Unearthing Forest History: Integrating Soil Charcoal Macrofossils and Pollen Reveals Delayed Forest Responses to...

Studying historical dynamics and compositional changes of ecotones is essential for improving predictions of their responses to anthropogenic global change. However, the small size and geographic isolation of marginal ecotonal stands require retrospective analyses at the local scale. Here, we combine soil macrofossil charcoal analysis (SMCA), a lake-sediment pollen record, and contemporary forest inventories to reconstruct past dynamics and assess contemporary trajectories of forest stands located at the northern boundary of the boreal-temperate ecotone (BTE) in the Rimouski hinterland (Lower St. Lawrence region, Canada). Radiocarbon-dated soil charcoal particles and regional pollen data reveal a delayed local establishment of thermophilous species, such as white pine (Pinus strobus), toward the end of the Holocene Climatic Optimum. This lagged expansion of temperate taxa coupled with a decline of boreal conifers led to the development of thermophilous assemblages that persisted into the early Neoglacial despite climatic cooling. Borealization of the landscape did not begin until the late Neoglacial (⁓1500 cal year BP), coinciding with shifts in the fire regime and the arrival of the eastern white cedar (Thuja occidentalis). At the stand-scale, comparisons among charcoal assemblages in buried and surface soil layers with dead and living trees reveal an unprecedented proliferation by trembling aspen, as well as a recent establishment of jack pine likely reflecting anthropogenic disturbances associated with European settlement. However, current size-class distribution suggests that these recent changes are likely to be transient in the absence of recurrent disturbances. Although predictive modeling remains necessary to forecast future forest trajectories under climate change, our results underscore the value of multi-proxy reconstructions for disentangling vegetation responses to interacting climate and disturbance drivers.