The study areas stretch along a west–east transect in a permafrost-free area in Tomsk Oblast, in the southern taiga of western Siberia (Bleuten and Filippov, 2008; Kirpotin et al., 2021). The region has a continental boreal climate. The mean annual temperature is 0 ∘C, the mean temperatures of the coldest and warmest season are -17 ∘C and 17 ∘C, respectively, and mean annual precipitation is ∼525 mm, falling predominantly in the warm season (https://climexp.knmi.nl, last access: 27 June 2019). Regionally, the forests are composed of both light (Pinus sylvestris, Betula pubescens, B. pendula, Populus tremula) and dark (Pinus sibirica, Picea obovata, and Abies sibirica) taiga species, the latter in greater proportions towards the east (Lashchinsky and Korolyuk, 2015). In the southern taiga of western Siberia, forests occupy 50 % of the area and peatlands 30 % with an equal proportion of forested and open peatlands (Vompersky et al., 1994; Liss et al., 2001). Our coring sites are in open peatlands surrounded by forested peatlands within a 2–6 km radius, depending on the site (Fig. 1). Fires, at present primarily ignited by humans (80 %), occur predominantly between spring and early autumn, with summer and autumn fires usually being the most severe (Kukavskaya et al., 2014, 2016; Kharuk et al., 2021).

We cored two sites, Rybnaya Mire (57.27566800∘ N, 84.48758900∘ E) located near the Rybnaya river, a tributary of the Ob river, and Ulukh–Chayakh Mire (57.34326197∘ N, 88.32306189∘ E) located on a terrace of the Chulym river, near the village of Teguldet, about 30–40 km from the border of the Krasnoyarsk Krai (Fig. 1). The local peatland vegetation at both coring sites includes mesotrophic open-sedge Sphagnum communities with small birch and pine trees at Rybnaya Mire and standing dead Pinus tree trunks at Ulukh–Chayakh Mire. These dead trees reflect a rise in water levels across the mire due to road construction in the 1960s. Peat cores (50 cm length, 5 cm diameter) totalling 400 cm in length at Rybnaya Mire and 348 cm at Ulukh–Chayakh Mire were extracted with a Russian-type corer in 2017 and 2019, respectively.

We established the chronology of both cores based on accelerator mass spectrometry (AMS) radiocarbon measurements performed at Isotoptech, Debrecen, Hungary (Sect. S1 in the Supplement). The 14C AMS age estimates were converted to calendar years before present (BP) using the IntCal20 dataset of Reimer et al. (2020), and we constructed the age–depth models using the smooth spline method implemented in CLAM software (Blaauw, 2010). In the age–depth models, we assigned a surface age of -67 cal yr BP (coring year 2017 for Rybnaya) and -69 cal yr BP (coring year 2019 for Ulukh–Chayakh) to the surface samples of each sequence.

To reconstruct past vegetation dynamics, we analysed pollen and non-pollen palynomorphs (NPPs) mainly at 4 cm intervals in both profiles following the protocol of Bennett and Willis (2001). A pollen sum exceeding 500 terrestrial pollen grains, excluding Cyperaceae, was counted for most samples and converted into percentages based on the total terrestrial pollen sum. The percentages of Cyperaceae, ferns, and mosses were calculated relative to the terrestrial pollen sum and their respective individual sum. The bottom part of the Ulukh–Chayakh profile, i.e. from 300 to 348 cm (age >6500 ka), contained extremely poorly preserved pollen and was excluded from further analysis. Microscopic charcoal particles longer than 10 µm were also counted alongside pollen, and their influx was estimated relative to the added Lycopodium marker and the sediment deposition rate.

We assigned pollen-derived tree taxa to one of the fire-related functional PFT groups – resisters (Pinus sylvestris, Larix spp.), avoiders (Picea obovata, Abies sibirica, Pinus sibirica), or invaders (Betula spp.). The two tree birch species (B. pubescens and B. pendula) and the dwarf shrub (B. nana) could not be differentiated by pollen analysis and were considered to be Betula spp. However, at present day, B. nana tends to be found only sparsely in this region. Due to its poor pollen preservation, Populus, a fire endurer, is mostly absent from our pollen records. The necessity to combine the Betula pollen signal and the paucity of pollen of Populus may have resulted in the endurer PFT group failing to be represented in our records. The PFT-specific fire-coping adaptations relate to a taxon's strategy to complete its life cycle in the context of a given fire regime (Gill, 1981; Wirth, 2005). We also separately grouped the tree pollen into taxa associated with dark taiga (Pinus sibirica, Abies sibirica, Picea obovata) and light taiga (Pinus sylvestris, Betula spp., Larix spp.) and created an index for dark-to-light taiga composition, which is an approximation of forest density, by calculating the ratio between the relative abundance of dark and light taiga tree taxa (Higuera et al., 2014). Additionally, we pooled the non-arboreal pollen types as shrubs, herbs, ferns, and mosses. To determine the regional changes in tree community composition, we created composite records of PFTs. This was done by averaging the Z scores and smoothing them to 300 years with a locally weighted regression and a 95 % confidence envelope on the same sites used for composite biomass burning reconstructions in the R palaeofire package.

To determine changes in peatland hydrology at each site, we used testate amoeba-based water table reconstructions. The testate amoebae preparation and identification were conducted at 4 cm intervals following the methods of Hendon and Charman (1997) and Charman et al. (2000). Based on the available literature (Grospietsch, 1958; Ogden and Hedley, 1980; Mazei and Tsyganov, 2006), we identified and counted a minimum of 150 testate amoebae per sample. No testate amoebae were present in core segments below 285 cm (age >6000 ka) in the Ulukh–Chayakh sequence. To derive the water table depth, we used two transfer functions, one developed for the pan-European region (Amesbury et al., 2016) and the other for Asia (Qin et al., 2021). The sample errors were generated using 1000 bootstrapping cycles (Line et al., 1994). The two transfer functions applied to our testate amoebae records show similar trends but different absolute water table depth values (cm). To avoid potential misinterpretation of absolute water table depth values, we followed literature recommendations (Amesbury et al., 2016; Swindles et al., 2016, 2019; Qin et al., 2021) and standardised the centimetre values and presented them as residual values (Z score). For the composite water table reconstruction, we used the same setting as for pollen.

The concentration of the geochemical element Ti was measured using a non-destructive Niton XL3t 900 X-ray fluorescence analyser (fpXRF) to determine the potential influence of water influx (i.e. floods) on mire water table. Sedimentary Ti NCS DC73308 was used as a Certified Reference Material (CRM). Measurements followed the procedure described by Hutchinson et al. (2016).

To quantify the relationship between peatland moisture conditions, biomass burning, the relative abundances of PFTs, and dark-to-light taiga composition, we performed a correlation analysis. We used microcharcoal (CHAR particles <150 µm) and macrocharcoal (CHAR particles >150 µm) influx to determine regional and local biomass burning, pollen percentages as indicators of PFTs (resisters, invaders, avoiders, and others) at local to regional scales, and testate-amoebae-based water table depth as a proxy for local peatland moisture. The group “others” includes pollen and spores of shrubs, herbs, ferns, and mosses. Before the analysis, we interpolated all datasets fed into the model to a 100-year interval using linear interpolation.

The age–depth model of the Rybnaya sequence spans ∼ the last 8400 years and shows a mean peat accumulation rate of 25 yr cm-1 (ranging between 6 and 36 yr cm-1). The Ulukh–Chayakh sequence may cover ∼ the last 8500 years, but the chronology of the bottom part of this site (>6000 years) relies on linear extrapolation and is therefore highly uncertain (Sect. S1). The subsequence covering the last 6000 years has an average temporal resolution of 21 yr cm-1 (ranging between 3 and 37 yr cm-1).

The comparison of sedimentary charcoal from sub-recent samples with satellite imagery of the Plotnikovo Mire (sites SP and SD) confirmed that the marked increase in charcoal particles towards the present day at the SD location closely agrees with the occurrence of high-severity fires (Fig. 1; Feurdean et al., 2020a). Satellite images and official forest statistics show no evidence of recent fires in the vicinity of the Rybnaya and Ulukh–Chayakh sites, except for a small fire documented in 1993 near the Ulukh–Chayakh site (Fig. 1). This aligns with the scarcity of charcoal pieces from subsurface samples in both cores (Fig. 2; Sect. S3). A high-severity fire event occurred in 2015 ca. 30 km from Ulukh–Chayakh, which is not documented in our subsurface charcoal record and corroborates the localised origin of charcoal found in this sedimentary profile. Samples from the Ulukh–Chayakh site show a minor increase in charcoal (2–7 pieces; up to 0.6 cm-2 yr-1) ca. 50 years ago, possibly related to a local fire event in 1993. The first clear charcoal peak at this site (250 pieces; 7.3 cm-2 yr-1), with numerous wood fragments, was found ca. 200 years ago, whereas the preceding charcoal peak (400 pieces; 9.2 cm-2 yr-1) was ca. 900 years ago (Fig. 2; Sect. S3). These two peaks therefore indicate the occurrence of infrequent, high-severity local fires that produce large quantities of charcoal, embedded in what is otherwise predominately a surface-fire-generated low level of charcoal.

Both records displayed a high signal-to-noise index (Sect. S2) with almost all peaks well above 3 (a mean of 4.98 at Rybnaya and 6.45 at Ulukh–Chayakh), the theoretical minimum value for justification of peak analysis (Kelly et al., 2011). At Rybnaya, CHAR was highest between 7.5 and 6 ka and around 4.5 ka, whereas the frequency of high-severity fires inferred from the charcoal peak magnitude and abundance of woody morphologies ranged between one and two fires per 900 years (Figs. 2, 4; Sects. S2, S3). At Ulukh–Chayakh, CHAR was highest >6 and 3.5 ka, and the fire frequency varied between two and five fires per 900 years. Both profiles showed a decline towards a minimum in CHAR and frequency of severe fires, i.e. low peak magnitude and reduced occurrence of woody charcoal morphologies, approximately between 4 and 1.5 ka (3–1.5 ka at Ulukh–Chayakh). The CHAR increased towards the present at both sites, but the charcoal peak amplitude was considerably higher at Ulukh–Chayakh.

The composite CHAR record showed augmented regional biomass burning between 7.5 and 4 ka (positive Z scores) with peaks centred at 7.5 and 4.5 ka (Fig. 5). We found a pattern of CHAR decline towards a minimum in the composite profiles between 4 and 2 ka (negative Z scores), followed by another increase between 2 ka and the present (positive Z scores). The composite fire frequency showed an increase until ca. 5–4.5 ka and then again from 2 ka to the present.

Early Holocene data are only available from the Rybnaya site (8.5 and 7.5 ka), indicating that light taiga and the fire invader Betula were most abundant (up to 60 %) during that period, whereas other light taiga fire resisters (Pinus sylvestris) and dark taiga fire avoiders (mainly Picea obovata) had an abundance of 10 % and 5 %, respectively (Fig. 3; Sect. S4a). The high proportion of light taiga resulted in a low dark-to-light taiga ratio of 0.1 and perhaps a low forest density. An increased dark-to-light taiga ratio (>0.2) was identified between 7.5 and 4 ka at Rybnaya and between 5 and 2.5 ka at Ulukh–Chayakh (Fig. 5; Sect. S4a, b). The light taiga was composed of up to 70 % fire resisters (Pinus sylvestris and Larix spp.), while the dark taiga, with its fire avoiders (Pinus sibirica, Picea obovata, and Abies sibirica), made up 25 % of the forest. A decline in fire resisters (down to 50 %), avoiders (below 10 %), and dark-to-light taiga ratio as well as increasing invader abundance (up to 60 %) occurred after 4.5 ka at Rybnaya and after 2.5 ka at Ulukh–Chayakh. Between-site variability was highest within the avoider group. Picea obovata had higher values at Rybnaya (up to 10 %) than at Ulukh–Chayakh (3 %), whereas Abies sibirica was more abundant at Ulukh–Chayakh (15 %) than at Rybnaya (7 %; Fig. 4). The composite vegetation record showed a higher proportion of fire resisters between 7.5 and 5 ka, fire avoiders between 7.5 and 3 ka, and fire invaders over the last 4.5 kyr (Fig. 5).

Understory vegetation primarily reflects local peatland development. It was predominantly composed of herbs–sedges (Cyperaceae; up to 20 %), ferns (Polypodiaceae, up to 20 %), and Equisetum (up to 25 %) at both sites until ca. 4.5 ka. An increase in the abundance of shrubs (Ericaceae; up to 10 %) and moss (Sphagnum; highly fluctuating between 10 % and 100 %) at Rybnaya as well as Ericaceae (up to 5 %) and moss (Sphagnum; up to 100 %) at Ulukh–Chayakh was noted over the last 4.5 kyr (Fig. 3; Sect. S4a, b). The sum and individual levels of herbaceous taxa, including those potentially related to human impact (Artemisia, Asteraceae, Chenopodiaceae, Urticaceae, Brassicaceae, Plantago lanceolata), were constant throughout the profiles. However, their abundance increased slightly over the last 1.5 kyr (Fig. 3; Sect. S4a, b). Coprophilous dung spores, Podospora and Sporormiela, were present throughout both records, although they became more abundant between 4.5 and 3.5 ka and between 3.2 and 1.0 ka at Ulukh–Chayakh as well as over the past millennium at Rybnaya (Sect. S4a, b).

The two testate amoebae transfer functions returned similar trends for the water table depth, but the absolute values (cm) were different; the Asian transfer function yielded higher reconstructed water tables than the European transfer function (Fig. 4; Sect. S5). At Rybnaya, the standardised values of water table depth indicated relatively high levels (negative Z score) between 8.5 and 7.5 ka, between 4 and 2.5 ka, and over the last 1 kyr (Fig. 4). We reconstructed low water levels (positive Z score) for the time interval between 7.5 and 4.5 ka and between 2.5 and 1 ka (Fig. 4). At Ulukh–Chayakh, the water level was high (negative Z score) between 3.5 and 2.5 ka, between 2 and 0.5 ka, and towards the present day. There, times of low water levels (positive Z score) occurred between 6 and 3.5 ka, between 2.5 and 2 ka, and around 0.5 ka. The composite record of the water table showed high levels at approximately 8.5–7.5, 3.5, and 3 ka as well as during the last 1.5 kyr and low levels between 7.5 and 4.5 ka as well as around 2.5 ka (Fig. 5).

The correlation analysis at Rybnaya showed that microcharcoal and macrocharcoal were highly correlated and positively associated with the proportion of fire avoiders and light-to-dark taiga index as well as negatively with resisters and invaders (Fig. 6a). However, not all these relationships were statically significant (Fig. A1). At Ulukh–Chayakh, microcharcoal and macrocharcoal were weakly correlated. Microcharcoal was positively associated with fire avoiders, resisters, others, and light-to-dark taiga index and negatively with invaders (mostly statically significant; Fig. 6a, b). All other relationships, i.e. between PFTs and macrocharcoal as well as depth to the water table, were statistically non-significant (Figs. 6a, A1).

Stand-replacing fires combust substantial amounts of biomass because they burn entire trees. Such fires manifest themselves in a greater abundance of charcoal particles that exceed the quantities typically produced by surface fires (van Marle et al., 2017). This feature allows a distinct separation of charcoal peaks from charcoal background levels, which yields a higher signal-to-noise index (Higuera et al., 2005, 2009; Courtney Mustaphi and Pisaric, 2014). The comparison of sedimentary charcoal from sub-recent samples with satellite imagery of the Plotnikovo Mire (Feurdean et al., 2020a), Rybnaya, and Ulukh–Chayakh shows that high-severity local fires produce large quantities of charcoal; thus, the charcoal peak extraction method applied here reliably reflects the occurrence of such fires.

We reason that the prevalence of a high charcoal influx with well-defined charcoal peaks and abundant woody morphotypes found between ca. 7.5 and 4(3) ka at both sites, as well as over the past 1.5 kyr at Ulukh–Chayakh, is indicative of the predominance of high-severity local fires (Fig. 2; Sect. S3). Between 4(3) and 1.5 ka, the observed low charcoal influx and peak magnitude were coupled with a lower abundance of woody morphologies at both sites, although the latter feature was more pronounced at Ulukh–Chayakh (Fig. 2; Sect. S3). For this period, we inferred predominately low-temperature surface fires that mostly burned understory biomass. Experimental production of charcoal additionally provides evidence that graminoids and Sphagnum retain a high charred mass only during low-temperature fires and that their charred mass declined rapidly with increasing fire temperature (Hudspith et al., 2018; Feurdean, 2021). Measurements of aspect ratio (L:W) and surface area on selected charcoal samples at Ulukh–Chayakh show that the aspect ratio only partially agrees with the predominant fuel type inferred from charcoal morphologies. A lower aspect ratio of charcoal particles, which is typical for wood and leaf morphotypes, was what we expected at times of increased woody morphologies and an increased aspect ratio with a rise in the relative proportion of graminoids (Feurdean, 2021; Vachula et al., 2021). This is probably the result of a strongly mixed morphology of charcoal types in the same sample (Fig. 2). Our measurements of charcoal surface area indicate high values at times of an increased abundance of leaves (Fig. 2; Sect. S3) with a typically high surface area (Crawford and Belcher, 2014; Feurdean, 2021).

The reconstructed fire return interval is ∼450 years (two fires per 900 years) for both sites throughout most of the Holocene (Fig. 4). However, fire frequency (at Rybnaya) and severity (at Ulukh–Chayakh) increased over the last 1.5–2 kyr compared to the long-term averages of both records, suggesting an intensification of fire activity during the most recent 2 millennia. The composite fire frequency derived from all four records also shows increasing biomass burning and fire frequency over the last 2–1.5 kyr (Fig. 5). The reconstructed fire return interval (FRI) in the study area is largely in line with other sites located in forested peatlands (Pinus–Betula-dominated, Picea obovata, Abies sibirica) in Russia, which report an FRI range between 100 and 600 years (Barhoumi et al., 2019, 2020, 2021). In contrast to a study in the northern Ural region (Barhoumi et al., 2019), we have not found a gradual increase in fire frequency from the early Holocene towards the present, but rather two distinct periods of enhanced activity between 7.5 and 4 ka and approximately over the last 2 kyr (Fig. 5). This is a pattern more similar to sites in the western Siberian Plain (Rudaya et al., 2020) and Lake Baikal area (Barhoumi et al., 2021).

Climate conditions influence peatland hydrology, fuel dryness, and flammability, as well as vegetation composition with regard to moisture- and fire-related PFTs. The relative abundances of the fire-related PFTs, in turn, influence the prevalence of specific fire regime types, e.g. low-intensity vs. high-intensity regimes. Below, we discuss how hydrological conditions in peatlands affect fuel dryness and flammability as well as feedback between tree community composition (PFTs) and fire regime.

Climate conditions affect the relative abundances of PFTs. Fire-related PFTs, in turn, create positive or negative feedbacks for fire regimes via specific fire-related traits (Feurdean et al., 2020a; Kharuk et al., 2021). We found a dominance of open light taiga and fire-invader species (Betula spp.) between 8.5 and 7.5 ka and over the past 4.5 kyr. The occurrence of Betula-dominated forests coincides with marked variability in the frequency and severity of fire, i.e. moderate between 8.5 and 7.5 ka, low between 4.5 and 1.5 ka, and high over the past 1.5 kyr (Figs. 2, 4). B. pubescens grows on wet peatlands, whereas B. pendula grows more frequently on drier sandy soils and peatlands with higher fire incidence (Wirth, 2005; Groisman et al., 2012; Blyakharchuk, 2003). The prevalence of birch species with contrasting hydrological and fire requirements (unable to be differentiated by pollen) may explain such variable association of Betula and fire regime. Denser forests of light taiga and fire resisters (Pinus sylvestris), interspaced with dark taiga and fire avoiders, primarily Pinus sibirica at Rybnaya and Abies sibirica at Ulukh Chayakh (only from 5.5 ka), prevailed between 7.5 and 4.5 ka (Fig. 3; Sect. S4a, b). This forest composition coincides with increased fire severity. Visual trends and correlation reveal that biomass burning increases at intermediate dark-to-light taiga index values (ranging from 0.15 to 0.3; Figs. 4, 6). This is consistent with emerging findings pointing to a higher fire hazard in less dense as opposed to denser boreal forests, which allow fuel to dry (Scheffer et al., 2012; Feurdean et al., 2020b). The dominance of dark taiga forests in western Siberia was associated with increased moisture conditions (Blyakharchuk and Sulerzhitsky, 1999; Zhang and Feng, 2018; Barhoumi et al., 2021). Our correlations, however, suggest a greater tolerance of deciduous trees to shallow water level (wetter conditions) compared to conifers (Fig. 6a, b). Note that testate amoebae and macrocharcoal reveal local changes in peatland hydrology and fire activity, whereas pollen and microcharcoal reflect local to regional changes in vegetation composition (the tree composition on the peatlands and adjacent forests) and fire activity (Bennet and Willis, 2001; Whitlock and Larsen, 2001; Lamentowicz et al., 2015; Blyakharchuk and Kurina, 2021). The deviating scale dependency of the reconstructions may induce some uncertainty in linking the local peat moisture conditions and the dominant local to regional PFTs.

The presence of the fire resisters (i.e. Pinus sylvestris) in Siberia is associated with a light surface fire regime (mean FRI of 28 years) that is occasionally interrupted by longer-term (mean FRI of 200 years) stand-replacing crown fire events (Ivanova, 2005; Kukavskaya et al., 2016). Fire avoiders typically experience severe stand-replacing crown fires at relatively long return intervals (mean FRI 150 years; range 99 and 300 years; Goldammer and Furyaev, 1996; Wirth, 2005; Kharuk et al., 2021). The post-fire regeneration pathway of conifer species involves recruitment directly after burning (P. sylvestris) or after an early successional phase initiated by Betula (Pinus sibirica, Picea obovata, and Abies sibirica). A mixed taiga forest with deciduous trees and dark taiga conifer taxa typically develops between 60 and 120 years after a stand-replacing fire, followed by the dominance of conifers as a late successional stage (Tautenhahn et al., 2016; Coop et al., 2020; Kharuk et al., 2021). We reason that the prevailing periodicity of severe fires (every 180 to 450 years) between 7.5 and 4.5 ka provided fire-free periods that were long enough for the fire avoiders Pinus sibirica, Picea obovata, and Abies sibirica to reach reproductive age. Alternatively, burning may have been too patchy to create sufficiently large forest gaps to limit seed dispersal and inhibit post-fire recovery. Pinus sibirica is typically classified as a fire avoider but behaves like a fire resister when reaching old age, which could also explain its increased presence at times of high fire activity. Fire also allows dark taiga species to compete with P. sylvestris and become established beyond poor soils and boggy areas (Kharuk et al., 2021).

A different fire regime and altered vegetation feedbacks emerged over the last 2 millennia at Ulukh–Chayakh, the site that experienced more severe fire episodes. The intensification of local fire severity at Ulukh–Chayakh paralleled a decline in the proportion of fire avoiders (mostly Abies sibirica and Picea obovata), an increase in fire invaders (Betula), and the abundant occurrence of heathland shrubs (Figs. 3, 4). Shrubs burn hotter than other surface fuel types and reach temperatures that kill most conifer seedlings (Tautenhahn et al., 2016). High-severity burned areas have also been found to be more prone to repeated burning and have a more negative impact on conifers (Kukavskaya et al., 2016; Whitman et al., 2019; Coop et al., 2020). We propose that the intensification in fire severity at Ulukh–Chayakh may have eliminated mature Abies sibirica and Picea obovata, limited their seed dispersal, and disrupted their successional pathways. These dynamics resulted in a shift in tree community composition towards more post-fire-adapted invader communities with better dispersal capabilities, recruitment strategies for burned areas, and rapidly maturing taxa. Our results support emerging findings of increases in deciduous trees (Betula or Populus) at the expense of evergreen conifers that are associated with contemporary warming and increasing fire severity in boreal forests and forested peatlands in Siberia and North America (Kelly et al., 2013; Tautenhahn et al., 2016; Mekkonen et al., 2019; Whitman et al., 2019; Kharuk et al., 2021). At Rybnaya, the modest increase in biomass burning over the past 2 millennia coincided with wet peatland conditions, the occurrence of Sphagnum, and a forest composition that remained dominated by Betula, indicating that fire has played a more limited role in tree community dynamics at this site. The post-disturbance (fire, insect outbreak) forest regeneration pathway in Siberia also involves Populus tremula. Eurasian aspen forest communities can reach hundreds of years in age and change forest composition to Populus dominance where regeneration of dark taiga conifers is absent (Tautenhahn et al., 2016; Kharuk et al., 2021). Although we have not encountered Populus in our pollen record due to its poor pollen preservation, the possibility that Populus may have been a part of post-fire forest succession pathways in the past cannot be discounted.