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.

Invasive insects have been shown to facilitate secondary invasive species by altering forest structure and function. Specifically, invasive insect herbivores may promote the establishment and growth of invasive plants by creating canopy gaps. Such secondary invasions may be influenced by ecological integrity—the degree to which ecosystem composition, structure, and function deviate from their natural or historical range of variation. Here we investigate 1) whether emerald ash borer (Agrilus planipennis; EAB) induced ash (Fraxinus spp.) mortality facilitates European buckthorn (Rhamnus cathartica)—an invasive shade-tolerant shrub; 2) the role of ecological integrity in this relationship. We use a principal component analysis (PCA) to calculate an index of ecological integrity and a zero-altered negative binomial generalized linear mixed model (GLMM) to describe European buckthorn occurrence and abundance. European buckthorn occurrence is influenced by canopy gaps, independent of EAB-induced gap formation. Ecological integrity and EAB-induced ash mortality interact to control European buckthorn abundance, with high ecological integrity limiting EAB-facilitated buckthorn invasion. This is the first evidence for EAB-facilitated buckthorn invasion and for an interaction between a secondary invasion and ecological integrity. Thus, ecological integrity plays an important role in the EAB-buckthorn system and may be used to manage the impacts of secondary invasions.

ContextIn fire-excluded forests across western North America, recent intense wildfire seasons starkly contrast with fire regimes of the past. The last 100 years mark a transition between pre-colonial and modern era fire regimes, providing crucial context for understanding future wildfire behavior. ObjectivesUsing the greatest time depth of digitized fire events in Canada, we identify distinct phases of wildfire regimes from 1919 to 2019 by evaluating changes in mapped fire perimeters (>20-ha) across the East Kootenay region (including the southern Rocky Mountain Trench), British Columbia. MethodsWe detect transitions in annual number of fires, burned area, and fire size; explore the role of lightning-and human-caused fires in driving these transitions; and quantify departures from historical fire frequency at the regional level. ResultsRelative to historical fire frequency, fire exclusion has created a significant fire deficit in active fire regimes, with a minimum of 1-10 fires missed across 46.4-percent of the landscape. Fire was active from 1919 to 1939 with frequent and large fire events, but the regime was already altered by a century of colonization. Fire activity decreased in 1940, coinciding with effective fire suppression influenced by a mild climatic period. In 2003, the combined effects of fire exclusion and accelerated climate change fueled a shift in fire regimes of various forest types, with increases in area burned and mean fire size driven by lightning. ConclusionsThe extent of fire regime disruption warrants significant management and policy attention to alter the current trajectory and facilitate better co-existence with wildfire throughout this century.

The past 100 years marks a transition between pre-colonial and modern era fire regimes, which provides crucial context for understanding future wildfire behavior. Using the greatest depth of digitized fire events in Canada, we identify distinct phases of wildfire regimes from 1919 to 2019 by evaluating changes in mapped fire perimeters (>20-ha) across the East Kootenay forest region (including the southern Rocky Mountain Trench), British Columbia (BC). We detect transitions in annual number of fires, burned area, and fire size; explore the roles of lightning- and human-caused fires in driving these transitions; and quantify departures from historical fire frequency at the regional level. We found that, relative to historical fire frequency, fire exclusion created a significant fire deficit across 89% of the flammable landscape. Fire was active from 1919 to 1940 with frequent and large fire events, but the regime was already altered by a century of colonization. Fire activity decreased after 1940, coinciding with effective fire suppression influenced by a mild climatic period. After 2003, the combined effects of fire exclusion and accelerated climate change fueled a shift in fire regimes of various forest types, with increases in area burned and mean fire size driven by lightning.