The role of fire in the late Holocene forest decline in semi-arid North China

Based on a sediment core from Lake Hulun Nuur in the semi-arid forest-steppe ecotone in North China, we reconstructed the history of climate change, vegetation development, and fire frequency in this region during the mid- to late Holocene. Four stages were identified: (1) 6300–4900 cal. yr BP, a warm-humid phase dominated by forest vegetation with moderate fire incidence and a gradual drying trend; (2) 4900–4250 cal. yr BP, a warm-dry phase dominated by forest vegetation with increased aridity and high fire occurrence; (3) 4250–1750 cal. yr BP, a gradual drying phase with forest decline and low fire occurrence; and (4) 1750 cal. yr BP–present, a cool-dry phase dominated by steppe with some decrease in aridity. Building on previous studies in this region, the successive timing of forest replacement by steppe along the precipitation gradient in the late Holocene indicates a gradual retreat of the forest margin under climate drying. Vegetation succession was predominantly determined by climate, but forests showed strong resilience to mid-Holocene drought events. Fire was suggested as a triggering disturbance that acted to reduce the resilience of forests to drought and prevent rapid forest recovery under subsequent climatic amelioration. No simple anti-correlation between fire and Pacific monsoon intensity suggested by previous studies was found here, highlighting vegetation as a critical factor for fire occurrence through the accumulation of fuel in the semi-arid region, where vegetation growth is strongly constrained by precipitation.