Holocene variations of wildfire occurrence as a guide for sustainable management of the northeastern Canadian boreal forest

Abstract

Background: Cumulative impacts of wildfires and forest harvesting can cause shifts from closed-crown forest to open woodland in boreal ecosystems. To lower the probability of occurrence of such catastrophic regime shifts, forest logging must decrease when fire frequency increases, so that the combined disturbance rate does not exceed the Holocene maximum. Knowing how climate warming will affect fire regimes is thus crucial to sustainably manage the forest. This study aimed to provide a guide to determine sustainable forest harvesting levels, by reconstructing the Holocene fire history at the northern limit of commercial forestry in Quebec using charcoal particles preserved in lake sediments. Methods: Sediment cores were sampled from four lakes located close to the northern limit of commercial forestry in Quebec. The cores were sliced into consecutive 0.5 cm thick subsamples from which 1 cm 3 was extracted to count and measure charcoal particles larger than 150 microns. Age-depth models were obtained for each core based on accelerator mass spectroscopy (AMS) radiocarbon dates. Holocene fire histories were reconstructed by combining charcoal counts and age-depth models to obtain charcoal accumulation rates and, after statistical treatment, long-term trends in fire occurrence (expressed as number of fires per 1000 years). Results: Fire occurrence varied between the four studied sites, but fires generally occurred more often during warm and dry periods of the Holocene, especially during the Holocene Thermal Maximum (7000-3500 cal. BP), when fire occurrence was twice as high as at present. Conclusions: The current fire regime in the study area is still within the natural range of variability observed over the Holocene. However, climatic conditions comparable to the Holocene Thermal Maximum could be reached within the next few decades, thus substantially reducing the amount of wood available to the forest industry.

Fires of the Last Millennium Led to Landscapes Dominated by Early Successional Species in Québec’s Clay Belt Boreal Forest, Canada

“…It is estimated that this period corresponds to a dominance of Picea mariana and A. balsamea forests in the studied landscape, and perhaps in the majority of landscapes belonging to Québec's Clay Belt. The reconstructions of fire history from lake sediments in the western Picea mariana-moss domain also show a general decrease in fire frequency during this period, but with a certain variability among sites [10,28,31,34].…”

“…1000 BP for some lakes (Profond, Raynald, Richard and Nans; Figure S1) but low for others (Geais, Loutre, Twin, Aurélie). These differences could be associated with the short duration of the Medieval Warm Period, which caused a varied fire pattern in the western portion of Picea mariana-moss domain [28]. Differences in physical features probably do not explain this variability because lakes with high and low fire frequency are located both in peatlands (Profond, Raynald, Geais, Loutre) and till environments (Richard, Nans, Twin, Aurélie).…”

Major postglacial summer temperature changes in the central coniferous boreal forest of Quebec (Canada) inferred using chironomid assemblages

“…Radiocarbon dates [calibrated at 2 sigma ranges based on the Intcal13.14C data set (Reimer et al ., )] and chronology have already been published by El‐Guellab et al . (). A summary is presented in Table .…”

“…Annual and August precipitation averages are 961 and 105 mm, respectively (Environnement Canada, 2017). Further details about the study site characteristics can be found in El-Guellab et al (2015).…”

Coniferization of the mixed‐wood boreal forests under warm climate

“…The last 2000 years included both the warm Medieval Climate Anomaly (MCA; approximately 1000 ± 300 cal a BP), and the cold and wet Little Ice Age (LIA; approximately 300 ± 100 cal a BP) (Kerwin et al, 2004;Booth et al, 2006). The MCA was a rapid, intense climate change recorded by several studies in eastern Canada (Bajolle et al, 2018), in Greenland (Kobashi et al, 2010), and more largely in the Northern Hemisphere (PAGES 2k Consortium, 2013;Ljungqvist, 2010), and suspected to have increased the fire activity for a short duration in some boreal forest regions (Kelly et al, 2013;El-Guellab et al, 2015). These more frequent fires were not recorded in our fire reconstructions, but were observed at other sites in north-eastern Canada (Ali et al, 2012;El-Guellab et al, 2015;Remy et al, 2017).…”