Fire is an important ecosystem process in California

that influences ecosystem composition, structure,

and function.

The influence of fire depends on ecosystem characteristics,

as well as on fire type and frequency.

California's Mediterranean-type climate

predisposes the landscape to fires.

Mild rainy winters lead to abundant plant growth,

which can yield densely vegetated landscapes

of potential fuels.

The warm annual summer drought in California

makes this vegetation highly flammable.

In Central California, the color of the foothills

reflects the changing seasons.

Fire regime, the frequency and severity of wildfires,

varies across California ecosystems.

Historical fire regimes, shown here,

illustrate the relationship between fire frequency

and severity in different regions.

The most frequent low-severity fires

are shown in green, in the Central Valley,

while the least frequent and most severe

fires are shown in red, on the eastern slopes of the Sierras.

Fire regime has changed to varying degrees

in different locations because of human activities,

including direct effects of fire suppression

and intentional and accidental fire ignitions,

and indirect effects of land use and land cover changes

that alter the flammability and connectivity

of wildfire-prone landscapes.

Finally, there are global activities

that alter the climate in ways that

influence fire risk, fire behavior, and natural ignitions

by lightning.

For all of these reasons, fire is both a crucial

ecological process and an area of great complexity

for ecosystem management and conservation in California.

This is a simple schematic of the relationship

between two key fire characteristics

and the impacts of fire.

Intensity is the measure of energy released during a fire.

Because it needs to be measured directly during a fire,

intensity measurements are rarely available.

A commonly used surrogate for fire intensity

is fire severity, which is a measure of biomass loss

and can be assessed after the fire.

Fire severity can be calculated with remote sensing imagery

or as soil burn severity, which considers

the loss of soil organic matter and direct effects

on soil structure.

Fire intensity and severity both have important implications

for ecosystem responses to fire and their impacts

on adjacent natural and urban resources or societal impacts.

Relevant responses include soil erosion, resprouting

and other vegetation regeneration,

restoration of community structure,

and faunal recolonization.

Factors influencing historical and modern patterns

of fire activity are diverse and have likely changed over time.

Native Californians occupied lowland landscapes

in the state that had dense impassable vegetation.

And fire was the only available tool

to modify vegetation in these landscapes.

By the 1800s, Mexican and European settlers

introduced cattle and used rangeland burns

to increase livestock production.

In this Northern California landscape,

the contrast between vegetation in the valley and the foothills

illustrates how fire and ranching

have influenced the system.

Rangeland improvement became more risky with urban expansion

and fires were suppressed to protect timber resources.

By the 1970s, fire policy shifted

from one of immediate suppression

to management as the important ecological role of fires

gained recognition.

So fire frequency has been greatly modified

by humans in many California ecosystems.

First, especially in densely populated coastal landscapes,

but also in California forests, humans

have greatly increased fire frequency

by providing ignitions.

These can be accidental, such as from escaped campfires

or discarded cigarettes, or they can be intentional.

These have led to recent fire frequency.

The number for fires, regardless of size,

outpacing previous natural fire frequency.

Conversely, aggressive fire suppression

has reduced the frequency of large burns

in forested landscapes.

Prior to Euro-American settlement,

montane forests burned every 10 to 30 years in California.

And foothills burned every 10 to a hundred years.

These fires were driven by lightning ignitions,

which are more frequent at higher elevations.

Today, humans override this natural frequency gradient,

causing more ignitions where populations are

more dense at lower elevations.

Suppression for many decades produced

a decline in the total area burned each year.

However, the cumulative effects of increasing fuel loads

in long unburned forests and increased drying and tree

mortality associated with climate trends

now mean that large catastrophic fires are more likely

and pose risks to both ecological and social

dimensions of these systems.

The Valley Fire, shown here, began in September of 2015.

It burned over 76,000 acres or 30,000 hectares

in less than two weeks, destroyed almost 2,000 homes,

killed four civilians, and reduced large expanses

of the landscape to bare ground, with few seed sources nearby

to spur rapid vegetation.

Human activity has also changed the duration and timing

of annual fire seasons.

Historically, fires ignited by lightning

were restricted to summer and early autumn, when

occasional monsoon conditions brought unstable air

into California from the east.

Large fire events were concentrated in late summer

and autumn as a result. Human fire ignition

has spread the burn season throughout the year.

Compared to lightning fires, human-ignited fires

have a greater potential to start during severe fire

weather conditions and are more likely to give rise

to large catastrophic fire events.

Although most natural ignition fires occur during drier summer

months, the 2013 Pfeiffer Fire in Big Sur

burned 370 hectares or 917 acres in December,

in the middle of what's usually the wet, fire-free season.

Heating of electrical control wires

adjacent to a water pipeline at the Pfeiffer Ridge Mutual Water

Company provided an ignition source

for dry leaf litter and the fire quickly spread.

Both topography and weather have always played major roles

in driving fire behavior.

For instance, Southern California

experiences severe fire weather conditions

in the autumn, when the strong, warm Santa Ana

winds develop in the desert and blow westward.

Following summer droughts, Santa Ana

wind events result in some of the state's worst fire events.

Today, humans provide a more reliable and widespread source

of ignitions.

And evidence indicates that Santa Anna wind-driven fires

are more frequent today than in the past.

In the fall of 2003, the Cedar Fire in San Diego,

driven by Santa Anna winds, burned over 280,000 acres

or 113,000 hectares.

The Cedar Fire destroyed over 2,000 homes,

resulted in 14 fatalities and 104 firefighter injuries,

and cost roughly $27 million.

In summary, humans have altered fire regimes through ignitions

and suppression in two major ways,

decreased fire frequencies in vegetation types

naturally typified by frequent low-severity fires

and increased fire frequencies and vegetation types naturally

characterized by infrequent, but severe fire.

Fire-return interval departure analysis

quantifies the difference between

current and pre-settlement fire frequencies.

This map shows changes in fire regimes

for areas with Forest Service and national park lands.

Warm colors on the map, like yellow and orange,

illustrate negative fire-return intervals or places

where fires are more frequent now

than historical pre-settlement regimes.

Cool colors, the blues, show areas

where fire frequencies have decreased.

While we generally think of fire as a disturbance,

fire suppression itself is a major perturbation

with profound ecological effects.

Now, let's dig into how fires and ecosystems interact

at a more localized scale.

Organic materials consumed by fire are referred to as fuels.

Fires are grouped into broad types, surface, crown,

and ground fires, as a function of the types of fuels consumed.

Low-intensity surface fires consume fuels

only on the ground.

These typically include downed dead material and litter,

as well as understory vegetation such as grasses

and other herbs.

Mixed conifer forests in the Sierra Nevada, for example,

historically had frequent surface fires

that stayed in the understory, clearing it, but retaining

canopy vegetation.

In contrast, high-intensity crown fires

burn in the canopies of shrubs and trees.

Fire can spread in surface fuels and can then jump to the canopy

through ladder fuels, like dead branches,

or they may burn only in the canopy.

Lower elevation Southern California chaparral shrublands

typically burn in high-intensity crown fires, as pictured here.

Both surface and crown fires are characterized

by flaming combustion.

A third type, ground fires, spread slowly

through smoldering combustion.

Flames often are not visible at all in a ground fire.

Ground fires and surface fires can

smolder for weeks or longer, but can later

erupt into surface or crown fires as the weather changes.

Many contemporary California plant species

have had a long evolutionary association with fire,

extending back up to 50 million years.

Surface fires and crown fires are each

associated with the evolution of very different plant traits.

For example, chaparral is a well-studied hotspot

of evolutionary adaptation to infrequent high-severity fires

that burn most or all above-ground vegetation.

These intense fires generate a range

of specialized post-fire regeneration niches

or strategies to recolonize after a fire.

Here, you can see fire poppies blooming in spring

after a winter fire at Fort Ord.

They exemplify many chaparral species

that spend the vast majority of their life cycles

as seeds in the soil and only emerge to flower, fruit,

and reproduce after a severe fire has

removed the dense shrub canopy.

Plant populations exhibit two modes

of recovery following fire.

The first mode is endogenous regeneration from resprouts

or from fire-triggered seedling recruitment from dormant seed

banks in the first post-fire growing season.

The second mode is non-endogenous regeneration,

such as delayed seedling recruitment

from resprouts or surviving parent plants

and colonization from unburned metapopulations.

Some plants, like the poppies we just saw,

establish through fire-triggered seedling recruitment

or resprout from below-ground plant parts.

Both of these strategies again are forms

of endogenous regeneration.

So while the poppies emerge from an endogenous seed bank,

chamise, shown here, can regrow from either an endogenous seed

bank or via resprouting from endogenous vegetative

structures, as seen here, making them facultative cedars.

Resprouting from vegetative structures that

survive fire can occur from stem bases, rhizomes, bulbs, corms,

roots, or on above-ground stems.

Resprouting occurs in most crown fire regimes,

where all or most above-ground stems are killed by fire.

Obligate resprouters, such as toyon, shown here,

are present in the first year after fire

as vegetative resprouts without seedling recruitment.

In contrast to plants that seed after fire,

obligate resprouters have seeds designed

for more widespread dispersal and their seedling recruitment

tends to be restricted to the understory

of the vegetation canopy on sites that remain free of fire

for extended periods of time.

In contrast to resprouting, some plants

depend on the seed bank for seedling recruitment.

Obligate seeders include woody species

that lack resprouting capacity and depend entirely

on post-fire seedling recruitment triggered

by fire-related cues.

Both heat and chemical cues from the combustion of biomass

can cue germination.

For example, some pines, such as this bigcone Douglas fir,

have seratonous cones that delay opening

until triggered by fire.

In the case of delayed seedling recruitment,

fire triggers copious seed production

and first-year post-fire plants recruit en masse.

These include many sage, scrub, subscrub species

and herbaceous perennials such as bunchgrasses.

If cones or fruits are present at the time of fire,

recruitment can occur in the first growing

season after fire.

However, recruitment is often delayed.

Many trees in surface fire regimes

have masting cycles of reproduction,

like this coast live oak, where massive amounts of cones

and fruits are produced periodically.

When fires coincide with mast years,

there can be abundant post-fire recruitment.

In contrast to resprouting and delayed recruitment,

parts of the semi-arid Western US,

dominated by woodlands, pinion, Juniper, and sagebrush,

all recover slowly from ground fires through recolonization.

In the Sierra Nevada, the sagebrush

that burned in the 2010 Pumice Fire

will likely recover slowly by a colonization

from other populations.

Moderate patches can be quickly filled with seedlings.

But large patches may be dominated

by shrubs that without reburning can stay forest-free

for decades.

This is a non-endogenous approach to recolonization.

Like plants, animals exhibit a diversity of strategies

for dealing with fires.

Some invertebrates can persist as dormant diaspores

in the soil.

Smaller mammals can shelter in place

and survive by seeking refugia, such as rock

outcrops, moist ravines, and burrows

within the fire perimeter.

Others, including birds and larger mammals, flee the fire

and subsequently must recolonize from the unburned landscape.

Changes in fire seasonality and fire regime

can threaten some animal species.

For example, the sage grouse, once a very common bird

in the western US and Canada, is now

a candidate for listing under the Endangered Species Act.

This bird depends on Great Basin shrublands,

including habitat along California's eastern edge,

that's dominated by sagebrush and historically experienced

summer and fall burns.

With invasion by the exotic cheatgrass, which

produces contiguous fine fuels, the fire season

both begins earlier and lacks historical patchiness.

The new fire regime disadvantages

both the sagebrush and the grouse that depends on it.

Once fire has passed, animal recovery

is influenced by the magnitude of changes

in vegetation structure.

This graph shows how animal population size, on the y-axis,

responds over time since a fire for different classes

of wildlife.

The solid line shows species that dominate early,

such as the coastal whiptail lizard,

which prefers open habitat.

While the dotted line shows species

that depend more on later successional stages,

such as the garden slender salamander, which prefers

forested habitat.

Herbivores and granivores may be food limited after a fire.

But other species, like some woodpeckers and bark beetles,

are attracted to recently burned areas.

Species-specific animal traits result

in changing peak abundances with time since fire.

In addition to effects on vegetation and animal

populations, fire influences soils, water,

and carbon storage.

Fires' most significant impacts to soil chemistry

revolve around soil organic matter and macronutrients.

Fire consumes soil litter and carbon compounds,

causing changes in soil structure and reduced soil

water-holding capacity.

Here, tree roots are exposed after a fire

that consumes soil organic matter and reduced soil depth.

Reduced vegetation cover and soil infiltration

can increase soil erosion and runoff.

Major sediment and nutrient pulses after a fire

can kill aquatic organisms.

But most aquatic effects of fires

are ephemeral or short-lived.

The loss of carbon also produces major changes

in soil microflora.

And this in turn can typically cause

a release of soluble nitrogen available to plants.

Nitrogen is mostly volatilized by a fire.

But this loss is often offset by an increase

in the soluble bio-available forms of nitrogen,

including ammonium and nitrate.

A pulse of nutrients is available to plants

that can use them quickly, such as some respouters

and seedlings that germinate in the year-after fires.

These lupins, in addition to being

able to fix atmospheric nitrogen themselves,

can use the soil nutrients quickly

before they're washed away during the rainy season.

Fire is an important ecological process

in California, that now must be carefully managed.

Its become a central focus of federal, state,

and local agencies.

Pre-fire management now involves manipulating fuel loads

from accumulating in an effort to reduce fire severity.

Typical management practices in forest systems

include thinning small and medium trees,

reducing surface fuels, and using prescribed burns

or burning fuel piles.

While fuel treatments in forests have

neutral to positive effects on fire severity,

fuel treatments in shrublands can cause ecological damage

by paving the way for invasive species to establish.

Post-fire management focuses on recovery.

Here, a US Forest Service employee

is planting a seedling after the 2007 Angora

Fire near Lake Tahoe.

A primary concern after fire is soil loss and excessive water

flow off of recently burned slopes

that can lead to floods and debris flows.

Federal management agencies focus

on short-term post-fire issues, such as identifying

and treating areas of high erosion hazard and nonnative

plant invasion.

In the longer term, tree planting

can stabilize populations of rare species,

accelerate successional processes,

and begin to resequester carbon lost to fire.

Projections based on dynamic vegetation models linked

to downscale general circulation models, or GCMs,

suggests that the geographic distributions

of major ecosystem types in California

could change substantially by the end of the 21st century,

with much of the change mediated by changes in fire activity

and severity.

Since most western US forests will likely

experience even more potential for fire,

climate management foci include reintroducing fire and creating

forest structures resistant or resilient to large fires.

In this lecture, we've established

that fire is an important ecological process

and that ecosystems and organisms are

adapted to fire and recover from fire in complex ways.

Let's review the main themes from the lecture.

Fire is an important ecosystem process in California

that influences ecosystem composition, structure,

and function.

Historically, humans have played a substantial role

in perturbing natural fire regimes through changes

in fire frequency, intensity, and burn seasons.

Many California forests have had a long history

of frequent low and moderate intensity surface fires.

And the primary human impact has been suppression

of this natural fire regime.

One consequence has been an anomalous accumulation

of surface fuels and in-growth of young trees, both of which

have contributed to the potential

for high-intensity crown fires.

Shrublands and other nonforested landscapes in the state

have historically burned in high-intensity crown

fires ignited by lightning.

An increase in human ignitions has

resulted in more frequent fires in these landscapes

in Central and southern Coastal California.

This increase in fire frequency has had negative ecosystem

impacts by type-converting native shrublands

to nonnative grasslands throughout many parts

of the region.

Finally, future global changes are

likely to have very different impacts on these two landscape

types, with global warming playing a significant role

in forests and demographic growth

and urban development playing larger roles in coastal plains

and foothills.

Fire

Ecosystems of California

与讲师见面