On a day in early June, as the air temperature creeps toward 80 degrees, longtime DNR forestry technician Dan Hertle and his prescribed burn crew light the first fire lines in 13 acres of pine woods in north-central Minnesota, near Lake Winnibigoshish. Crew members wear hard hats and full-sleeved, flame-resistant clothing. They are sweating in no time. Shed pine needles from seasons past fuel the flames, which are slowed to a crawl by the season's fresh greenery. The sluggish speed of late spring and summer fires is an advantage for land managers.

Prescribed burns have typically been done early in spring or in fall, says Greg Hoch, prairie habitat team supervisor. But the DNR has been doing more burns from late spring into summer.

In the state's fire-dependent native landscapes, growing-season fire does a better job of reducing woody-stemmed plants than does dominant-season fire. In the prairies, flames keep trees at bay. Where Hertle works in Deer River, late-spring fire helps regenerate large stands of white and red pines. A century of fire suppression has made brushier forests with shady understories where pines struggle to get started. Forest regeneration through fire uses natural seed sources and increases plant diversity. In pine forests, a growing-season burn will check deciduous shrubs, often beaked hazel, a native plant that can dominate the understory in the absence of fire.

In spring, a hazel moves its energy stores from its roots to its branches to make leaves. The fire starves the hazel by burning its cambium—the nutrient-carrying cell layer between the bark and the wood—and preventing the shrub from replenishing its roots. It also exposes soil by reducing the needle litter to fertile ash.

Growing-season burns are conducted before the hazel leaves have fully emerged and created a microclimate too humid for fire.

After the burn, more sunlight hitting the ground results in what DNR forest ecologist John Almendinger calls "a burst of ground-layer plant diversity." A survey Almendinger conducted of the site Hertle burned shows that an area treated with fire had 80 plant species present, versus 50 on a neighboring site without fire. Species that often appear after a fire include yellow panic grass and the black morel fungus. Increased plant diversity means more resistance to invasive species and climate change.

A grant through the state's Legacy Amendment enabled growing-season burns of the pine woods near Lake Winnibigoshish with the goal of establishing a naturally regenerating, mixed-age pine stand. The DNR's Restoration Evaluations Program, which assesses the success of Legacy-funded projects, found that two years after the last burn, hazel growth remained patchy while white pine seedlings numbered 500 per acre. This density is adequate, Hertle says, adding that the site may naturally recruit more pines from future cone crops in areas where hazel is sparse.

In Minnesota's northwestern quarter, Jason Ekstein leads an eight-person DNR crew that uses summer fire to reduce woody plants in the state's largest remnant prairies. Summer fires are especially effective at combatting woody plants including willows, dogwoods, and aspens.

Ekstein conducts 5 to 10 burns during a typical summer. Last year, he was limited to one burn by poor weather and smoke from Canadian wildfires that created air quality advisories. The burn was a whopper, though: 7,000 acres were burned within a 12,000-acre area. Patchiness is a trademark of summer fire. Unburned fragments provide refuge and a more varied mosaic of habitats for wildlife.

Slow summer fires keep flames flickering longer against woody stems. This longer burn time coupled with increased air temperature means more thorough cambium girdling. By autumn, non-woody plants will have bounced back to create winter cover for prairie wildlife.

Another benefit from the extended burning season is that it gives the few crews operating around the state more time to restore and maintain landscapes by disrupting plant succession.

"Prairie communities evolved with disturbance that was drought, fire, or grazing," says Ekstein. "Variability is what gives the prairie its diversity."