When a Redbud member visited Glacier National Park several summers ago, the first thing she asked a ranger was, “Where are the best places to see wildflowers?” He quickly replied, “Where there have been fires.”

Those areas did have great swaths of blackened trees. Our native-plant enthusiast also saw wondrous displays of wildflower species and colors. Their seeds opened by fire heat, the soil enriched by ash from burned trees, and the mountainsides opened once again to more sun, the wildflowers grew vigorously. Their turn had come once again.

Native Ecosystems Need Fire

A similar cycle occurs in our local areas when they experience fire, as long as human activity has not knocked natural systems out of kilter.

“Lightning and human-caused fire ignitions occur frequently in the area. Vegetation grows, dies and produces organic matter. Fire consumes organic matter, kills some vegetation, stimulates many types of plants, and recycles nutrients. Without fire to perform these functions at the intensities and frequency under which they evolved, ecosystems can fall out of balance.” (Community Wildfire Protection Plan, Nevada County, California, updated April 2016.)

When fire takes out all or part of a forest, as did the River Fire in the Bear River drainage this summer and other fires across Nevada and Placer Counties in recent years, many changes happen that benefit native plants:

low-intensity fire --- singed trees and ground cleared by fire
Woodland adjoining Highway 174 experienced low-intensity fire in the recent River Fire. Trees are singed.

● Heat opens up cones of several local conifer species, releasing their seeds.
● Some native seeds are fire-activated by heat, smoke, or post-fire soil nutrients.
● Fire removes the overstory and permits sunlight to reach the soil.
● Wood ash increases nutrient levels in the soil.
● Non-native grasses and other invasive plants are killed. They may return with less vigor or (a detriment to native plants) greater vigor.
● Surviving trees grow stronger and bigger, without competition from undergrowth and weaker trees.
● Fire kills off diseases and pests that can otherwise devastate landscapes. According to CalFire, “More trees die each year from insect infestation and disease than from fire.” (Source: Benefits of Fire, CalFire.)

Though we think most often of wildfires in forests, other habitats also experience and need fire. In chaparral habitats, for instance, manzanita, ceanothus, and scrub oak have fire-activated seeds. As resinous plants, they increase the potential for the fire they need.

Fire Severity and Recovery

Active fire with flames, silhouettng trees, some still green and alive
In medium-intensity burn, some trees survive. (Photo: NPS)

The regeneration trajectory for native plants after a wildfire depends on the severity of the fire:
● In a low-intensity fire, native trees are singed but often not killed; fire burns primarily along the ground. Trees no longer have to compete with underbrush; as burned residue breaks into nutrients, established trees grow stronger, bigger, and healthier. Mature trees may drop leaves because of the fire but continue to provide food (e.g., for insects) and protection for native fauna.
● A moderate-intensity fire reaches the canopy, causing more tree mortality. Open spaces emerge, and sun-loving pioneer species appear the following spring, followed in subsequent years by shrubs and new young trees.
● Where fire kills all or virtually all the trees, high-intensity burning has occurred. Extreme heat has a devastating effect on the mycorrhizae and soil bacteria that make nitrogen and other nutrients available to plants. The soil damage may take decades to recover (if left undisturbed) or be so intense that, coupled with climate change, the habitat changes type; a former forest may regrow into chaparral. Though chaparral naturally burns hot, historically the fire interval was significantly longer than it is today. Too many fires in too short a time span can destroy chaparral plant communities so the area converts to grassland, usually primarily or wholly non-native grasses and other weeds. If fire does not return too often, many chaparral plants recover or reseed and benefit from the reduction or elimination of invasive weeds.

Results of a high-intensity fire in Southern California; trees burned & soil quite damaged

Often, in habitat recovery, fire-following plants come first. (So, too, do fire-following fungi.) They help heal the land by generating deep roots quickly, which reduces erosion potential. Some of fire-following species fix nitrogen via bacteria on their roots. Plants such as legumes (such as lupines and chaparral pea) fix nitrogen with Rhizobia bacteria, ceanothus species and bladderpod with Frankia bacteria. Many of us have seen fireweed (Chamerion angustifolium) spring up after fires in the mid- and high Sierra. It can quickly cover a slope with stalks 1 to 5 feet tall filled with bright pink to magenta blossoms.

Clarkias, lupine, poppies, and monkeyflowers also return. Annual grasses (sometimes native, too often not!) usually show up at this time. Interestingly, ferns and fungi are among early returnees to areas they have historically occupied.

Shrub about one-ft tall with many new leaves, and burned branche sticking out
Mountain mahoghany (Cercocarpus betuloides) resprouts after fire

Next come woody shrubs whose seeds or other reproductive mechanisms survived the fire. Some crown-sprout from roots not killed by the fire. These include toyon, deerbrush, and some manzanitas. Some partially-burned shrubs may resprout from remaining branches. Traditionally, in forested areas, trees then return. Hardwood trees with deep roots, such as many native oaks and the California buckeye (Aesculus californica), resprout. Finally, come pines and other conifers.

Many fires are a mosaic of patches of varying severity.

Charcoal and Ash After Fire

Forests (and other plant-rich habitats) have evolved with fire. Their nutrient cycles depend on fire to periodically make more minerals available to plants.

When fire burns trees and woody plant material incompletely, it yields natural charcoal. Charcoal is almost pure carbon, formed when wood (such as trees) burns incompletely in the absence of oxygen. This carbon tends to remain in the soil rather than releasing to the air, providing a carbon sink that can last centuries. Charcoal affects the biota in the soil and their effect on nutrient cycling, how plants receive and release energy. The charcoal provides a matrix for many kinds of life, including mycorrhizae, fungi, and microbes; this helps the soil absorb and retain moisture and retain nutrients.

When fire burns those materials completely, it yields mineral ash. This ash contains such soluble plant nutrients as potassium, phosphorus, calcium, and magnesium, released by the plants that died in the fire.

Though nitrogen is usually depleted into the air during a fire, ammonium produced by combustion often remains, particularly in low-intensity fires. This may quickly convert to nitrates. These nitrates have a positive effect, as they are quickly taken up by growing plants, but they damage water ecosystems.

Wood ash and smoke chemicals in soil after a fire may also create conditions that some native seeds require in order to germinate, after long lying dormant.

Fires Are Different Today

Until recent decades, high-intensity fire seldom covered more than 400 acres; now this occurs with increasing frequency. Seven of the 10 largest fires in California recorded history have occurred since 2019 including the Dixie Fire, largest in California history and still growing at the time of this writing (“The Largest Wildfire in California’s History Is Continuing to Grow,” Time, August 11, 2021.)

Today, human actions, not natural forces such as lightning, cause 95 percent of California fires (California’s Catastrophic Wildfires in 3 Charts. PBS NewsHour, posted Sep 14, 2020.) Most occur in areas where fires have been suppressed for decades, leading to fuel build-up that can mean fires burn hotter than fires before European colonization. These super-hot temperatures not only kill trees that would survive low-intensity fires but destroy both seeds and microscopic soil life. Where such human-caused conditions occur, they make fire recovery more difficult.

blackened, empty forest, in a
High-intensity fire, by destroying so much plant life that protects from erosion, leaves soil and waterways vulnerable.

Particularly in a high-intensity fire, overstory, shrub and understory plants that once slowed water on slopes are gone, so rain hits the ground hard. On slopes, nutrients and topsoil wash away more easily; that material may impact waterways, sometimes imperiling the health of plants and animals living in the water, in the burn area and downstream.

Moreover, climate change is turning western U.S. air, soil and plants hotter and drier than in the past; these factors are making fires more likely, faster, larger and hotter. Such fires may result in permanent loss of ecosystems. Scientists who compared forests that had burned early this century with those that burned late last century found that “The proportion of sites with no regrowth almost doubled after 2000, from 19% to 32%, coinciding with increasing temperatures and more droughts.” (Science, Ecosystems could once bounce back from wildfires. Now, they’re being wiped out for good. Dec 19, 2017. )