Factors Responsible for the Co-occurrence of Forested and Unforested Rock Outcrops in the Boreal Forest |
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| Authors: | Hugo Asselin Annie Belleau Yves Bergeron |
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| Institution: | (1) NSERC/UQAT/UQAM Industrial Chair in Sustainable Forest Management, Université du Québec en Abitibi-Témiscamingue, 445, boulevard de l’Université, Rouyn-Noranda (Québec), J9X 5E4, Canada |
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| Abstract: | Rock outcrops in the boreal forest of Québec 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 Québec. 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. |
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| Keywords: | Alternative stable states Boreal forest Diverging types Fire Primary succession Secondary succession |
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