Origin of the lichen woodland at its southern range limit in eastern Canada: the catastrophic impact of insect defoliators and fire on the spruce-moss forest

The lichen woodland is one of the most important forest ecosystems in North America, dominating the central part of the boreal forest. The southernmost lichen woodland is paradoxically in the heart of the southern boreal forest. This distribution prompted this study aiming to identify the factors responsible for the inception and development of the lichen woodland at its southern range limit in eastern Canada. We tested the hypothesis that the southern lichen-spruce woodland is a regressive, post-fire type of the sprucemoss forest. We studied adjacent lichen-spruce and spruce-moss stands growing under similar soil conditions. We reconstructed the recent history of spruce-moss forest transformation to lichen woodland using tree size, tree ring patterns, and macrofossil analysis of organic soil. All the plant macrofossils buried in the unburned organic mat below the charcoal layer of the last fire corresponded to a feather-moss forest assemblage and included head capsules of the spruce budworm (Choristoneura fumiferana (Clem.)) or European spruce sawfly (Gilpinia hercyniae Hartig.). The impact of combined insect and fire disturbances translates into a dramatic decrease in post-fire tree regeneration of the forest inducing the shift to lichen woodland. The inception of the southern lichen woodland highlights the fragility of the spruce-moss forest even in the core area of the southern commercial boreal forest.