Fire Ecology and Effects

This photo of fire-killed conifers in the background, with some living conifers and an aspen stand in the foreground illustrates the patchy nature of fire - where some areas burn and other areas inside the fire footprint remain unburned.

 

Fire is an agent of change in forests, shrublands, grasslands, and other ecosystems. Every year, thousands of lightning strikes occur in and near Lassen, and when it is dry and warm, and there is enough fuel to burn, fires may ignite. When conditions are safe, a lightning fire may be sometimes be allowed to burn to benefit forest health.

Fire Ecology – the study of the interactions between fire and living organisms and their environment – informs us about how fire burns in different types of vegetation, and how plants and other organisms respond to fire. In the southern Cascades and the Sierra Nevada mountains, fires have occurred for thousands of years, and are as much a part of this ecosystem as are wind, rain, erosion, and other natural processes. The native plants and animals are well adapted to fire, and some plants will not regenerate well without it.

 

Two fire effects monitoring staff record post-fire fuels and vegetation data in a monitoring plot.

How Do We Learn about Fire Effects?

A National Park Service Fire Monitoring Program began in the 1980s. This program uses long-term monitoring locations or plots, to track how trees and shrubs respond to lightning-caused fires and prescribed burns ̶ how many individuals of each species are present before and after fire. They also measure the amount of fuel (dead branches, logs, pine needles, and other materials on the ground that can burn). During the fire, scientists measure current weather conditions, how much moisture is in the woody fuels, and how the fire is burning. The plots they install are re-measured over time, and provide information to managers about how fires affect woody plants, if prescribed burn objectives have been achieved, and when an area may need to be burned again.

In addition to NPS monitoring programs, we also rely on the research of scientists from other organizations who conduct studies to more fully understand the role of fire in these ecosystems, especially as climate and fire patterns are rapidly changing. This research also provides information about fire effects on plants, animals, soils, water, and larger landscape patterns.

 

How Do Fire Regimes Influence Fire Effects?

As all forests are not alike, neither are the types of fire that burn in them. Fire managers need to know about the characteristics of a fire regime to effectively manage and restore fire. A fire regime is the naturally-occurring pattern of fire in a particular vegetation type or specific location. Fire regime characteristics include:

• Fire return interval (how often fire burns)
• Season (when it burns)
• Fire severity (level of damage to plants and soil affected by fire)
• Fire intensity (the quantity of heat produced)
• Fire size and pattern (large areas burned, or small patches)
• Fire type (burning on ground surface below trees, or as crown fires that burn from tree to tree in branches and treetops)

Historically, Lassen’s fire regime was classified as “mixed severity” and fires occurred an average of every 5-15 years, depending on elevation and forest type. Lower elevation forests had more frequent fire than higher elevation forests because they are warmer and drier and typically have more flammable fuels. Mixed severity means there are many small patches of burned area and a few large patches. Some patches are high severity – killing most vegetation – while others are unburned, low, or moderate severity. This mix creates what fire managers call a mosaic, almost like a quilt pattern of different fire effects. These diverse patches help make a landscape more resilient to drought and future wildfires. However, in the past decade, due to dense forests from lack of fire, and a drier, warmer climate, fires are burning larger areas at high severity.

 

Aerial view of an area burned in the 2012 Reading Fire in the park. Patches that are black or have trees with brown needles burned at higher severity, while areas that still look green may have had fire only under the tree canopies, or are unburned.

Calvin Farris / National Park Service

 

Aspen seedlings re-sprout in burned area.

Fire Effects on Plants

Fire creates opportunities for plant growth by burning logs, sticks, and tree needles on the forest floor and releasing nutrients into the soil; fire also opens up gaps in the forests, creating more light for tree seedlings, shrubs, and other plants that may not thrive under a shady forest. For trees and shrubs that survive the fire and are not injured, growth may increase from the extra nutrients and light and the reduced competition from plants that died in the fire.

Plants have different adaptations to fire. Some types of trees have thick bark to protect them from fire’s heat, or may have their branches high off the ground, out of reach of the fire. Other plants may die in the fire, but have seeds in the soil or deposit them from cones during or after the fire; many seedlings must establish for a few to survive to adulthood! Other plants may re-sprout from deep roots or the base of their stems. Aspen trees typically sprout back from their network of lateral roots.

 

Manzanita shrubs grow back from seed on this burned slope.

Some species of shrubs, like common manzanita, will germinate readily after fire from a soil seed bank, with seeds stimulated to grow by the fire’s heat. Many herbaceous plants may also grow from seeds in the soil, or sometimes re-sprout from deep roots. Conifer trees store seeds in cones, and some species like lodgepole pine have increased seed release after fire. Most pines need open, sunny areas for their seedlings to take root and grow. In forests lacking fire for many decades, shade-tolerant fir trees become more common and may gradually replace pines, which need open, sunny conditions.

 

Female black-backed woodpecker on a pine probing for insects under the bark. This type of woodpecker is attracted to recently burned forests where dead or injured trees host many insects they feed on.

Mike Laycock, National Park Service. Flickr (CC BY-NC 2.0),

Fire Effects on Animals

Like native plants, most animals in the park have evolved with fire. Many large mammals like deer and bears may be able to move out of the way or flee a fire. Most small animals such as rodents, amphibians, and reptiles avoid fire by seeking refuge in tunnels or burrows, under large downed logs, or in damp areas such as meadows or streams. But many animals may die in a high-severity fire. Later, wildlife populations re-establish, in parallel with post-fire growth of vegetation.

The burned, dead, and decaying trees, whether fallen as logs or standing upright as snags, are important habitat for insects and various mammals and birds. Birds such as black-backed and Lewis’s woodpeckers, are attracted to burned forests, and will probe burned trees to feed on insects and excavate nest cavities. Other cavity-users that depend on woodpeckers to carve out a cavity may follow – tree squirrels, chickadees, swallows, bluebirds, and kestrels are all examples.

 

Mule deer returns to burned forest. Browsers like deer may need to feed in unburned or lightly burned areas initially, but as shrubs re-grow, deer return to take advantage of the high-quality food source.

Grazing animals (rodents, ground squirrels, rabbits) and browsers (such as deer) may find more vegetation to feed on as shrubs and other plants thrive in areas made more favorable to plant growth with increased sun and soil nutrients. Even spotted owls, a species favoring old-growth forest, may benefit from more open burned patches where they can hunt for rodents, as long as enough old growth forest remains for them to roost and nest in. While some animals die in a fire, or lose part of a forest they were living in, others will benefit in the longer term from diverse habitats and new growth. More research is needed to better understand the impacts of large, high-severity fires on wildlife.