It’s 7:30 am, and the morning air is crisp and damp as the Fire Regime field crew unloads their bags and the chainsaw from their truck. They a long way to hike across rugged terrain to get to their first plot, and they’ll have to move fast. Traveling off-trail takes time, and so does the process of painstakingly searching the mountain slopes and ridges for signs of ancient fires, captured in the rings of living and dead trees. Cameron Naficy leads the way, guiding his team through open pine forests as the gray jays observe their progress from the trees.
By the time they get back to the truck nearly twelve hours later, they will be tired, hungry, and carrying 45 new tree cores and 5–15 tree cross-sections to add to the pile. It’s a tough job, but they wouldn’t have it any other way.
All the kilometers traveled and samples collected were motivated by the question: What was the pre-industrial (i.e., pre-fire suppression) fire regime of the Landscapes in Motion study area? To answer this question, the team used tree cores and fire scars to document when trees grew and burned, and learn how fires shaped the forests of these dynamic landscapes.
Reconstructing historical fire regimes
In 2018 and 2019, the Fire Regime team collected thousands of tree cores and hundreds of cross-sections of fire-scarred trees across six sites in Alberta’s Foothills. These samples were later processed and crossdated using “master chronologies.” This process made it possible to learn each tree’s year of establishment, age, early growth rates, and for dead trees, the year of death. It also allowed them to assign an exact calendar year to each fire scar found in the cross-sections.
Map of the Landscapes in Motion study area, with the Fire Regime team’s plots represented by dots. Inset: map showing plot boundaries for one of the sites; red dots indicate plots where tree cores were collected and “x” indicates where fire scars were collected. Image courtesy of Cameron Naficy.
This landscape has been shaped by a mixed-severity fire regime with very frequent fire
The Fire Regime team used their on-the-ground data to reconstruct the fire history of the area. They found that fires were quite frequent within the fire-sensitive forest types of the study area, with median fire return intervals between 25–40 years for most cover types. Douglas-fir forests were the one cover type that had a notably frequent fires, with median return intervals ranging between 15–20 years between successive fires in a patch of forest.
This high historical fire frequency could have implications for how these forests are managed. Modern timber management practices are largely modeled after an ecosystem’s natural disturbance regime, and this approach has traditionally focused on stand-replacing disturbances with longer return intervals. If fires were more frequent and not always severe, this raises interesting questions about the ways in which these historical fire regimes could inform management approaches. It may also have implications for prescribed burning and fire suppression policies.
The study area was characterized by frequent fires, shown here as the percent of patches in which each fire year was documented. Regional fire years were defined as fires that were recorded in over 20% of patches in the study area, with fires that burned more than 50% of patches colored in red to highlight major and well-documented fire years. “Widespread fire years” were those recorded in more than 10–20% of patches. Overall, local fires (those recorded in less than 10% of patches) comprised 74% of all fire years.
Lodgepole pine and Douglas-fir zones show different mixed-severity fire regimes
Douglas-fir and lodgepole pine forests both had mixed-severity fire regimes, but these regimes—i.e., their fire frequency and severity—showed important differences.
Douglas-fir forests were characterised by very frequent, low- and moderate-severity fires, with occasional high-severity fires. In lodgepole pine forests, high-severity fires were most common but there were more low- and moderate-severity fires than expected.
The historical mixed-severity fire regimes of these two forest types are both more complex and included more frequent fire than previously assumed. While lodgepole pine forests do experience many high-severity fires, the heterogeneity of fire severities and burn patterns led to a greater degree of structural complexity than expected. Likewise, the mix of fire severities in Douglas-fir forests produced stands that are extremely variable and many that are complex, open, patchy, and often interspersed with grassy meadows or aspen.
Examples of forests that result from mixed-severity fire regimes. A) A complex lodgepole pine stand with trees of multiple sizes and ages. B) An older, more complex Douglas-fir forest with a sparsely-vegetated understory. Photos by Cameron Naficy.
The results of the Fire Regime team’s work (and that of the other teams) suggests that disturbance regimes in southwestern Alberta were more complex than previously assumed—fires were both more frequent and less severe, on average.
The team also found evidence suggesting that Indigenous burning played an important role in shaping the forests of southwest Alberta before these practices were formally suppressed through provincial policy.
These findings will have implications for forest management that strives to emulate the natural disturbance regimes of the region as our understanding of these regimes changes.
Learn more
How they collected data from tree rings and fire scars
Summary of the team’s final results
