Dr Étienne Boucher - Widespread ecosystem shifts and carbon stock depletion during the Little Ice

The fate of ecosystem services provided by Eastern Canada’s taiga–tundra transition is tightly linked to complex, yet poorly understood, interactions among climate variability, fire history, and forest structure. This study investigates how present-day forest attributes and services reflect such long-term ecological trajectories over the past millennia. We combined (i) a novel, two-millennia-long tree-ring-based reconstruction of growing degree days (GDD), (ii) a large-scale semi-random forest inventory of current forest attributes (structure, composition, and carbon stocks), and (iii) radiocarbon and dendrochronological estimates of the time elapsed since the last fire. Our results suggest that most forest stands that burned after ~1450 CE, during a period when GDD values fell below 800, regenerated to more open stands following fire. Forest that burned during the last 200 years systematically shifted to treeless tundra. This led to substantial depletion of both above- and belowground carbon stocks across the region. In contrast, stands that escaped fire over the past 500 years remain carbon-rich ecosystems and serve as refugia for pyrophobic species such as balsam fir. In the absence of the Little Ice Age, forests in this region would thus likely be denser, more carbon-rich, and more diverse than they are today.