preliminary results – do not cite ecosystem controls on the relationship between climate...

PRELIMINARY RESULTS – DO NOT CITE

Ecosystem Controls on the Relationship between Climate

Variability and 20th Century Fire in the American West

Jeremy LittellUW College of Forest

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Climate Impacts GroupPRELIMINARY RESULTS – DO NOT CITE


Area Burned in the Western U.S.

• More area burned does not necessarily equate to more ecological impact.

• More area burned DOES equate to more money spent fighting fires and more air pollution.

• Area burned is a metric used to effect change on everything from ecosystem management to EPA air quality standards.


Reported Area Burned

Fire Suppression Fire Exclusion “Catastrophic” Fires

Some Fire Much less fire A lot of fire

Cool PDOWarm PDO Warm PDO


Recent Climate / Fire Work• Long-term (1916), coarse

scale, seasonal relationships

• Short-term (1980), fine-scale, monthly climate and synoptic relationships

• Short-term, coarse scale ecological relationships

• No long-term, modern, ecologically specific work

McKenzie et al. 2004. Conservation Biology

Westerling et al. 2003. BAMS.


Climate Drivers of Fire Area Burned

• Increasing temperature (winter and summer) appears related to increasing area burned trend.

• Inter-annual wet/drought “cycles” also appear related to small/big fire years.

• Combination of long-term soil fuel moisture (“set-up”) and short term weather (sub-seasonal blocking ridges and storm fronts) implicated


Objectives

• Derive a set of more ecologically-relevant time series of wildland fire area burned.

• Relate these to 20th century seasonal

climate.

• Evaluate ecological underpinnings of strongest relationships.


Objectives


• Relate reconstructions to 20th century

seasonal climate.



Reconstructing Area Burned

State Data: 1916-2003 Gridded Data: 1980-2003


Caveats

• Is the state-level data correctly normalized for reporting area in the early 20th century?

• Is the proportion of area burned attributable to climate (vs. land use, vegetation dynamics, or fire exclusion) reasonably stationary?

NO!

?????


Area Reporting

• Between 1916 and 1964, most states contained between 10% and 60% of the public land they do today

• Acres in 1916 ≠ Acres in 2002, so each state series adjusted by smoothed area reporting


Reconstructing Area Burned

• Area burned time series almost never have any significant autocorrelation

• Noisy, non-normal data

• Several orders of magnitude in observations


Useful Distributions for Modeling Fire

Most Gridded, State, and Eco-province data are log-normal, but the log of the variance is proportional to the mean squared.


Reconstructing Eco-province Area Burned from State and

Grid Data• Regression models, training period is 1980-2003:

Log models:

ln (Cascade Mixed (gridcells))= ln (WA) + ln(OR) + ln(WA):ln(OR)

Gamma models:

Cascade Mixed = WA + OR + WA:OR | σ ~ μ2

• Use modeled relationship for best model to backcast Cascade Mixed for full 1916-2003 dataset.


Ecoprovince Reconstruction Results


Reconstructing Area BurnedA

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Objectives



seasonal climate.



Developing Seasonal Ecoprovince Climate

• Several constituent HCN divisions for most provinces

• Standardized, area-weighted, pre-whitened time series of T, PPT, PDSI:– Annual (Oct. - Sep.)– Winter (NDJFM)– Spring (MAM)– Summer (JJA)– GS (MJJAS)

Ecoprovince T, PPT, PDSI Climate Time

Series


Climate – Fire Relationships: 1980-2003 Summary

• Reconstructed and observed fire area burned relationships with climate similar in magnitude and seasonal pattern

• Year of fire summer precipitation, temperature significant in most ecoprovinces

• Broad, regional patterns in climate/fire correlations emerge


Southwest Regional Pattern

• Four southwest eco-provinces with similar climate correlations

• + Summer / GS T• - Summer / GS PPT• + lag 1 Winter / Spring

PPT • AM.Des (orange) has

Annual relationships for same variables


Northern Mountain Ecosystem Pattern

• Four mountain, forested eco-provinces with similar climate correlations

• - All Seasons PPT• + Summer / GS T• Year of fire relationships

very strong • Lag 1 relationships weak,

but same sign


Southern Mountain Ecosystem Pattern

• Three mountain, mixed eco-provinces with similar climate correlations

• + Summer / GS T• - Summer / GS PPT• Lag 1 relationships mixed;

some like desert SW, others like northern Mountains


Mixed Dry Ecosystem Pattern

• Three dry eco-provinces with similar climate correlations

• Strong Year-of, - PDSI

• Lag 1 relationships mixed –T and +PPT, PDSI


Objectives



seasonal climate.



Fuels and Ecosystem Pattern

• Different fuel types respond differently to climate

• Two mechanisms: drying of fuels and production of fuels

• Fuel - limited systems• Climate - limited systems• Ignition - limited systems


1916-2003 Climate and Fire

• Correlations not as strong for full period

• Pattern is ecosystem dependent

• 21 yr. Moving Correlations show why


Conclusions

• Late 20th century area burned is not unprecedented.

• Much (between 30% and 70%) of the decrease-increase pattern is related to climate.

• Ecosystem – specific models are useful for determining the climatic mechanism(s) responsible for fire area burned


Prospectus

• Use ecoprovince-specific fire ~ climate models to forecast future area burned given projected climate.

• Develop Pacific Basin Climate + Weather fire process models


Acknowledgements

Tony Westerling

Don McKenzie

Dave Peterson

Phil Mote

Tom Swetnam

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