The impact of wildfires on the dynamics of vegetation cover in the middle taiga subzone of Western Siberia during the Holocene

In the article we present new results on the influence of paleo-fires on the dynamics of vegetation cover and the connections between them using the example of bottom sediments of Lake “S14” in the middle taiga subzone of Western Siberia (Khanty-Mansiysk Autonomous Okrug). The change in vegetation cover is influenced by both climate and fire activity, which acted as a trigger for the evolution of vegetation cover. This is evidenced by the obtained paleoecological data based on the analysis of identified particles of charcoal and pollen in lake sediments. According to the radiocarbon dating, sedimentation of lake "S14" began at 11920 cal. yr. BP. Based on the macro-charcoal analysis and statistical processing of the obtained data in the CharAnalysis program in R, the Holocene history of paleo-fires in the study area was reconstructed. 16 local fire episodes, their time, frequency and intensity were identified (11400, 11100, 10700, 10400, 9800, 9400, 7400, 6100, 5150, 4500, 3800, 2800, 1400, 1100, 400, 250 cal. yr BP).

Using spore-pollen analysis, the dominant landscapes were reconstructed for the entire period of the existence of lake “S14”: 12000-11500 cal. yr BP – larch-spruce forests with an admixture of birch; 11500-9850 cal. yr BP – larch-spruce-birch forests; 9850-4700 cal. yr BP – spruce-pine-birch forests; 4700-3500 cal. yr BP – birch-pine forests; 3500-2250 cal. yr BP – birch-cedar-pine forests; 2250-1000 cal. yr BP – cedar-pine forests with an admixture of birch; 1000 cal. yr BP to present – cedar-birch-pine forests. The resulting reconstruction of the dynamics of vegetation cover is compared with the history of paleo-fires of the study lake and with the climatic periods of the Holocene. This made it possible to identify three periods with maximum pyrogenic activity (11500-10400, 7500-6800 and 400-250 cal. yr BP), as well as to consider the conditions contributing to the intensification of Holocene wildfires. To determine the degree of impact of fires on the change in vegetation cover and the connections between them, a correlation analysis was carried out using the Pearson method in the PAST program. The analysis was made based on a comparison of micro- and macro-particles of charcoals with the pollen content of the predominant plant taxa for lake “S14”.

The most powerful paleo-fires were noted at the end of the Preboreal – beginning of the Boreal periods of the Holocene (11500-10400 cal. yr BP) with 4 local fire episodes and a high rate of accumulation of charcoal particles (1.1 per cm²/year). At the same time, larch-spruce forests with an admixture of birch grew near the lake area. The next maximum of pyrogenic activity was recorded in the mid-Atlantic period of the Holocene (7500-6800 cal. yr BP) with one local fire. The rate of charcoal accumulation decreased slightly compared to the previous period – 0.9 particles per cm²/year. At this time, the territory of the middle taiga subzone was covered with spruce-pine-birch forests. The third peak of local fires occurred at the end of the Subatlantic Holocene period (400-250 cal. yr BP) with a macro-charcoal accumulation rate of 0.6 particles per cm²/year. The vegetation cover included Siberian cedar, birch and pine forests at this time. It was found that the most intense fires occurred during dry climatic periods. The longest fire-free periods (9400-7400, 2800-1400 cal. yr BP) were observed precisely during the period of increasing precipitation.

According to the results of correlation analysis, wildfires had an impact on vegetation dynamics throughout the Holocene. A positive correlation of micro- and macro-charcoal particles with each other was revealed, which confirms the presence of fires at the local and regional levels and connection of local fires with regional fire situation. It has been determined that micro- and macro-charcoals simultaneously have a negative correlation with birch (Betula pendula), Siberian cedar (Pinus sibirica), Scot’s pine (Pinus sylvestris) and fir (Abies sibirica), and a positive correlation with grasses (Poaceae) and spruce (Picea obovata). A positive correlation with grasses and a negative correlation with tree pollen reflects the effect of fires on vegetation cover, probably, the suppression of tree species and the growth of grasses in the first stages of post-pyrogenic succession. The positive correlation with spruce is most likely due to the greater burning of landscapes at the beginning of the Holocene, when larch-spruce forests dominated the landscape and the climate was drier. This confirms the direct influence of fires on the formation of vegetation landscapes in the study region.