Fire in the boreal forest: how black spruce burns and regrows

The boreal forest, also called taiga, is the world's largest forested habitat, about one third of its forested area. In North America it runs from Alaska across Canada into the Great Lakes region of the United States. A National Park Service page on fire in ecosystems explains why fire is a natural part of it, and what is changing.

A wildfire burning in lowland black spruce in Denali National Park and Preserve. NPS/Y Matsui.

What it is like

Winters are long and cold and summers short, with 50 to 100 frost-free days. Typical trees are black and white spruce, with tamarack, jack pine and balsam fir; birch, aspen and poplars are the deciduous trees. The harsh conditions mean relatively few plant species. Soils are often acidic and cold, so a mature boreal forest usually has a thick mat of moss, decaying moss and litter on the ground.

Fire's historic role

The boreal forest is adapted to fire, which makes a mosaic of young and old trees. In central Alaska the spruce forest usually burns every 60 to 150 years. Fire clears the thick floor mat of old forests so that new young forests can begin. Summers can be warm and lightning commonly starts fires.

Black spruce forests are highly flammable, almost designed to burn. Fire carries well through feather mosses, lichens and heather-family shrubs, and low branches that drape into the moss give the fire a ladder to the crown, where the seed cones are. Thin bark and shallow roots mean low or high intensity fires easily kill the trees. Yet black spruce is adapted too: its tight cluster of seed cones at the top is semi-serotinous, opening in a fire's heat to release seeds, and seedlings usually establish within 5 to 10 years afterward.

What comes back

• Birch and aspen return quickly, sprouting from roots below ground or from windblown seed.
• Willow, blueberry, Labrador tea and shrub birch re-sprout if the fire only lightly burns the soil; after a deep burn that kills roots, they depend more on seed.
• Non-woody plants such as fireweed and grasses become more plentiful soon after.
• Pioneer mosses and liverworts start to grow where the ground cover burned, and feather mosses and lichens return as the spruce canopy grows back.

One year after a fire in a black spruce forest in Yukon-Charley Rivers National Preserve, grasses and willows have sprouted where the fire did not burn deep into the soil; fourteen years after, willow and birch are nearly head high and young spruce seedlings below the photo board will take longer to mature. NPS.

Young forests feed moose, which browse the deciduous shrubs and trees, and voles and other small animals that eat new green plants and in turn feed owls, foxes and martens. Mature forests matter for caribou and many birds.

Now

Boreal forests have burned naturally for thousands of years. Fire managers protect people, communities and certain areas, and in some remote areas fires are managed for their natural benefit. Temperatures are warmer and summers longer, and the page says the area burning in Alaska and Canada has increased over the past 15 years. Some fires are burning in recently burned areas, shortening the time between fires there. Such changes can alter which plants and animals live in a place, thaw permafrost and release carbon stored in soil, and scientists continue to study them. Park Service fire managers use long-term observations of plants and fire to make decisions meant to protect resources and keep ecosystems in balance.

Sources

• National Park Service, "Fire in Ecosystems: Boreal Forest" (https://www.nps.gov/articles/000/fire-in-ecosystems-boreal-forest.htm), public domain as a work of the U.S. government. A map credited to NPS with data from The Nature Conservancy and Esri is not used.
• National Park Service; rewritten in hubnx's own words.