Is the closed-crown boreal forest resilient after successive stand disturbances? A quantitative demonstration from a case study

Question Recent studies revealed by indirect evidence that lichen-spruce woodlands in the closed-crown boreal forest are an alternative stable state of black spruce-feathermoss stands. This paper aims to demonstrate and quantify black spruce forest cover regression following successive wildfire events in the closed-crown forest zone during the 20th century. A case study in north-eastern North American closed-crown forest zone is presented. Location The study area is at the centre of the closed-crown boreal forest zone of eastern Canada (49 degrees 59N, 71 degrees 59W). Methods A burned area of 25ha was selected in which three tree cohorts were visible. The first cohort contains partly buried trunks and stumps of a stand destroyed by a forest fire in 1920, the second contains standing dead trees from a 1995 forest fire and the third contains seedlings established after the 1995 fire. Fires were dated by scar analyses of living trees at the edge of the site. An array of sample plots was established in autumn 1997 to evaluate each cohort in terms of stocking and tree density. Results Black spruce stocking decreased significantly between cohorts, from 43.5% in Cohort 1 (all coniferous trees) to 25.6% in Cohort 2 and 11.5% in Cohort 3 (2011). Jack pine showed an inverse tendency, from 0% in Cohort 1 (not determined) to 0% in Cohort 2 and 3.8% in Cohort 3. Black spruce stems per hectare also dropped significantly from a minimum of 1322 in Cohort 1 to 1088 in Cohort 2 and 976 in Cohort 3. Conclusions This study is the first quantitative demonstration that lichen-spruce woodlands in the closed-crown boreal forest zone are an alternative stable state of black spruce-feathermoss stands. Stocking levels and densities markedly declined after each of the two fires (1920 and 1995) that both occurred after spruce budworm outbreaks (1910 and 1980) and are most likely responsible for the shift from a closed-crown to an open woodland.