Factors responsible for the co-occurrence of forested and unforested rock
outcrops in the boreal forest.
Hugo Asselin, Annie Belleau, Yves Bergeron.
Rock outcrops in the boreal forest of Quebec can show either of two different states: a forested state with
>25% tree cover, and an unforested state (<25% tree cover). We tested three different hypotheses that
might explain the co-occurrence of forested and unforested rock outcrops: (1) differences in bedrock
geology, with unforested outcrops associated to bedrock types inimical to tree growth; (2) unforested
outcrops as recently disturbed sites undergoing secondary succession towards a forested state; (3) unforested
outcrops as an alternative stable state to forested outcrops, induced by post-fire regeneration failure.
Digitized forest inventory maps were used along with bedrock geology maps and time-since-fire maps to
compare forested and unforested outcrops for bedrock geology type and date of the last fire. Field surveys
were conducted on 28 outcrops (14 forested, 14 unforested) to gather information regarding tree species
composition and site characteristics (thickness of the organic matter layer, percent cover of lichens, mosses
and ericaceous shrubs). None of the three hypotheses explain the co-occurrence of forested and unforested
rock outcrops in the boreal forest of Que´ bec. Both outcrop types occur on the same bedrock geology types.
Unforested outcrops are not recently disturbed sites in early-successional states, as no clear distinction
could be made in tree species composition and date of the last fire between the two outcrop types. Forested
and unforested outcrops are not alternative stable states, as unforested outcrops are unstable and cannot
maintain themselves through time in the prolonged absence of fire. Hence, unforested rock outcrops could
be viewed as degraded, diverging post-fire types maintained by the late Holocene disturbance regime,
characterized by high fire frequencies.