Fire Ecology on Isle Royale: Trees, Pests, Moose and Weather

Fire can destroy, but to nature it is also a force for necessary change. Its role in an ecosystem is called fire ecology. On Isle Royale, some species have adapted to depend on fire, while other species and several local factors can dampen or intensify it.

Two forests

The island holds two distinct forest biomes:

Common fire-dependent species on the island include bearberry, blueberry, fireweed, jack pine and black spruce.

Some trees burn more readily than others. NPS.

Trees

• Balsam fir, extremely common on the island, is very flammable: dry needles, shallow roots, thin resinous bark and a dense understory of low dead branches.
• Jack pine needs heat to reproduce. Its cones are serotinous: they open and release seed only at a certain temperature, reached through direct sunlight or fire.
• Black spruce, with semi-serotinous cones, is a "torching" tree: long dry limbs sweep the duff-covered floor and carry flame up to the crown. On Isle Royale it is sparse and grows mostly in swamps, so it seldom burns except in prolonged drought.
• Other trees have evolved defenses such as thick bark.

Spruce budworm is a pest, but a native one. NPS.

Forest health

Spruce budworm, a native pest, feeds on spruce and balsam fir branches, stripping the trees and sometimes killing them. Dead and damaged trees raise fire risk and make fire effects more intense.

• Cycles: outbreaks come in waves; on the surrounding mainland they recur every 25–40 years. On the island the interval is unknown.
• Past fires: budworm was a factor in the 1936 Fire, the 2021 Horne Fire and the 2022 Mount Franklin Fire.
• Other pests: spruce coneworm, large aspen tortrix, white satin moth, saddled prominent caterpillar and Bruce spanworm.

Moose

A large moose population reshapes the vegetation, and with it the fire.

• Yew: moose favor American yew and overbrowse it. Yew is highly flammable, so less yew means less chance of a stand-replacing crown fire. Thimbleberry, which follows it, is disliked by moose and seldom burns except in prolonged drought.
• After a fire: balsam fir and aspen, staples of the moose diet, are among the first to colonize burned ground, so moose do well in young, recently disturbed forest.

Other animals

• Large mammals can easily avoid a fire of any size; small mammals may die in an intense, fast fire, but populations recover quickly.
• Nesting birds are more likely to be disturbed than killed.
• Bald eagles: active nests are mapped and monitored every year, and fire crews defend them as far as possible.
• Wolf pups can't move far while denning, but that is in spring, when fires are unlikely; by peak fire season they are usually mobile.

What drives fire here

• Climate: Lake Superior keeps shores cool and humid. Relative humidity usually recovers to high levels at night, less so on ridge tops — a marine inversion that leaves inland heights warmer and drier. High overnight humidity works against fire.
• Weather: fires are most common in July and August, the hottest months, and more likely in drought and lightning storms.
• Topography: the island has little complex terrain, so fires are mainly wind-driven — usually slow surface fires with occasional torching when winds stay below 15 mph.
• Natural firebreaks: Lake Superior, inland lakes, wetlands and rock outcrops.
• Soil: thin mineral soils under thick organic layers, where fire can creep unseen, look extinguished, and flare up again when conditions allow.

More: fire on Isle Royale — fire management, fire history, fire towers and a glossary.

Sources

Based on "Fire Ecology on Isle Royale," Isle Royale National Park, National Park Service, including the page's tree flammability table; a work of the United States government in the public domain.