Fire Effects

Patches of green aspen and conifers blend with burned trees to create a mosaic of fire effects following the 2012 Reading Fire.

Fire has affected wildlands in the southern Cascades and Sierra Nevada mountains for thousands of years. Historically, lightning-ignited fires resulted in mixed-severity effects on small sections of the Lassen landscape. Today, the cumulative effects of fire suppression and climate change are fueling larger and hotter wildfires resulting in more high-severity effects. Fire ecology research within the park and Lassen region help us to understand historic and present-day fire regimes and anticipate the effects of fire into the future.

 

A mosaic of burned and unburned areas on Raker Peak.

Fire Regimes and Effects

Pre-suppression fire regimes in the Lassen region were classified as mixed-severity with fires occurring an average of every 5 to 15 years. The introduction of wildfire suppression activities in 1904 resulted in significant changes to fire regimes within the park and region. These shifts in fire regimes resulted in significantly longer fire cycles, increased forest density, and changes in types of trees.

Historic Period

1656-1904 (Pre-Suppression)

Prior to 1904, lower elevation forests naturally had more frequent fire than higher elevation forests because they are warmer and drier and typically have more flammable fuels. Fires generally resulted in mixed-severity effects on vegetation. This creates a mosaic, or quilt-like pattern, of low-, moderate-, and high-severity, and unaffected patches. These diverse patches help make a landscape more resilient to drought and future wildfires.

1905-1980s (Suppression)

Fire suppression activities initiated in 1904 resulted in a dramatic decline in fire frequency. During this time, land managers actively suppressed most lightning-ignited fires. Over time, this resulted in denser forests with larger numbers of younger trees (which previously killed by fire) and changing proportions of each species type.

Current Period

1980s-2000s

In the 1980s, Lassen Volcanic began to limit its suppression efforts of lightning-ignited fire within its newly established Wilderness area. In the decades that followed, Lassen Fire Management used minimal intervention to allow the return of natural fire regimes in Lassen Volcanic Wilderness. These lightning-ignited and prescribed fires ranged from 18 to 3,500 acres in size and resulted in mixed-severity effects.

2010->

In the past decade, the park and Lassen region have seen dramatic increases in the size and severity of wildfire. The cumulative effects of fire suppression and climate change now fuel larger and hotter wildfires. The 2012 Reading Fire (16,098 park acres) and 2021 Dixie Fire (73,240 park acres) each far exceeded the size of previous wildfires. More than half of the Dixie Fire footprint in the park experienced low- to moderate-severity effects due in part to fire management in the previous decades. However, one-third of the affected park area experienced high-severity effects that are concentrated within the southeast area. Since historic fire regimes did not reach this magnitude of size or severity, Lassen Volcanic and its partners within the Lassen region are using scenario planning to consider implications for the future.

 

What is a Fire Regime?

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)

 

The initial 2021 Dixie Fire vegetation burn severity map for the park shows concentrated high-severity effects along the southern boundary, where high winds pushed the fire into the park.

What is Burn Severity?

The National Park Service uses vegetation burn severity to measure the effect on vegetation resulting from fire, specifically mortality and biomass consumption. One measure of burn severity is called the Composite Burn Index (CBI) and is based on the combined effects to the understory vegetation (grass, shrub layers), midstory trees, and overstory trees. CBI measures the loss of basal area (number of live trees) and changes in canopy cover (green layer of branches and crowns of plants or trees).

An initial burn severity assessment helps managers understand the intensity of the fire (the actual temperature and heat yield of the flaming front). An extended assessment completed a year or more following a fire often captures delayed effects on vegetation. Trees that are green immediately following a fire can die later as a result of the heat that may have been intensified around their roots. Vegetation burn severity data like the CBI helps park biologists, botanists, and fire ecologists understand what to expect from the changed landscape for wildlife habitat, invasive weeds, and eventually forest regeneration.

 

Ecologists record fuels and vegetation data in plots to monitor and evaluate fire effects over time.

Tracking Fire Effects over Time

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. Ecologist record how many individuals of each species are present before and after fire and measure the amount of fuel (dead branches, logs, pine needles, and other materials on the ground that can burn).

During a 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 remeasured over time. Plot data 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.