Origin of the lichen-spruce woodland in the closed-crown forest zone of eastern Canada

We investigate the timing and factors responsible for the transformation of closed-crown forests into lichen-spruce woodlands. The study area extends between 70 degrees and 72 degrees W in the closed-crown forest zone from its southern limit near 47 degrees 30' N to its northern limit at the contact with the lichen-spruce woodland zone around 52 degrees 10' N. A total of 24 lichen-spruce woodlands were selected. Radiocarbon dating of charcoals at mineral soil contact and within the organic horizons allowed the principal factors causing the degradation of the closed-crown forest to be identified, i.e. light fires, successive fires and the occurrence of a spruce budworm epidemic followed by a fire. Charcoals dated in the organic horizon were less than 200 years old, suggesting a recent transformation of the closed-crown forest following surface fires. Before their transformation into lichen-spruce woodlands, stands were occupied by old, dense forests that originated from fires dating back to 1000 yr bp. The radiocarbon dating of charcoals in the organic horizon indicated that several stands burned twice in less than 50 years, while others burned shortly after a spruce budworm epidemic. Light fires are frequent within the lichen-spruce woodlands according to multiple charcoal layers found within the organic matter horizon. While closed-crown forests are predicted to expand under climate warming, compound disturbances diminish the natural regeneration of the closed-crown forests in the south and favour the expansion of lichen-spruce woodlands. As black spruce germinates on mineral soils, surface fires accentuate the expansion of the lichen-spruce woodlands southward. Under global warming, warmer springs will lead to earlier low-intensity fires that do not remove as much organic matter, and hence prevent conditions suitable for black spruce regeneration. Also, spruce budworm reduces seed production for a certain time. The occurrence of fire during this period is critical for regeneration of black spruce.