Boreal forest cover was reduced in the mid-Holocene with warming and recurring wildfires

The hemi-boreal climate-vegetation-fire nexus, marking the southern boreal forest boundary in North America, has evolved post-glaciation. This transitional zone hosts diverse ecosystems, including mixed forests with a wide array of tree species, savannas, grasslands, and wetlands. While the interplay of humans, climate, and fire shapes continental vegetation dynamics, specific influences remain unclear. Integrating archaeological records with climate, vegetation, and fire data, we present a 12,000-year historical account of the hemi-boreal zone. Postglacial biomass burning exhibited episodes of persistent elevated activity, and a pivotal shift around 7,000 years ago saw the boreal forest transition to an oak-pine barren ecosystem for about 2,000 years before reverting. This mid-Holocene shift occurred during a context of lower fire return intervals and sudden uptick in mean annual temperatures. Population size of Indigenous peoples mirrored these wildfire fluctuations, increasing during longer fire intervals and decreasing during elevated fire activity. Anticipated increases of fire activity with climate change are expected to echo transformations observed 7,000 years ago, reducing boreal forest extent and impacting land use.