Drivers of regional and local boreal forest dynamics during the Holocene

In this work, the Holocene history of the western taiga forests, at the modern western range limit of Siberian larch (Larix Sibirica) in northern Europe, is investigated using fossil pollen and stomata records from small forest hollow sites. The relative importance of the potential drivers of long-term boreal forest composition is quantitatively assessed using novel approaches in a palaeoecological context. The statistical method variation partitioning is employed to assess relative importance of climate, forest fires, local moisture conditions and human population size on long-term boreal forest dynamics at both regional (lake records) and local scales (small hollow records). Furthermore, wavelet coherence analysis is applied to examine the significance of individual forest fires on boreal forest composition. The results demonstrate that Siberian larch and Norway spruce (Picea abies) have been present in the region since the early Holocene. The expansion of spruce at 8000 7000 cal yr BP caused a notable change in forest structure towards dense spruce dominated forests, and appears to mark the onset of the migration of spruce into Fennoscandia. The mid-Holocene dominance of spruce and constant presence of Siberian larch suggests that taiga forest persisted throughout the Holocene at the study sites in eastern Russian Karelia. Climate is the main driver of long-term changes in boreal vegetation at the regional scale. However, at the local scale the role of local factors increases, suggesting that intrinsic site-specific factors have an important role in stand-scale dynamics in the boreal forest. When the whole 9000 year study period is considered, forest fires explain relatively little of the variation in stand-scale boreal forest composition. However, forest fires have a significant role in stand-scale forest dynamics when observed in shorter time intervals and the results suggests that fires can have a significant effect on short-term changes in individual tree taxa as well as a longer profound effect on forest structure. The relative importance of human population size on variation in long-term boreal vegetation was statistically assessed for the first time using this type of human population size data and the results showing unexpectedly low importance of human population size as a driver of the changes in long-term boreal vegetation may be biased because of the difference in spatial representativeness between the human population size data and the pollen-derived forest composition data. Although the results strongly suggest that climate is the main driver of long-term boreal forest dynamics, the local disturbances, such as fires, species interactions and local site specific characteristics can dictate the importance of climate on stand-scale boreal forest dynamics.Väitöstutkimuksessa selvitetään läntisten taigametsien kehitystä viimeisten 10 000 vuoden aikana. Tutkimusalue sijaitsee Siperian lehtikuusen läntisimmällä luontaisella esiintymisalueella Luoteis-Venäjällä. Kasvillisuuden kehitystä tutkitaan sedimenttikerroksissa säilyneistä fossiilisista siitepölyhiukkasista. Lisäksi alueen metsäpalohistoriaa tutkitaan sedimenttikerroksissa säilyneiden hiilipartikkelien avulla. Tutkimuksessa selvitetään tilastollisten hajonnan ositus- ja aikasarja-analyysi -menetelmien avulla ilmaston, metsäpalojen, paikallisten kosteusolosuhteiden sekä ihmispopulaation koon merkitystä pohjoisessa havumetsäkasvillisuudessa tapahtuneisiin muutoksiin sekä paikallisella että alueellisella tasolla viimeisten 9 000 vuoden aikana. Yksi tutkimuksen keskeisistä tuloksista on Siperian lehtikuusen yhtäjaksoinen esiintyminen tutkimusalueella jo viimeisten 10 000 vuoden ajan. Tämä eroaa aiemmasta oletuksesta, jonka mukaan lehtikuusi olisi leviämässä kohti länttä. Toinen merkittävä tulos on metsän rakenteessa tapahtunut selkeä muutos 8 000 7 000 vuotta sitten, kun kuusen populaation huomattavan kasvun seurauksena mäntyjen, koivujen ja lehtikuusten luonnehtimat avoimemmat metsät muuttuivat tiheämmiksi kuusivaltaisiksi metsiksi. Tämä kuusen huomattava yleistyminen tutkimusalueella tukee aiempia tuloksia, jotka osoittavat kuusen leviämisen idästä Fennoskandian alueelle alkaneen noin 7 000 6 500 vuotta sitten. Tilastollisten analyysien tulokset osoittavat, että ilmasto on merkittävin boreaalista kasvillisuutta säätelevä tekijä alueellisessa mittakaavassa. Paikallisella tasolla ilmaston merkitys jää vähäisemmäksi ja paikallisilla tekijöillä, kuten metsäpaloilla ja kasvupaikan ominaisuuksilla, on suurempi vaikutus kasvillisuuteen. Metsäpalot ovat merkittävä tekijä yksittäisten lajien lyhytaikaisissa muutoksissa. Tulokset osoittavat etenkin kuusen olevan herkkä metsäpaloille, kun vastaavasti pioneerilajit koivu ja leppä hyötyvät metsäpaloista. Metsäpalojen seurauksena metsän rakenteessa voi tapahtua myös pidempiaikaisia, perustavanlaatuisia muutoksia. Siperian lehtikuusen esiintyminen ja kuusimetsien vallitsevuus osoittavat taigametsien säilyneen tutkimusalueella viimeisten 10 000 vuoden ajan, eikä alueella ole havaittavissa jalojen lehtipuiden yleistymistä holoseenin lämpökauden aikana kuten vain hiukan lännenpänä Fennoskandian alueella. Tämä ja tilastollisten analyysien tulokset osoittavat, että vaikka ilmasto on merkittävin pohjoista havumetsäkasvillisuutta säätelevä tekijä, synnyttävät paikalliset tekijät, kuten metsäpalot ja paikalliset kasvupaikkaolosuhteet, alueellisia eroja boreaalisessa kasvillisuudessa tapahtuviin muutoksiin

Editorial : Past interactions between climate, land use, and vegetation

Peer reviewe

Changes in species composition and diversity of a montane beetle community over the last millennium in the High Tatras, Slovakia : Implications for forest conservation and management

Montane biomes are niche environments high in biodiversity with a variety of habitats. Often isolated, these non-continuous remnant ecosystems inhabit narrow ecological zones putting them under threat from changing climatic conditions and anthropogenic pressure. Twelve sediment cores were retrieved from a peat bog in Tatra National Park, Slovakia, and correlated to each other by wiggle-matching geochemical signals derived from micro-XRF scanning, to make a reconstruction of past conditions. A fossil beetle (Coleoptera) record, covering the last 1000 years at 50- to 100-year resolution, gives a new insight into changing flora and fauna in this region. Our findings reveal a diverse beetle community with varied ecological groups inhabiting a range of forest, meadow and synanthropic habitats. Changes in the beetle community were related to changes in the landscape, driven by anthropogenic activities. The first clear evidence for human activity in the area occurs c. 1250 CE and coincides with the arrival of beetle species living on the dung of domesticated animals (e.g. Aphodius spp.). From 1500 CE, human (re)settlement, and activities such as pasturing and charcoal burning, appear to have had a pronounced effect on the beetle community. Local beetle diversity declined steadily towards the present day, likely due to an infilling of the forest hollow leading to a decrease in moisture level. We conclude that beetle communities are directly affected by anthropogenic intensity and land-use change. When aiming to preserve or restore natural forest conditions, recording their past changes in diversity can help guide conservation and restoration. In doing so, it is important to look back beyond the time of significant human impact, and for this, information contained in paleoecological records is irreplaceable.Peer reviewe

What evidence exists for temporal variability in Arctic terrestrial and freshwater biodiversity throughout the Holocene? A systematic map protocol

Background: The Arctic tundra is subject to the greatest climate change-induced temperature rises of any biome. Both terrestrial and freshwater biota are responding to recent climate warming through variability in their distribution, abundance, and richness. However, uncertainty arises within models of future change when considering processes that operate over centennial timescales. A systematic evidence synthesis of centennial-scale variability in biodiversity does not currently exist for the Arctic biome. Here, we sought to address the primary research question: what evidence exists for temporal variability in Arctic terrestrial and freshwater biodiversity throughout the Holocene (11,650 years before present (yBP)-OyBP)? Methods: Consultation with stakeholders informed key definitions, scoping and the appropriateness of the research question. The research question was structured using a PECO framework-Arctic biota (P), a timestamped year in the Holocene (E), another year in the Holocene (C), and the dimensions of biodiversity that have been measured (O)-to inform the search strategy. Search strings were benchmarked against a test list of 100 known sources to ensure a specific and comprehensive return of literature. Searches will occur across 13 bibliographic databases. The eligibility criteria specify that sources must: (a) use 'proxy' methods to measure biodiversity; (b) fall within the spatial extent of the contemporary Arctic tundra biome; and (c) consist of a time-series that overlaps with 11,650yBP to OyBP (1950AD). Information coded from studies will include proxy-specific information to account for both temporal uncertainty (i.e., the characteristics of age-depth models and dating methods) and taxonomic uncertainty (i.e., the samples and processes used for taxonomic identification). We will assess temporal uncertainty within each source by determining the quality of dating methods and measures; this information will be used to harmonise dates onto the IntCa120 calibration curve and determine the available temporal resolution and extent of evidence through space. Key outputs of this systematic map will be: (1) a graph database containing the spatial-temporal properties of each study dataset with taxonomic harmonisation; and (2) a geographical map of the evidence base.Peer reviewe

Abrupt high-latitude climate events and decoupled seasonal trends during the Eemian

The Eemian (the Last Interglacial; ca. 129-116 thousand years ago) presents a testbed for assessing environmental responses and climate feedbacks under warmer-than-present boundary conditions. However, climate syntheses for the Eemian remain hampered by lack of data from the high-latitude land areas, masking the climate response and feedbacks in the Arctic. Here we present a high-resolution (sub-centennial) record of Eemian palaeoclimate from northern Finland, with multi-model reconstructions for July and January air temperature. In contrast with the mid-latitudes of Europe, our data show decoupled seasonal trends with falling July and rising January temperatures over the Eemian, due to orbital and oceanic forcings. This leads to an oceanic Late-Eemian climate, consistent with an earlier hypothesis of glacial inception in Europe. The interglacial is further intersected by two strong cooling and drying events. These abrupt events parallel shifts in marine proxy data, linked to disturbances in the North Atlantic oceanic circulation regime.Peer reviewe

Warm summers and rich biotic communities during N-Hemisphere deglaciation

Detailed studies on fossil remains of plants or animals in glacial lake sediments are rare. As a result, environmental conditions right at the moment of deglaciation of the large N-Hemisphere ice-sheets remain largely unknown. Here we study three deglacial phases of the Fennoscandian Ice Sheet as a unique, repeated element in a long sediment record preserved at Sokli in northern Finland. We summarize extensive multi-proxy data (diatoms, phytoliths, chironomids, pollen, spores, non-pollen palynomorphs, macrofossils, lithology, loss-on-ignition, C/N) obtained on glacial lake sediments dated to the early Holocene (ca. 10 kyr BP), early MIS 3 (ca. 50 kyr BP) and early MIS 5a (ca. 80 kyr BP). In contrast to the common view of an unproductive ice-marginal environment, our study reconstructs rich ecosystems both in the glacial lake and along the shores with forest on recently deglaciated land. Higher than present-day summer temperatures are reconstructed based on a large variety of aquatic taxa. Rich biota developed due to the insolation-induced postglacial warming and high nutrient levels, the latter resulting from erosion of fresh bedrock and sediment, leaching of surface soils, decay of plant material under shallow water conditions, and sudden decreases in lake volume. Aquatic communities responded quickly to deglaciation and warm summers and reflect boreal conditions, in contrast to the terrestrial ecosystem which responded with some delay probably due to time required for slow soil formation processes. Birch forest is reconstructed upon deglaciation of the large LGM ice-sheet and shrub tundra following the probably faster melting smaller MIS 4 and MIS 5b ice-sheets. Our study shows that glacial lake sediments can provide valuable palaeo-environmental data, that aquatic biota and terrestrial vegetation rapidly accommodated to new environmental conditions during deglaciation, and that glacial lake ecosystems, and the carbon stored in their sediments, should be included in earth system modeling.Peer reviewe

Multi-proxy temperature and environmental reconstruction during the Late Glacial and Early Holocene in the Bohemian Forest, Central Europe

Multi-proxy temperature reconstructions can provide robust insights into past environmental conditions. By combining different proxies we can disentangle the temperature signal from the indirect climate effects on the environment. This study uses a multi-proxy approach to reconstruct temperature and palaeoenvironmental conditions during the Late Glacial and Early Holocene (13.5–8 cal. ka BP) in the Bohemian Forest, Central Europe. We assessed the similarity of the temperature signal based on chironomids, isoprenoid glycerol dialkyl glycerol tetraether lipids (isoGDGTs), and pollen within a comparison with locally modeled temperature data generated by the CHELSA_Trace21k dataset. Pollen, macroscopic charcoal remains, and geochemistry were further used to reconstruct past environmental conditions such as vegetation dynamics, fire activity, the input of lithogenic material (Titanium), nutrient content (Total Nitrogen) and the sources of organic matter (C/N and δ13Corg). All temperature reconstructions based on independent proxies were positively correlated and followed the same long-term trend. However, results also showed that chironomids-inferred July temperature had lower amplitude variations compared to the other temperature curves. IsoGDGTs showed the most pronounced decrease in temperature values at the onset of the Younger Dryas (YD), corroborating that this cooling event was more marked during winter than summer. However, a decrease of less than 1 °C during summer and two short-term warm events at 12.6 and 12.2 cal ka BP provoked a modest and asynchronous response of the vegetation to the onset of the YD. Nevertheless, isoGDGTs appeared to react to changes in both temperature and organic carbon sources, particularly between 11.2 and 10.6 cal yr BP. These environmental changes, characterized by high values of the GDGT-0/crenarchaeol ratio, recorded an increase in methanogenic activity in the lake sediments, which likely altered the recorded climatic signal. The corresponding anoxic episodes in the lake sediments might be caused by an increasing input of organic carbon from the catchment, related to the development of the vegetation and catchment soils at the beginning of the Holocene. Finally, pollen-based temperature reconstruction showed a lag in the response to major climatic events, such as the onset of YD and Holocene. Our study increases the understanding of the climate-vegetation-environmental feedback during the Late Glacial and Early Holocene in the Bohemian Forest, Central Europe

The Reading Palaeofire Database : an expanded global resource to document changes in fire regimes from sedimentary charcoal records

Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using IntCal20 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1676 records (entities) from 1480 sites worldwide. The database (RPDv1b - Harrison et al., 2021) is available at https://doi.org/10.17864/1947.000345.Peer reviewe