Reconstruction of Holocene and Last Interglacial vegetation dynamics and wildfire activity in southern Siberia

Abstract. Wildfires are a rapidly increasing threat to boreal forests. While our understanding of the drivers behind wildfires and their environmental impact is growing, it is mostly limited to the observational period. Here we focus on the boreal forests of southern Siberia, and exploit a U-Th dated stalagmite from Botovskaya cave (55.2994° N, 105.4445° E), located in the upper Lena region of southern Siberia, to document wildfire activity and vegetation dynamics during parts of two warm periods; the last interglacial (124.1 – 118.8 ka BP) and the Holocene (10 – 0 ka BP). Our record is based on levoglucosan (Lev), a biomarker sensitive to biomass burning, and on lignin oxidation products (LOPs) that discriminate between open and closed forest and hard- or softwood vegetation. In addition, we used carbon stable isotope ratios (δ¹³C) to evaluate soil respiration and local infiltration changes. While the δ¹³C record reflects a dominant control of the host rock, the Lev and LOP time series show fire pattern and vegetation type differences between the last interglacial and the Holocene. Our LOP data suggest that during the last interglacial, the region around Botovskaya cave was characterised by open forest, which by ca. 121.5 ka BP underwent a transition from fire-resistant hardwood to fire-prone softwood. The Lev record indicates that fire activity was high and increased towards the end of last interglacial just before 119 ka BP. In contrast, the Holocene was characterised by a closed-forest environment with mixed hard- and softwood vegetation. Holocene fire activity varied but at a much lower level than during the last interglacial. We attribute the changes in wildfire activity during the intervals of interest to the interplay between vegetation and climate. The open forests of the last interglacial were more likely to ignite than their closed Holocene equivalents, and their flammability was aided by warmer and drier summers and a stronger seasonal temperature contrast compared to the Holocene. Our comparison of the last two interglacial intervals suggests that with increasing global temperatures the boreal forest of southern Siberia becomes progressively more vulnerable to higher wildfire activity.