Are the old-growth forests of the Clay Belt part of a fire-regulated mosaic?

Abstract

Old-growth forests make up a substantial proportion of the forest mosaic in the Clay Belt region of Ontario and Quebec, Canada, despite fire cycles that are presumed to be relatively short. Two hypotheses have been suggested as explanations for this phenomenon: (1) the old-growth forests in question are located on sites that are protected from fire or (2) the fire hazard is just as great there as elsewhere, and that part of the mosaic is simply the tail of the distribution, having been spared from fire merely by chance. The tree-ring method has proven inadequate as a means of determining the date of the most recent fire in these old-growth forests, as the time that has elapsed since that date probably exceeds the age of the oldest trees. Accordingly, a paleoecological study was conducted with a view to determining the date of the last fire in these forests. Charcoal horizons were located and radiocarbon dated in six oldgrowth forests. The possibility that these forests have never burned at all is ruled out by the fact that macroscopic charcoal fragments were found at all sites. The proximity of potential firebreaks has a significant influence in the survival model, suggesting fire-cycle heterogeneity throughout the landscape. However, the proportion of old-growth forests observed is in agreement with what would be expected assuming that fire hazard is independent of stand age. Oldgrowth stands could thus be incorporated into natural disturbance based management, although the great variability of the intervals between catastrophic disturbances should be carefully considered.Résumé : Les forêts anciennes constituent une proportion importante de la mosaïque forestière de la ceinture d'argile (Ontario et Québec, Canada) en dépit de cycles de feu que l'on présume relativement courts. Deux hypothèses ont été avancées pour expliquer ce phénomène : (1) Les forêts anciennes sont localisées sur des sites protégés des feux ou (2) le risque d'incendie y est équivalent et cette portion de la mosaïque ne constitue que la queue de la distribution, épargnée par le feu simplement par hasard. La dendrochronologie s'est avérée inadéquate pour dater le dernier feu dans ces forêts anciennes puisque le temps écoulé depuis le dernier feu est probablement supérieur à l'âge des plus vieux arbres. Une étude paléoécologique a donc été effectuée dans le but de dater le dernier feu dans ces forêts. Les horizons contenant du charbon de bois ont été localisés et datés au carbone 14 dans six forêts anciennes. Le fait que des fragments de charbon macroscopiques aient été retrouvés dans tous les sites indique que ces forêts ont bel et bien été affectées par les feux dans le passé. La proximité de coupe-feu potentiels accroît la durée du cycle des feux. Toutefois, la proportion de forêts anciennes observée indique que le risque d'incendie est indépendant du temps écoulé depuis le dernier feu. Les forêts anciennes pourraient donc être intégrées à des aménagements forestiers inspirés par la dynamique des perturbations naturelles. Toutefois, la grand...

Soil oxygen within boreal forests across an age gradient

“…Spruce trees were able to establish in the understory of the forest in the absence of fire, and the initial cohort died out, resulting in a maximum tree age that in some cases significantly underestimates time since fire. Therefore, the forests that we have dated as being approximately 250 yr since fire may in fact be closer to 1000 or even 2000 (Cyr et al 2005) yr since fire, and the process of paludification on the Clay Belt would proceed at a pace similar to that found in Siberia.…”

“…This rate differs dramatically from the approximately 800 yr that has been suggested in Siberia by Glebov and Kurzukhin (1992). However, recent C 14 dates of charcoal and organic forest floor immediately above the mineral horizon in the research area established by Cyr et al (2005) indicate that dendrochronology dates may substantially underestimate the age of the forest. Spruce trees were able to establish in the understory of the forest in the absence of fire, and the initial cohort died out, resulting in a maximum tree age that in some cases significantly underestimates time since fire.…”

Ecosystem management in paludified boreal forests: enhancing wood production, biodiversity, and carbon sequestration at the landscape level

“…The disturbance regime and forest dynamics of the Clay Belt determine the characteristics of the forest landscape that is the habitat for forest-dependent species (Harper et al 2003;Bergeron et al 2004;Cyr et al 2005). Forest-dependent species of bryophytes, lichens and invertebrates make up the most species rich groups across the landscape (Paradis and Work 2011;Bergeron and Fenton 2012).…”

Using knowledge of natural disturbances to support sustainable forest management in the northern Clay Belt

“…2) shows that the natural mosaic contains more than 50% of forests older than 100 years, and more than 15% of forests older than 200 years, meaning that over-mature and old-growth forests historically are a major part of the forested landscape in these regions. Moreover, Cyr et al (2005) report that large tracts of black spruce forests have not burnt for thousands of years in the Clay Belt region. Although trembling aspen or jack pine can establish themselves following fire (see Gauthier et al 2004 for detailed succession pathways), stand establishment following fire is often dominated by an initial cohort of spruce, which can give rise to a dense even-aged forest originating principally from seed (Fig.…”