Invasive Plants that Alter Fire Regimes in the Deserts of ...archive.rangelands.org/.../plenary/Brooks_Chambers_Fire_Regimes.pdf · Invading Plants Mesquite, Tarbush, creosotebush

Invasive Plants that

Alter Fire Regimes in the

Deserts of North America

Matt Brooks

U.S. Geological Survey

Western Ecological Research Center

Yosemite Field Station, El Portal CA

Jeanne Chambers

U.S.D.A. Forest Service

Rocky Mountain Research Station

Reno NV

Presentation Outline

• Explain what a fire regime is and how

invasive plants can alter it.

• Describe how fire regimes are altered in

major desert vegetation types.

• Present recommendations on how best to

manage altered fire regimes.

Fire Regimes

• Time (seasonality, return interval)

• Space (size, complexity)

• Magnitude (intensity, severity, type)

It is relatively simple to comprehend the characteristics

and effects of an individual fire. However, it is much more

difficult to do the same for fire regimes, which are defined

as patterns of burning across time, space, and magnitude.

Sugihara et al. 2006

Fire Regimes

Fire regimes can be strong forces in the evolution of

species traits. When fire regimes are rapidly altered,

individual species and species assemblages may be

significantly affected.

photo by Lesley DeFalco

Plants

Fuels

Fire Regime

Climate Topography

Nonnative Plants

Fuels

Fire Regime

Native Plants

Climate Topography

Nonnative Plants

Fuels

Fire Regime

Native Plants

+ –

–

Brooks et al. 2004

Invasive Plant / Fire Regime Cycle

Determining that an invasive plant/fire

regime cycle has become established

1. Plant invasion has changed fuel structure

and fire behavior

2. The invasive species benefit from these

changes, often to the detriment of natives

3. These changes lead to an altered fire regime

- this last step can be very difficult to document

- must show a change in some fire regime factor

Brooks 2008

Nonnative PlantsAnnual grassland

(dominated by Bromus spp.)

Fuels↓ Coarse load

↑ Fine load

↑ Continuity

↑ Ignitability

Fire Regime↑ Frequency

↑ Extent

↑ Seasonal window

Native PlantsSagebrush-steppe, Blackbrush,

Creosotebush scrub

+ –

–

Grass / Fire Cycle

in Hot and Cold Desert Shrublands of North America

adapted from Brooks 2008

Resiliency of Non-native Grasses and

Native Perennials Following Fire

Non-native grasses have high resiliency to fire.

Cover of Perennial Plants

fire history (times burned 1984-1993)

0 1 2 3

cover

(%)

0

5

10

15

20

25

30

Bromus rubens Above-Ground Live Biomass

recurrent fire (times burned 1984-1993)

0 1 2 3

dry

kg/h

a

0

100

200

300

400

500

600

700

800

900

Biomass of Non-native Grasses

Native perennials have low resiliency to fire

Cover of Native Perennials

fire history (times burned 1984-1993)

0 1 2 3

sp

ecie

s r

ichn

ess (

# o

f spe

cie

s)

0

2

4

6

8

600 m2 scale

0

2

4

6

8

10

12

60 m2 scale

Species Richness of Native Perennials

Brooks in prep

Annual grasses invading desert shrublands

• Perennial shrubs are gone

• Landscape has low resistance to

fire

• Annual grasses and other non-

natives are the dominant species

left and are resilient to fire

Invading PlantsMesquite, Tarbush, creosotebush

Fuels↓ Fine surface fuels

↑ Coarse canopy fuels

↓ Ignitability

Fire Regime↓ Frequency

↑ Intensity

↓ Seasonal window

Surface fire to crown fire

Native PlantsPerennial grasses

+ –

–

Woody Plant / Fire Suppression Cycle in

Hot Desert Grasslands of North America

adapted from Brooks 2008

Woody plants invading hot desert grasslands

• Perennial grasses are gone

• Landscape has high

resistance to fire

• Woody species are the

dominant species left to

recover if a fire happens to

occur

adapted from MacMahon and Wagner 1985

Sonoran

annual precipitation (cm)

% d

uring w

inte

r

0

100

50

10 3020 40

Mojave

Chihuahuan

woody perennials

perennial grasses

increasing productivity

Great Basin

50


Sonoran


% d

uring w

inte

r

0

100

50

10 3020 40

Mojave

Chihuahuan

Great Basin

50

Native vegetation insufficient to

fuel historical fires

Native vegetation sufficient to




% d

uring w

inte

r

0

100

50

10 3020 40

Cold Desert Shrublands

50






Hot Desert

Shrublands

Hot Desert Grasslands


% d

uring w

inte

r

0

100

50

10 3020 40 50






Riparian

Riparian

Riparian

Riparian

Hot Desert Shrublands

Low elevation shrubland

Middle elevation shrubland

High elevation shrubland/woodland

Brooks and Minnich 2006

Low

elevation

Middle

elevation

High

elevation

Low

elevation

Middle

elevation

High

elevation

Current Hot Desert Shrubland Fire Regimes

Non-native Annual Fuel Regime

– Driven by single- or consecutive-year rainfall

patterns causing episodic fine fuel buildup (e.g. due

to ENSO)

– Low and middle elevations where native woody

fuels are typically too sparse to carry fire

– Fires were historically rare to non-existent and

native vegetation has low resiliency to fire

– Significant fire management intervention may be

warranted to break the invasive plant / fire regime

cycle

Brooks in prep

Current Hot Desert Shrubland Fire Regimes

Native Perennial Fuel Regime

– Driven by decade and century scales of rainfall

patterns causing gradual woody fuel buildup (e.g.

due to PDO)

– High elevations where native woody fuels can carry

fire

– Fires occurred historically and native vegetation has

some resiliency to fire

– Fire suppression may be warranted at the WUI, but

on a landscape scale periodic fire on 75-100+ years

intervals may be desirable

Brooks in prep

Cold Desert Shrublands

Woodland

10-20+

Salt Desert Shrub

4 – 7”

Sagebrush steppe

7-16”

• Historically, salt desert shrublands

(mostly chenopods) rarely if ever

burned.

• In sagebrush types (Artemisia

species), fire return intervals were

moderate to long (~50 to 200++

yrs) depending on precipitation/

productivity. Fire size and

patchiness were variable.

• Pinyon and juniper woodlands were

largely restricted to “fire safe” sites

on ridge tops and steep slopes.PJ woodland

10-20+”

Low to Mid Elevations• Invasive annual grasses (cheatgrass, red

brome, medusa head) were introduced in late 1800s and rapidly spread. Increase in flammable fine fuels with high rate of spread.

Initiation of annual grass fire cycle -

shorter FRIs, larger, more contiguous fires

Other Factors - longer and more severe fire

seasons, more human fire starts

High Elevations

• Overgrazing by livestock beginning in mid-late

1800s decreased native perennial grasses and

forbs (fine fuels).

• Decreased competition from herbs, favorable

establishment conditions and fire reduction led

to increases in shrubs and pinyon and juniper

(woody fuels).

Less frequent fires until last decades –

increased fire size and severity since then

Altered Fire Regimes and

Invasive Species

• High severity fire and factors that deplete

herbaceous understory species increase

susceptibility to invasion.

• Chambers et al. (2007) found effects on

cheatgrass biomass and seed production

are additive:– Herb Removal = 2 to 3 fold increase

– Fire = 2 to 6 fold increase

– Herb Removal + Fire = 10 to 30 fold increase

Resistance to cheatgrass and resilience or

recovery after fire is increased by perennial

grasses and forbs

Recently, secondary weeds are spreading

through region (knapweeds, yellow star

thistle, rush skeletonweed).


Invasive Species

2003

No. S

eeds/P

lant

020406080

100120140160180

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mass/p

lant (g

)

0.00.20.40.60.81.01.21.4

Undershrub

Interspace

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100%

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0%

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50%

0%

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50%

0%

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50%

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Burn Control Burn Control Burn Control

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2003

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How might future increased temperatures

hot and cold desert fire regimes?

• Climatic models predict increasing temperatures.

• Increasing temperatures may drive cold desert conditions more towards hot deserts (e.g. more extreme fire weather conditions and lower live fuels moistures).

• Conditions in higher elevation and more mesic shrublands may become more conducive to dominance by non-native grasses and the grass/fire cycle, and reduced resistance and resiliency of native vegetation to fire.

Brooks and Matchett in prep

How might future changes in precipitation affect

hot and cold desert shrubland fire regimes?

Brooks and Matchett in prep

• Potential future precipitation is much more difficult to predict than temperature.

• Proportions of summer:winter and rainfall:snow, in addition to increases or decreases and interannual variations, all have differing implications for invasive plants, fire regimes, and interactions between the two.

• Increasing temperatures will mean increasing evapo-transpiration rates, so increased rainfall may still lead to decreased soil moisture and concomitant effects on fine and woody fuels.

Hot Desert Grasslands

• Historically, most hot desert grasslands burned at an

average return interval of 5-15 years (range of 2-30 years).

• Fires typically occurred at the beginning on the monsoon

season in late June to early July.• Most woody species are killed by

fire and don’t reproduce until they

are >10 years old.

• Most perennial grasses survive fire

and only have reduced productivity

for a few years.

• As a result, perennial grasses

dominate landscapes that are

burned about every decade.Brooks and McPherson 2008

Hot Desert Grasslandspost-settlement

• Livestock grazing has reduced fire fuel loads to the point that fires cannot spread even under extreme fire weather conditions.

• Fire suppression activities and landscape scale fuel fragmentation from roads have further reduced the amount of area burned.

• As a result, fire return intervals have increased, woody plants have established, and perennial grasses and other herbaceous species have declined, converting grasslands to shrublands.

Brooks and McPherson 2008

Hot Desert Grasslandspost-settlement

• Fires have been reintroduced in an effort to convert

shrublands back to grasslands.

• However fire is typically applied in March or April, outside

of the historical summer fire season.

• While these may favor grasses over shrubs, early-season

fires tend to favor the dominance of a widespread non-

native perennial grass (Lehmann lovegrass), which leads

to low diversity vegetation stands.

Brooks and McPherson 2008

• Historically, FRIs of desert riparian areas likely were influenced by drought, lightning strikes, FRIs of the surrounding landscape, and Native Americans, but in most cases were probably very long.

• Fire size and patchiness were influenced by fuel and fire characteristics, geomorphic setting, and hydrologic regime.

Desert Riparian Ecosystems

• Tamarisk (8 Tamarix sp.) and Russian olive (Elaeagnus angustifolia) were introduced in 1800s.

• Altered flow regimes resulted in drier floodplain environments where the more drought tolerant invaders are replacing native cottonwoods and willows.

• Floods that provided conditions for native species establishment and cleared live and dead vegetation are suppressed.

• Tamarisk and Russian olive now form dense thickets of contiguous fuels with high amounts of woody debris and leaf litter, and create volatile fuel ladders under native cottonwoods.

Fires of higher severity now occur every 10 to 20 years in some riparian areas (Lovich et al. 1994).


Invasive Species

• Tamarisk resprouts after fire, and both invaders have longer-lived seeds and less specific establishment requirements than native cottonwoods and willows.

• Established native vegetation can suppress tamarisk seedlings and decrease susceptibility to invasion (Sher et al 2000, 2002).

Changes in flow regimes have increased both flammability of riparian areas and the spread of invaders

Fires have replaced floods as the primary disturbance in many southwestern riparian ecosystems


Invasive Species

Breaking Invasive Plant / Fire Cycles

Must manage both the invasion process and the fire

regime at landscape scales

• Managing for more desirable FRIs requires maintaining or

increasing ecosystem resistance to invasion and

ecosystem resilience or the ability to recover after fire

• Inherent differences exist in resistance and resilience

among desert vegetation types

– Abiotic and biotic characteristics

– Current ecological conditions

• Management activities need to consider likely FRIs for

target vegetation types and current ecological conditions

Effective Management Hinges on

Resistance and Resilience to FireFactors that decrease resistance and resilience

• Low and highly variable precipitation

• High resource fluctuations due to variable precipitation and fire

• Low abundance and life form diversity of native species

• Transformer species present as adults or propagules

• Abundant and contiguous woody or fine fuels

Low resistance & resilience - hot desert shrub, salt desert, Wyoming sage

Moderate resistance & resilience – mid to high elevation sage, mountain brush

Prevention of Altered Fire Regimes

Routinely assess current ecological conditions to prioritize areas for

management.

Increase resistance in areas with intact native communities

• Prevent and minimize the size of wildfires

• Control invasion vectors and corridors (roads, trails, etc.)

• Eliminate/reduce ongoing stressors (repeated fire, overgrazing)

• Increase early detection and eradication efforts

Increase resilience in areas with intact native communities

• Decrease woody fuel loads if appropriate

• Seed with native species if appropriate

• Actively manage to minimize invasion and stressors

• Minimize other stressors both before and after fire

Restoration of Areas with Altered Fire Regimes

Increase resistance and resilience of transitional or converted

vegetation stands in high priority management areas (e.g. near

intact native vegetation stands, areas of high resource value, the

WUI)

• Use an integrated management approach

– Eliminate or reduce invader abundance and propagule supply

– Restore native species in areas where necessary and feasible

– Create communities with high resilience and desirable FRI

if necessary

• Actively manage to minimize invasion, disturbance, and stressors

Adaptive Management

• Monitor changes occurring and results of management activities

– Link into new regional efforts like NEON and the new UNR and USGS climate change networks

• Use an adaptive management approach focused at landscape scales

– Base on feedback from resource assessments and monitoring

Promote active and effective collaboration among all

stakeholders

Documents

Invasive Plants that Alter Fire Regimes in the Deserts of ...archive.rangelands.org/.../plenary/Brooks_Chambers_Fire_Regimes.pdf · Invading Plants Mesquite, Tarbush, creosotebush