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

Ahmed El-Guellab,Sylvie Gauthier,Adam A Ali,Yves Bergeron,Hugo Asselin

doi:10.1186/s40663-015-0039-2

Abstract

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 cm3 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.

Highlights

Cumulative impacts of wildfires and forest harvesting can cause shifts from closed-crown forest to open woodland in boreal ecosystems
Forestry operations have expanded in a south-north fashion over the last few decades (Bouchard and Pothier 2011; Kivinen et al 2012), and the cumulative impacts of natural disturbances and forest harvesting may affect the ecological integrity of boreal ecosystems
In a comparative study of Holocene fire regimes along a transect crossing the limit of commercial forestry, Oris et al (2014b) noted that all regions were characterized by considerably higher fire occurrences during the Holocene Thermal Maximum (HTM; ca. 7000– 3000 cal. years BP), followed by a sharp decrease in fire occurrence at the beginning of the Neoglacial period in forests located close to and north of the limit, compared to a more gradual decrease south of the limit

Summary

Introduction

Cumulative impacts of wildfires and forest harvesting can cause shifts from closed-crown forest to open woodland in boreal ecosystems. In Quebec (eastern Canada), government authorities have established a northern limit to commercial forestry operations in the boreal forest (Jobidon et al 2015). North of this limit, forest productivity is deemed too low, and fire frequency too high, to support a profitable forest industry. They concluded that fire occurrence was more sensitive to climate change close to and north of the limit than further south These regional patterns, hide important between-site variability (see Figure S2 in Oris et al (2014b)), and additional studies are needed to verify if the same regional fire history can be reconstructed from other sites

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