Increasing spring dryness accelerates transitions toward pyrogenic vegetation in eastern boreal North America

Ongoing climate change is increasing vegetation flammability, intensifying fire activity in the boreal forests of eastern North America. This situation suggests a potential tipping point in fire regimes, raising critical questions about their impact on the biodiversity and structure of these ecosystems. To gain a deeper understanding of landscape dynamics and ongoing environmental changes, it is essential to understand how this climate, vegetation and fire linkages operate across various temporal and spatial scales. By integrating paleo-datasets (charcoal, pollen, chironomids, and testate amoebae) with model simulations of vapor pressure deficit (VPD) and plant-available soil water (ASW) over the past 8,000 years, we show that drier spring conditions over the last 3,000 years led to fewer but larger and more severe fire episodes, peaking within the last 250 years. This shift in fire regimes promoted an increase in fire-adapted conifer species, particularly Pinus banksiana, across the landscape. These findings challenge previous projections of increased dominance by thermophilous species under climate change scenarios and instead suggest an expansion of pyrophilous vegetation. Such ecological transitions are set to drive significant environmental and socio-economic consequences.