A Disrupted Historical Fire Regime in Central British Columbia

Abstract

In the 2017 and 2018, 2.55 million hectares burned across British Columbia, Canada, including unanticipated large and high-severity fires in many dry forests. To transform forest and fire management to achieve resilience to future megafires requires improved understanding historical fire frequency, severity, and spatial patterns. Our dendroecological reconstructions of 35 plots in a 161-hectare study area in a dry Douglas-fir forest revealed historical fires that burned at a wide range of frequencies and severities at both the plot- and study-area scales. The 23 fires between 1619 and 1943 burned at intervals of 10–30 years, primarily at low- to moderate-severity that scarred trees but generated few cohorts. In contrast, current fire-free intervals of 70–180 years exceed historical maximum intervals. Of the six widespread fires from 1790 to 1905, the 1863 fire affected 86% of plots and was moderate in severity with patches of higher severity that generated cohorts at fine scales only. These results indicate the severity of fires varied at fine spatial scales, and offer little support for the common assertion that periodic, high-severity, stand-initiating events were a component of the mixed-severity fire regime in these forest types. Many studies consider fires in the late 1800s relatively severe because they generated new cohorts of trees, and thus, emphasize the importance of high-severity fires in a mixed-severity fire regime. In our study area, the most widespread and severe fire was not a stand-initiating fire. Rather, the post-1863 cohorts persisted due disruption of the fire regime in the twentieth century when land-use shifted from Indigenous fire stewardship and early European settler fires to fire exclusion and suppression. In absence of low- to moderate-severity fires, contemporary forests are dense with closed canopies that are vulnerable to high-severity fire. Future management should reduce forest densities and to restore stand- and landscape-level heterogeneity and increase forest resilience. The timing and size of repeat treatments such as thinning of subcanopy trees and prescribed burning, including Indigenous fire stewardship, can be guided by our refined understanding of the mixed-severity fire regime that was historically dominated by low- to moderate-severity fires in this dry forest ecosystem.

The contribution of Indigenous stewardship to an historical mixed‐severity fire regime in British Columbia, Canada

“…This MSFR had a high proportion of low-severity fires indicated by fire scars that burned at local to widespread (for this study site) spatial scales, punctuated by patches of higher-severity fire indicated by cohorts at the local (plot) scale only (Harvey et al, 2017;Heyerdahl et al, 2012). At the site level, the mean fire return interval between 1628 and 1982 at Ne Sextsine (21 years) is consistent with mean intervals in other MSFRs in dry forest ecosystems in BC (Brookes et al, 2021;Daniels & Watson, 2003;Harvey et al, 2017;Heyerdahl et al, 2012;Marcoux et al, 2013). Compared to MSFRs in higher-elevation montane forests in BC (Chavardès & Daniels, 2016;Marcoux et al, 2013), the fire regime at Ne Sextsine contained little evidence of spatially extensive high-severity fire events.…”

“…Low-severity fires were also a feature of the mixed-severity fire plots, but the cohorts (both fireinitiated and unattributed) survived one to 11 subsequent fires that must have been of low enough intensity to not kill vulnerable young trees. This interpretation is supported by a clear reduction in fire frequency, especially in the low-severity plots, starting in the 1870s and strong evidence of forest encroachment and increases in density in dry forest ecosystems in the absence of frequent fire in BC (Brookes et al, 2021;Harvey et al, 2017;Turner & Krannitz, 2001) and similar frequent-fire ecosystems in North America (Hagmann et al, 2021;Hessburg et al, 2016;Larson et al, 2020).…”

Cumulative disturbance converts regional forests into a substantial carbon source

“…deciduous species) and risk of MPB (e.g. diverse species planting at the site and landscape level) [62]. Other interventions such as prescribed burning and commercial thinning should also be considered part of the forest management toolbox to reduce the risk of carbon emissions from natural disturbances [60,63].…”

Diffusion of indigenous fire management and carbon-credit programs: Opportunities and challenges for “scaling-up” to temperate ecosystems

“…However, colonial fire exclusion through the forced relocation of Indigenous peoples onto reserves, and the prohibition of Indigenous burning, fundamentally altered the historical fire regimes and forests (Greene, 2021;Copes-Gerbitz et al, 2022). Several fire history reconstructions in central BC have documented the absence of low-to moderate-severity fires following colonization in the mid-to late-1800s, which altered forest structure and composition, leaving the landscape vulnerable to high severity fires (Harvey et al, 2017;Brookes et al, 2021;Copes-Gerbitz et al, 2022). Hence, revitalizing Indigenous fire practices offers an important landscape stewardship tool missing for almost a century .…”