What if a forest grew back after a fire?

Imagine a forest where a fire has passed through. The tree trunks are dark and the ground is bare.

Is the forest gone forever? Or can it grow back? Let's find out!

In 1988, fires spread across about 800,000 acres of Yellowstone National Park, around 36 percent of it. Afterward, many trunks stood dark and bare, and the Park Service says some visitors still feel the Yellowstone they knew is gone.

But the National Park Service describes what happened next: new, young trees came up from the burned ground, and today they are forests again. How does a forest grow back? This lab is a thought experiment. Follow three paths: the first plants, the cones that need fire, and the animals that come back.

The 1988 fires burned a mosaic of about 1.4 million acres in the Greater Yellowstone Ecosystem, including roughly 36 percent of the park, after extremely warm, dry and windy weather (NPS). The Park Service describes Yellowstone as a functioning ecosystem in which fire plays a vital role.

This thought experiment follows recovery after a stand-replacing fire, step by step. Which plants arrive first, what lets lodgepole pine reseed itself, and how do animals use the changed landscape? The steps are drawn from what the Park Service has observed; the exact path at any one site would vary.

Some forests have fires every hundred years or more. The plants and animals that live there are used to it.

Some trees even keep their seeds safe until a fire comes!

In Greater Yellowstone, the National Park Service says, lodgepole pine and mixed conifer forests tend to have big fires every 100 to 300 years. The plants and animals that live there have adapted to that pattern. In Grand Teton National Park's words, if they weren't fire resilient, they wouldn't live there.

Lodgepole pines have thin bark, and the Park Service says they die easily when a fire passes through. But they have a clever way to start the next forest, sealed inside their cones.

Every forest type has a fire regime, its normal pattern of fire recurrence. Ponderosa pine forests typically see frequent, low-intensity fires that clear undergrowth while sparing large trees; Greater Yellowstone's lodgepole and mixed conifer forests burn on a much longer interval, every 100 to 300 years, in large fires that may kill many mature trees (NPS, Grand Teton).

An NPS article puts stand-replacing fires in lodgepole at every 150 to 400 years, but as often as every 15 years when conditions are dry and windy enough, and notes that a warming climate is making fires in lodgepole more frequent and larger. Lodgepoles' thin bark gives little protection; their adaptation is in their seeds.

After the fire, grass and flowers grow back from roots under the ground.

Pine cones pop open and drop seeds. Tiny trees sprout in the sunshine. Year by year, the forest grows back!

Recovery starts fast. The Park Service says grasses and wildflowers often survive underground, because their roots are unharmed, and regrowth can begin within days once there is water. The fire releases nutrients from the burned wood and leaves, so for a few years these plants often grow even better.

Meanwhile, heat has opened the lodgepole pines' cones, scattering seeds onto bare, sunny ground. Aspen trees send up new shoots from their roots. Year after year the young trees grow, and decades later they are forests again.

Above-ground parts of grasses and forbs burn, but their root systems typically survive, and for a few years productivity often rises because fire rapidly releases nutrients from wood and litter; regrowth begins as soon as moisture allows, sometimes within days (NPS). Aspen stems may die, but their connected root networks survive even very hot fires, and fire stimulates root suckers and bare mineral soil for seedlings.

Lodgepole seedlings come up among surviving trees and standing and fallen snags, and many dead trees remain upright for more than a decade. Recovery varies: some soils respond quickly with nearly complete cover, while others continue to support little vegetation. The Park Service notes that the young trees that grew after 1988 are now renewed forests.

Some lodgepole pine cones are sealed shut with sticky stuff.

Only the heat of a fire melts it. Then the cone opens and the seeds fall onto the sunny ground.

Lodgepole pines make two kinds of cones. One kind is serotinous: it is sealed tight with resin, a sticky substance. The Park Service says the resin only melts at 113 °F (45 °C) or hotter, so the seeds can stay stored in the cone for years.

When a fire passes through, the resin melts and the cones open. The seeds land just when conditions suit them best: the forest floor has been cleared and sunlight pours through the open canopy. Lodgepole seedlings love direct sun.

Lodgepole pine produces serotinous and non-serotinous cones. Serotinous cones stay sealed with resin that melts only at 113 °F (45 °C) or above, so seeds can be stored for years and released when fire has reduced litter on the forest floor and opened the canopy to sunlight, conditions that favor lodgepole seedlings (NPS).

This timing is an adaptation: thin-barked adults die easily in fire, but their stored seeds arrive on a freshly cleared seedbed at the moment of least competition. Other species use different routes: whitebark pine seeds cached by red squirrels and Clark's nutcrackers give it a head start on exposed burned sites, and aspen resprouts from roots.

Animals come back to the new forest. Bears munch on fresh green plants.

Birds called flickers find ants to eat. Bluebirds make nests in old tree trunks.

What about the animals? The Park Service says wildfires do not greatly change the numbers of most wildlife in Yellowstone. After 1988, elk, bison and deer populations soon rebounded. Bears have often been seen grazing in burned areas once plants start to return.

Birds find new homes too. Bluebirds, flickers and ducks called Barrow's goldeneyes nest in holes in trees, and a burned forest has many standing dead trees for nests. Robins and flickers find ants and worms more easily. A burned forest is not an empty place: it is a young one.

The Park Service reports that wildfires do not significantly affect the abundance of most wildlife species in Yellowstone. After 1988, elk, bison and deer populations soon rebounded, the fires had no discernible effect on the number of grizzly bears in the Greater Yellowstone Ecosystem, and bears are frequently seen grazing in burned areas as vegetation returns. Grand Teton notes that many birds rely on insects that come to newly burned forests.

Cavity nesters such as Barrow's goldeneye, flickers and bluebirds gain nest sites in dead trees, and robins and flickers find ants and worms more easily; species that need mature forest, such as boreal owls, may lose habitat. Fire creates a landscape more diverse in age, which the NPS says reduces the chance of disease or fire spreading through large areas.