopic charcoal in pollen slides, a pollen and non-pollen palynomorph record as well as μXRF data. This multi-proxy approach adds valuable data about fire activity in the region and allows a comparison of different prevalent fire reconstruction methods. As the first record of its kind from Siberia, it provides a long-term context for current fire activity in central Siberian boreal forests and enables a better understanding of the environmental interactions occurring in the changing subarctic landscape

Siberian treeline dynamics in a warming climate - results from larch population genetics and vegetation modelling

A vegetation change from open tundra to dense taiga will fuel the global warming by positive feedback caused by albedo decreases. Yet, it is unclear how fast the arctic treeline, formed of Larix species, will advance north in the next decades. The most determinant factor of tree migration is the ability to disperse seeds (and pollen). Hence, to realistically forecast the migration of tree species in a dynamic vegetation model, it is crucial to incorporate reliable estimates of dispersal. Classical methods, for example counting seeds in seed traps, have been used to describe local dispersal abilities but are not applicable to give precise estimates on rare long-distance dispersal events. In this study we overcome this with the help of modern molecular techniques. By using a set of 16 nuclear microsatellites we inferred the cryptic signal of heritage among larch individuals to study the migration history among well-established tree stands and for different time-cohorts. We analyzed the genetic structure of larch populations for several latitudinal transects spanning north-to-south from tundra to open taiga forests in Siberia and additionally of several age cohorts which established throughout the last century in prevailing cold and warm periods. Finally, we present the results of simulations with our individual-based model LAVESI which was developed by us originally to study population dynamics of larch forest stands. Using downscaled global climate models and 'representative carbon pathway' (RCP) scenarios it is feasible to project the future treeline in Siberia

Holocene vegetation change and turnovers of treeline forming species

The prospected strong high-latitude warming is projected to cause a northward move of the arctic-boreal treeline. These changes will potentially have significant effects on climate, but their timing and mode are not well understood. Besides being governed directly by temperature increases, speed of treeline changes will also be influenced by the capacity of migration and establishment, and by competition between different treeline forming species. In Siberia, where about half of the global circum-arctic boreal treeline is located, the treeline is formed by monospecific stands of three species of larch, Larix sibirica, Larix gmelinii and Larix cajanderi. There is clear evidence for ecological separation, particularly between L. sibirica and the northeastern species, with only L. gmelinii and L. cajanderi being able to survive on permafrost with an active layer depth of less than 1-2 m, but L. sibirica being competitively superior at sites with low permafrost tables. Under a warming climate, the respective ranges of the treeline forming species are projected to shift to the Northeast, causing turnovers of forest tree species. The Siberian treeline has undergone several pronounced latitudinal fluctuations in the Holocene, and detailed analyses of the historical processes of vegetation change and competitive displacement would add to predictions for the projected future shift. We are using sedimentary ancient DNA to analyze lake sediment cores spanning most of the Holocene from the southern Taymyr peninsula, where the ranges of L. sibirica and L. gmelinii come together. Changes of the complete vegetation are revealed by DNA metabarcoding and pollen analyses, while diagnostic mitochondrial haplotypes trace the temporal dynamics in distribution of the two closely related larch species. We incorporated these two species into our larch population dynamics model LAVESI to understand the influence competition between these species might have on the speed and timing of treeline movement under changing climates. Simulations were forced with regional climate series at locations in the vicinity of the sampled lakes. These analyses offer a very high degree of resolution and shed light on the complicated ecological processes leading to a change in overall vegetation

Sedimentary DNA versus morphology in the analysis of diatom-environment relationships

The Arctic treeline ecotone is characterised by a steep vegetation gradient from arctic tundra to northern taiga forests, which is thought to influence the water chemistry of thermokarst lakes in this region. Environmentally sensitive diatoms respond to such ecological changes in terms of variation in diatom diversity and richness, which so far has only been documented by microscopic surveys. We applied next-generation sequencing to analyse the diatom composition of lake sediment DNA extracted from 32 lakes across the treeline in the Katanga region, Siberia, using a short fragment of the rbcL chloroplast gene as a genetic barcode. We compared diatom richness and diversity obtained from the genetic approach with diatom counts from traditional microscopic analysis. Both datasets were employed to investigate diversity and relationships with environmental variables, using ordination methods. Aftereffective filtering of the raw data, the two methods gave similar results for diatom richness and composition at the genus level (DNA 12 taxa; morphology 19 taxa), even though there was a much higher absolute number of sequences obtained per genetic sample (median 50,278), compared with microscopic counts (median 426). Dissolved organic carbon explained the highest percentage of variance in both datasets (14.2 % DNA; 18.7 % morphology), reflecting the compositional turnover of diatom assemblages along the tundra-taiga transition. Differences between the two approaches are mostly a consequence of the filtering process of genetic data and limitations of genetic references in the database, which restricted the determination of genetically identified sequence types to the genus level. The morphological approach, however, allowed identifications mostly to species level, which permits better ecological interpretation of the diatom data. Nevertheless, because of a rapidly increasing reference database, the genetic approach with sediment DNA will, in the future, enable reliable investigations of diatom composition from lake sediments that will have potential applications in both paleoecology and environmental monitoring

Sensitivity of ecosystem-protected permafrost under changing boreal forest structures

Boreal forests efficiently insulate underlying permafrost. The magnitude of this insulation effect is dependent on forest density and composition. A change therein modifies the energy and water fluxes within and below the canopy. The direct influence of climatic change on forests and the indirect effect through a change in permafrost dynamics lead to extensive ecosystem shifts such as a change in composition or density, which will, in turn, affect permafrost persistence. We derive future scenarios of forest density and plant functional type composition by analyzing future projections provided by the dynamic global vegetation model (LPJ-GUESS) under global warming scenarios. We apply a detailed permafrost-multilayer canopy model to study the spatial impact-variability of simulated future scenarios of forest densities and compositions for study sites throughout eastern Siberia. Our results show that a change in forest density has a clear effect on the ground surface temperatures (GST) and the maximum active layer thickness (ALT) at all sites, but the direction depends on local climate conditions. At two sites, higher forest density leads to a significant decrease in GSTs in the snow-free period, while leading to an increase at the warmest site. Complete forest loss leads to a deepening of the ALT up to 0.33 m and higher GSTs of over 8 ∘C independently of local climatic conditions. Forest loss can induce both, active layer wetting up to four times or drying by 50%, depending on precipitation and soil type. Deciduous-dominated canopies reveal lower GSTs compared to evergreen stands, which will play an important factor in the spreading of evergreen taxa and permafrost persistence under warming conditions. Our study highlights that changing density and composition will significantly modify the thermal and hydrological state of the underlying permafrost. The induced soil changes will likely affect key forest functions such as the carbon pools and related feedback mechanisms such as swamping, droughts, fires, or forest loss

Thermohydrological Impact of Forest Disturbances on Ecosystem‐Protected Permafrost

Boreal forests cover over half of the global permafrost area and protect underlying permafrost. Boreal forest development, therefore, has an impact on permafrost evolution, especially under a warming climate. Forest disturbances and changing climate conditions cause vegetation shifts and potentially destabilize the carbon stored within the vegetation and permafrost. Disturbed permafrost-forest ecosystems can develop into a dry or swampy bush- or grasslands, shift toward broadleaf- or evergreen needleleaf-dominated forests, or recover to the pre-disturbance state. An increase in the number and intensity of fires, as well as intensified logging activities, could lead to a partial or complete ecosystem and permafrost degradation. We study the impact of forest disturbances (logging, surface, and canopy fires) on the thermal and hydrological permafrost conditions and ecosystem resilience. We use a dynamic multilayer canopy-permafrost model to simulate different scenarios at a study site in eastern Siberia. We implement expected mortality, defoliation, and ground surface changes and analyze the interplay between forest recovery and permafrost. We find that forest loss induces soil drying of up to 44%, leading to lower active layer thicknesses and abrupt or steady decline of a larch forest, depending on disturbance intensity. Only after surface fires, the most common disturbances, inducing low mortality rates, forests can recover and overpass pre-disturbance leaf area index values. We find that the trajectory of larch forests after surface fires is dependent on the precipitation conditions in the years after the disturbance. Dryer years can drastically change the direction of the larch forest development within the studied period

Long-term thermokarst lake development and internal ecological feedbacks: A new reconstruction from Lake Satagay (Yakutia, Siberia)

The permafrost-shaped landscape of Central Yakutia is particularly rich in thermokarst lakes, which provide important cultural and ecosystem services to the local population. Climate warming and an intensification of agriculture in alaas systems (i.e. mostly drained basins of large thaw lakes formed during the early Holocene under warm climatic conditions) in the Central Yakutian Lowlands may lead to pronounced changes in water resources, water quality, nutrient loading and biodiversity. This could in turn threaten the livelihoods of affected communities, who depend on functional alaas ecosystems. To better foresee potential future impacts of environmental changes on internal lake ecological processes, it is important to gain a better understanding of how thermokarst lakes reacted to such changes in the past. Here, we present a new paleoenvironmental reconstruction of ecological changes within Lake Satagay (N 63.078, E 117.998, Nyurbinsky District), covering the last ca. 10,800 years. We use sedimentological and XRF-derived geochemical parameters, in addition to the metabarcoding of sedimentary ancient DNA (sedDNA) for diatoms and aquatic plants, and microscopic diatom analyses, to evaluate sedimentological and biodiversity shifts throughout the Holocene. Our study revealed 53 diatom DNA sequence types and 53 species morphologically. High distributions of Stephanodiscus and Fragilaria, among multiple other diatom genera in the early Holocene, indicate that initial formation of this typical alaas lake occurred earlier than expected (i.e. before 10,800 BP). In recent millennia diatom abundance decreased and their community is almost exclusively composed of Pseudostaurosira and Fragilaria. Composition of aquatic plants show an overall dominance of Ceratophyllaceae and strong fluctuations in Potamogetonaceae likely related to lake level and water chemical changes. All proxies investigated support that lake conditions and biotic composition has been resilient since 4,000 BP, but youngest samples since 47 BP indicate that land use influence has been crucial for the lake quality. This study represents a step towards a better understanding of climate and human-impacted alaas lake development and its consequences for their ecosystem services in eastern Siberia in the near future

Forest structure and individual tree inventories of northeastern Siberia along climatic gradients

We compile a data set of forest surveys from expeditions to the northeast of the Russian Federation, in Krasnoyarsk Krai, the Republic of Sakha (Yakutia), and the Chukotka Autonomous Okrug (59–73∘ N, 97–169∘ E), performed between the years 2011 and 2021. The region is characterized by permafrost soils and forests dominated by larch (Larix gmelinii Rupr. and Larix cajanderi Mayr). Our data set consists of a plot database describing 226 georeferenced vegetation survey plots and a tree database with information about all the trees on these plots. The tree database, consisting of two tables with the same column names, contains information on the height, species, and vitality of 40 289 trees. A subset of the trees was subject to a more detailed inventory, which recorded the stem diameter at base and at breast height, crown diameter, and height of the beginning of the crown. We recorded heights up to 28.5 m (median 2.5 m) and stand densities up to 120 000 trees per hectare (median 1197 ha−1), with both values tending to be higher in the more southerly areas. Observed taxa include Larix Mill., Pinus L., Picea A. Dietr., Abies Mill., Salix L., Betula L., Populus L., Alnus Mill., and Ulmus L. In this study, we present the forest inventory data aggregated per plot. Additionally, we connect the data with different remote sensing data products to find out how accurately forest structure can be predicted from such products. Allometries were calculated to obtain the diameter from height measurements for every species group. For Larix, the most frequent of 10 species groups, allometries depended also on the stand density, as denser stands are characterized by thinner trees, relative to height. The remote sensing products used to compare against the inventory data include climate, forest biomass, canopy height, and forest loss or disturbance. We find that the forest metrics measured in the field can only be reconstructed from the remote sensing data to a limited extent, as they depend on local properties. This illustrates the need for ground inventories like those data we present here. The data can be used for studying the forest structure of northeastern Siberia and for the calibration and validation of remotely sensed data. They are available at https://doi.org/10.1594/PANGAEA.943547 (Miesner et al., 2022).</p