Changes in Surface Albedo as a Result of Forest Fires in Northern Arizona

Ponderosa Pine Forests

Isaac Bickford, Kyle JonesGeorge Koch, Bruce Hungate,

Matthew Hurteau

National Institute for Climatic Change Research,

Western Region

Albedo

• We used a different definition than most astronomers use.

• For our purposes, albedo is the ratio of reflected to incoming shortwave radiation.

Surface Typical Albedo

Conifer Forest (Summer) 0.08

Bare Soil 0.17

Green Grass 0.25

Desert Sand 0.40

Fresh Snow .80 - .90

Influence of Albedo on Climate

Albedo

Solar radiation absorption

Cooling

Changes in Albedo

• How canopy cover affects albedo

– Forest canopies often have lower albedo values than the ground.

– Thinner canopies allow more of the ground to show through, raising average albedo for the area.

High Canopy Cover

Albedo

Low Canopy Cover

Albedo

Incoming and reflected shortwave radiation

How Fire Influences Albedo

• Stand replacing fire increased albedo in Alaskan boreal forests.

• Recovery to pre-fire albedo ~ 55 years

• Albedo had a larger impact on radiative forcing than did fire emissions (CO2, aerosols, etc.)

Randerson et al. (2006)

Winter

Summer

MODIS

• Moderate-resolution Imaging Spectroradiometer

• Captures data for 36 wavelengths, imaging the entire earth every 1-2 days

• Corrected for difference between viewing angle and solar angle

• Albedo can be calculated for any solar angle using the MOD43B1 data product

Fire Influence on Albedo at Lower Latitudes

• Ponderosa Pine dominated, coniferous forest system• Northern Arizona ponderosa pine forests have snow

cover for a shorter time period than Alaskan boreal forests.

• Fire can influence climate change in several ways, including aerosol emissions and albedo.

Research Question:• How does fire affect albedo in a lower latitude

evergreen coniferous forest system?

Data Sites

• 1996 Hochderffer fire - 11 years ago• 1996 Horseshoe fire - 11 years ago• 1973 Burnt fire - 34 years ago

• Control located in nearby ponderosa pine with no known recent fire activity

Specific Questions

• How does albedo differ between fire and control sites during winter (with snow) and summer?

• How long does it take albedo to return to pre-fire level?

Fire sites have higher average winter albedo(2000 - 2006 MODIS data)

Hochderffer(1996)

Horseshoe(1996)

Burnt(1973)

Control

Albedo positively correlated with snow depth for recent fire sites, but not for control site

Hochderffer(1996)

Horseshoe(1996)

Burnt(1973)

Control

Summer albedo also higher for recent fire sites than control sites

Conclusions• As in high latitude boreal forests, stand-replacing wild fires increase albedo in southwestern pine forests.

• Effect of fire on albedo is greatest in winter,when snow is present. Fire site albedo can benearly four times that of control sites.

• Effect of fire on winter albedo is still present after 34 years, consistent with slow recovery of thesesemi-arid forests.

Implications

• By increasing albedo, fire causes a cooling effect that is counter to the warming influence due to the loss of carbon by the fire.•To fully understand the effect of fire on climate change, more variables need to be analyzed.

Future Work• For our study:

– Include the data we collected from other fire sites in our analysis

– Construct a chronosequence illustrating the effect of stand replacing fire on albedo in southwestern ponderosa pine forests

• For another study:– Examine balance of albedo and CO2 emissions on net

radiative forcing.– Extend this approach to management strategies, e.g.,

forest thinning actions to reduce fire risk.