Simulated decline of a northern forest due to anthropogenic controls on the regeneration-mortality balance

The population structure of forests is shaped by balancing the opposing forces of regeneration and mortality, each of which influence C turnover rates and are sensitive to climate. Regeneration underlies the migrational potential of forests to climatic change and remains underserved in modeling studies. Our objective was to test the hypothesis that warming may reduce tree regeneration rates while amplifying fire regimes, producing forest loss. Absent sites within dispersal limits, trees may fail to track the velocity of warming, producing a decline in forested area. Long-term implications include changes to biogeochemical and energetic balances, species composition, and evolutionary trajectories. We performed hybrid model simulations to assess the resilience of forests to past-century conditions over the next fifty years in western Canada. We conducted simulations at a species-level taxonomic resolution to capture genotypic/phenotypic variability in response to climate. A recent shift toward small, frequent, human-caused fires and warming-reduced regeneration diminished species migration potential. The simulated rate of forest migration lagged behind temperature equilibria by 319 m yr-1. Understanding species migrational potential is particularly critical for northern forests, which have warmed at a rate twice the global mean. Our findings highlight the effect of diminished regeneration due to climatic change, a process neglected in current global-scale terrestrial biosphere models used in climate studies. We suggest that future terrestrial biosphere model studies incorporate these demographic rates in their findings on global change, as they carry substantial climatic and evolutionary implications.