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 southernmos t lichen woodland is paradoxically in the heart of the southern boreal fore st. This distribution prompted this study aiming to identify the factors re sponsible for the inception and development of the lichen woodland at its s outhern range limit in eastern Canada. We tested the hypothesis that the so uthern lichen-spruce woodland is a regressive, post-fire type of the spruce -moss forest. We studied adjacent lichen-spruce and spruce-moss stands grow ing under similar soil conditions. We reconstructed the recent history of s pruce-moss forest transformation to lichen woodland using tree size, tree r ing patterns, and macrofossil analysis of organic soil. All the plant macro fossils buried in the unburned organic mat below the charcoal layer of the last fire corresponded to a feather-moss forest assemblage and included hea d capsules of the spruce budworm (Choristoneura fumiferana (Clem.)) or Euro pean spruce sawfly (Gilpinia hercyniae Hartig.). The impact of combined ins ect 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 s pruce-moss forest even in the core area of the southern commercial boreal f orest.