ISABELLA LAKE— You can see forest succession in action here in the aftermath of the Pagami Creek fire, which reset the forest’s clock across more than 90,000 acres back in 2011.
While the Pagami burn nuked most everything in its path, Isabella Lake was the largest lake it encountered during its historic Sept. 12 run and it served as an effective fire line, protecting its southeast shore and areas of forest on its leeward side.
The contrast is stark, as I discovered during a day trip into the area with my son Max and his friend Eva. I’d hiked portions of the Powwow Trail at least twice since the fire, but this was the first time I’d explored the area by canoe.
I remember hiking the Powwow Trail from the Isabella Lake trailhead about two weeks after the burn and it was a blackened moonscape for as far as the eye could see. But just to the east along Isabella Lake’s south shore, where the fire never reached, we canoed past mature second-growth forest that the lake’s broad expanse had protected from the burn. It’s a mix of aspen, birch, spruce, and jack pine, with large patches of dead balsam fir from the ongoing budworm infestation— primed fuel for the next blaze.
The large red and white pine had been cut from this area beginning in the 1940s into the 1960s, back when Forest Center was a thriving logging community, once home to as many as 250 people who worked for the Tomahawk Lumber Company.
The contrast between the burn and unburned was especially stark on Boga Lake, a long narrow body of water that extends southwest to northeast just to the southeast of Isabella Lake. The southwest half was protected by Isabella Lake’s “fire shadow,” while the northeast half was consumed by fire and the forest is in recovery. Nearly 14 years after the fire, the forest is comprised of two different types, dense stands of young jack pine or mixed stands of young aspen and birch.
Here in the Northwoods, these are the sun-loving pioneers, all well adapted to respond quickly to land opened up by disturbance. Jack pine, in particular, is fire dependent. It’s cones remain tightly closed, often for decades, each tree storing up enormous amounts of seed as it matures. Among our pines, the jack is the shortest-lived (typically surviving no more than 150 years) and it tends to grow on rocky ground with shallower soils— in other words the places most prone to fire. When fire occurs, the intense heat unlocks the cones, which are bound up in resin until heated. While the parent trees almost certainly die in the blaze, they end up releasing vast amounts of seed, each equipped with a feathery wing, not unlike a small maple seed, that helps disperse the seeds in the days and weeks following fire. Those seeds fall on an ashy, sun-drenched ground that’s primed for their quick germination. The young seedlings all fight for the light and grow straight up, often in stands so dense it’s difficult to walk between them. Just a few scattered mature jack pine in an area are enough to create a new jack pine forest with thousands of young trees. Over the decades, most will die, leaving a few of the most successful to repopulate the area once again in the wake of the next fire.
Without fire, jack pine will mostly disappear as a component of a Northwoods forest stand, as other, more shade tolerant species— like balsam fir, white spruce, white pine, and red maple— grow up and shade the understory.
The aspen and birch, both of which are intolerant of shading, have a different approach, which is less dependent on fire, but just as quick to respond to the abundant light brought on in the wake of fire. Aspen can respond most quickly from its roots where the fire didn’t burn deeply into the soil. In those cases, the young aspen can dominate, growing several feet the first year after a fire, juiced by a mature root system that’s already waiting and the flush of nutrients released by the fire. These young sprouts are actually clones of the parent, which can sprout hundreds of genetically identical stems in the wake of a fire. Normally, aspen sprouting is kept in check by the production of a hormone, called auxin, in the growing tips of aspen branches. In the wake of fire, those branches are killed, which prompts the surviving roots to sprout as many stems as possible.
Where aspen didn’t grow before, their tiny seeds, embedded in gobs of fluff, can float for miles in the breeze and quickly seed a recent burn, offering new territory for aspen to colonize.
Paper birch have a somewhat similar approach in the wake of fire, relying on quick resprouting to restore themselves to a stand where they existed before. They also produce large amounts of seed, with wings that aid their dispersal.
While Pagami was a destructive fire, it fulfilled a natural function of the Northwoods ecosystem, which is highly dependent on fire to sustain the diversity of forest stands typically found here.
These pioneer species are particularly dependent on fire to sustain themselves as part of the forest ecosystem. These are short-lived, sun-loving species, so without fire they are eventually replaced by the shade tolerant species that can grow up in the understory and sustain themselves indefinitely.