Fire Regimes in the Deserts of North America

Invasive Plants that Alter Fire Regimes in the Deserts of North America

Matt Brooks Jeanne Chambers U.S. Geological Survey U.S.D.A. Forest Service Western Ecological Research Center Rocky Mountain Research Station Yosemite Field Station, El Portal CA 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

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.

• Time (seasonality, return interval) • Space (size, complexity) • Magnitude (intensity, severity, type)

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 Climate Topography

Fire Regime

Fuels

Plants Climate Topography

Fire Regime

Fuels

Nonnative Plants Native Plants Invasive Plant / Fire Regime Cycle

+ Fire Regime –

Fuels

Nonnative Plants Native Plants

–

Brooks et al. 2004 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 Grass / Fire Cycle in Hot and Cold Desert Shrublands of North America

Fire Regime ↑ Frequency + ↑ Extent – ↑ Seasonal window

Fuels ↓ Coarse load ↑ Fine load ↑ Continuity ↑ Ignitability

Nonnative Plants Native Plants Annual grassland Sagebrush-steppe, Blackbrush, (dominated by Bromus spp.) Creosotebush scrub

–

adapted from Brooks 2008 Resiliency of Non-native Grasses and Native Perennials Following Fire Non-native grasses have high Native perennials have low resiliency to fire. resiliency to fire 12 2 Species Richness600 mof scaleNative Perennials Bromus rubens Above-Ground Live Biomass Biomass of Non-native Grasses 10 900 8 800

700 6

600 4

500 2 400

dry kg/ha dry 300 0

200 8 2 100 60 m scale

0 Cover of Perennial Plants 0 1 2 3 30 Cover of Native Perennials 6 recurrent fire (times burned 1984-1993) (# of species) richness species 25

2 15

cover (%) cover 10 0 0 1 2 3

5 fire history (times burned 1984-1993)

0 0 1 2 3 Brooks in prep fire history (times burned 1984-1993) 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 Woody Plant / Fire Suppression Cycle in Hot Desert Grasslands of North America

Fire Regime ↓ Frequency ↑ Intensity + ↓ Seasonal window – Surface fire to crown fire

Fuels ↓ Fine surface fuels ↑ Coarse canopy fuels ↓ Ignitability

Invading Plants Native Plants Mesquite, Tarbush, creosotebush Perennial grasses

–

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 Deserts of North America increasing productivity

100 woody perennials

Great Basin Mojave

Sonoran % during winter during %

Chihuahuan perennial grasses 0 10 20 30 40 50 annual precipitation (cm)

adapted from MacMahon and Wagner 1985 Deserts of North America Native vegetation insufficient to Native vegetation sufficient to fuel historical fires fuel historical fires

100

Great Basin Mojave

Sonoran % during winter during %

Chihuahuan 0 10 20 30 40 50 annual precipitation (cm) Deserts of North America Native vegetation insufficient to Native vegetation sufficient to fuel historical fires fuel historical fires

100

Cold Desert Shrublands

Hot Desert 50

Shrublands % during winter during % Hot Desert Grasslands

0 10 20 30 40 50 annual precipitation (cm) Deserts of North America Native vegetation insufficient to Native vegetation sufficient to fuel historical fires fuel historical fires

100

Riparian Riparian

Riparian % during winter during % Riparian

0 10 20 30 40 50 annual precipitation (cm) Hot Desert Shrublands

Low elevation shrubland

High elevation shrubland/woodland Middle elevation shrubland

Brooks and Minnich 2006 High elevation

Middle elevation

Low elevation High elevation

Middle elevation

Low 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

• Historically, salt desert shrublands (mostly chenopods) rarely if ever Salt Desert Shrub burned. 4 – 7” • In sagebrush types (Artemisia species), fire return intervals were moderate to long (~50 to 200++ yrs) depending on precipitation/ Sagebrush steppe productivity. Fire size and 7-16” patchiness were variable. • Pinyon and juniper woodlands were largely restricted to “fire safe” sites on ridge tops and steep slopes. PJWoodland woodland 1010--20+20+” Altered Fire Regimes and Invasive Species 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 2002 2003 1.4 1.2 Undershrub 1.0 Interspace 0.8 0.6 0.4 0.2 0.0 Biomass/plant (g) Biomass/plant 2002 2003 1.4 Altered Fire Regimes and 180 1601.2 Undershrub 1401.0 Interspace Invasive Species 1200.8 1000.6 • High severity fire and factors that deplete 80 0.4 herbaceous understory species increase 60 0.240 susceptibility to invasion. 0.020

Biomass/plant (g) Biomass/plant 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% • Chambers et al. (2007) found effects on Seeds/Plant No. 0 cheatgrass biomass and seed production 180 160 are additive: 140 Burn Control Burn Control Burn Control Burn Control Burn Control Burn Control – Herb Removal = 2 to 3 fold increase 120 100 6400' 7180' 7800' 6400' 7180' 7800' – Fire = 2 to 6 fold increase 80 – Herb Removal + Fire = 10 to 30 fold increase 60 40  Resistance to cheatgrass and resilience or 20 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0% 100%50%0%

No. Seeds/Plant No. 0 recovery after fire is increased by perennial grasses and forbs Burn Control Burn Control Burn Control  Recently, secondary weeds are spreading Burn Control Burn Control Burn Control 6400' 7180' 7800' through region (knapweeds, yellow star 6400' 7180' 7800' thistle, rush skeletonweed). 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? • 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.

Brooks and Matchett in prep 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 Grasslands post-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 Grasslands post-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 nonnative perennial grass (Lehmann lovegrass), which leads to low diversity vegetation stands.

Brooks and McPherson 2008 Desert Riparian Ecosystems

• 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. Altered Fire Regimes and Invasive Species • 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). Altered Fire Regimes and 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 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 Fire Factors 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