Reconstructing past fire-regime metrics during the Holocene in the western Tianshan Mountains, Northwest China

Hitherto quantitative long-term reconstructions of key fire-regime parameters (e.g., fire frequency, fire return interval, fire episodes, peak magnitude and fuel type) over the Holocene are still limited across the vast grassland systems of Northwest China. To fill this gap in knowledge, this study presents quantitative characteristics of fire-regime components during the Holocene based on sedimentary charcoal assemblages from a wetland core situated at the steppe zone in the western Tianshan Mountains of Northwest China. A wide range of charcoal particle-size categories (10–30 μm, 30–50 μm, 50–100 μm and >100 μm) was extracted to detect the spatial patterns in past fire-regimes on local and regional scales. The results reveal a predominance of regional-scale fire-regimes in the grass-dominated landscape throughout the Holocene. Charcoal morphological records show that the dominant types of fuel source were herbaceous plants over the Holocene. Fire intensity and frequency exhibited an overall upward trend, when the amounts of herbaceous fuel loads gradually increased. The fire-regime reconstructions based on the integrated charcoal record with all particle-size categories indicate that the early-Holocene (10.0–7.2 cal kyr BP) was characterized by low charcoal accumulation rates (CHAR) averaging 1283 pieces cm −2 year −1 , 3 fire-episodes, moderate fire frequency (mean 2.13 fires 2000 year −1 ), low peak magnitude (mean 95,099 pieces cm −2 peak −1 ) and high fire return interval (FRI) averaging 553 years. The mid-Holocene (7.2–4.0 cal kyr BP) was marked by increased CHAR (mean 7892 pieces cm −2 year −1 ), higher peak magnitude (mean 547,174 pieces cm −2 peak −1 ), 3 fire-episodes, decreased fire frequency (mean 1.87 fires 2000 year −1 ) and shorter FRI (mean 538 years). The late-Holocene (4.0–0.7 cal kyr BP) witnessed a further escalation of fire activities, thereby reaching the highest values of CHAR (mean 10,227 pieces cm −2 year −1 ), fire-episodes (8), fire frequency (mean 4.30 fires 2000 year −1 ) as well as peak magnitude (mean 610,610 pieces cm −2 peak −1 ), and the shortest FRI (mean 403 years). The millennia-scale interactions between fire, climate, vegetation and human activities were assessed to explore the possible mechanisms affecting past fire-regime dynamics. The fire-regimes in the early-Holocene were likely fuel-limited due to the arid-warm climate and low biomass production. The fire-regimes in the mid- and late-Holocene were probably climate-limited because of increasing humidity, grass productivity, fuel loads and favorable weather conditions of fire season. Notably anthropogenic factors possibly affected the fire-regime changes in the late-Holocene. This study provides significant insights into long-term grassland fire management and fire-regime prediction under future climate change.