Holocene wildfire regimes in western Siberia: interaction between peatland moisture conditions and the composition of plant functional types
Authors: Feurdean Angelica , Diaconu Andrei-Cosmin , Pfeiffer Mirjam , Gałka Mariusz , Hutchinson Simon M. , Butiseaca Geanina , Gorina Natalia , Tonkov Spassimir , Niamir Aidin , Tantau Ioan , Zhang Hui , Kirpotin Sergey 
Published: June 2022

Abstract. Wildfire is the most common disturbance type in boreal forests and
can trigger significant changes in forest composition. Waterlogging in
peatlands determines the degree of tree cover and the depth of the burnt
horizon associated with wildfires. However, interactions between peatland
moisture, vegetation composition and flammability, and fire regime in forest
and forested peatland in Eurasia remain largely unexplored, despite their
huge extent in boreal regions. To address this knowledge gap, we
reconstructed the Holocene fire regime, vegetation composition, and peatland
hydrology at two sites located in predominantly light taiga (Pinus sylvestris Betula) with
interspersed dark taiga communities (Pinus sibirica, Picea obovata, Abies sibirica) in western Siberia in the Tomsk Oblast,
Russia. We found marked shifts in past water levels over the Holocene. The
probability of fire occurrence and the intensification of fire frequency and severity increased at times of low water table (drier conditions), enhanced
fuel dryness, and an intermediate dark-to-light taiga ratio. High water
level, and thus wet peat surface conditions, prevented fires from spreading on
peatland and surrounding forests. Deciduous trees (i.e. Betula) and Sphagnum were more
abundant under wetter peatland conditions, and conifers and denser forests
were more prevalent under drier peatland conditions. On a Holocene scale,
severe fires were recorded between 7.5 and 4.5 ka with an increased
proportion of dark taiga and fire avoiders (Pinus sibirica at Rybnaya and Abies sibirica at Ulukh–Chayakh) in a predominantly light taiga and fire-resister community characterised by Pinus sylvestris and lower local water level. Severe fires also occurred over the last 1.5 kyr and were associated with a declining abundance of dark taiga and fire avoiders, an expansion of fire invaders (Betula), and fluctuating water tables. These findings suggest that frequent, high-severity fires can lead to compositional and structural changes in forests when trees fail to reach reproductive maturity between fire events or where extensive forest gaps limit seed dispersal. This study also shows prolonged periods of synchronous fire activity across the sites, particularly during the early to mid-Holocene, suggesting a regional imprint of centennial- to millennial-scale Holocene climate variability on wildfire activity. Humans may have affected vegetation and fire from the Neolithic; however, increasing human presence in the region, particularly at the Ulukh–Chayakh Mire over the last 4 centuries, drastically enhanced ignitions compared to natural background levels. Frequent warm and dry spells predicted by climate change scenarios for Siberia in the future will
enhance peatland drying and may convey a competitive advantage to conifer
taxa. However, dry conditions will probably exacerbate the frequency and
severity of wildfire, disrupt conifers' successional pathway, and accelerate
shifts towards deciduous broadleaf tree cover. Furthermore,
climate–disturbance–fire feedbacks will accelerate changes in the carbon
balance of boreal peatlands and affect their overall future resilience to
climate change.