Pyrogeography of savanna fire intensity and severity

What are we measuring and why?

Daniel GodwinKruger National Park, 2014

Fuel

Oxygen Heat

fuel + oxygen + heat -> water + hydrogen + carbon dioxide + carbon monoxide + heat

For a given interaction of fuel + oxygen + heat…

How much energy is released?

Intensity explains p(topkill)

Demographic bottlenecks

Structurally defines savannas and allows for coexistence of trees, grass

Although not the only factor…

Fire limits tree establishment

If fire is a strong factor in limiting tree establishment in savannas…

In areas of high tree cover, you could expect either infrequent or low intensity fires

In areas of low tree cover, you could expect either frequent or high intensity fires.

I = Hwr

I – Fireline Intensity (kW m-1) H – Heat content (kJ g-1) w – Mass of combusted fuel (g) r – Rate of Spread (m s-1)

Why do we see increasing woody cover at the same time as increasing p(top kill)?

Confounding effects of granitic vs. basaltic?

Fire doesn’t limit as much as we think

Fire intensity as we measure it doesn’t capture the effects on woody cover that we think it should

Severity is the ecological impact of a fire

Plants

Animals

Soil

Intensity is energy released Intensity != Severity… …but no standard metric for severity in

savannas

Remote Sensing using MODIS Fire Product

Modeled fireline intensity

Analysis of fire scar reports from KNP

Local-scale intensity variation

Ten years of data (1 Jan 2004 – 1 Jan 2014

Subset to only include fires of > 90% accuracy

No clear relationship with MAP

No clear relationship with woody cover

FRP bad metric for savanna fires?

Mean Low RH

Wind Speed Range

MAP

Fuel Load: f(Time Since Fire, Rainfall)

MFRI

Fire Intensity:f(Fuel Load, RH, Fuel

Moisture, Wind Speed)

Fuel Moisture

Range

Probability of Top Killf(Intensity, Height)

Intuitive: hotter fires near PRET

Clear relationship with MAP…but that’s expected given the model.

No clear relationship with woody cover, though differences in intensity between geologic material.

Date Location Size Impact on Woody Cover Impact on Herbaceous Material

Impact on Woody Cover

Very severe – None

Impact on Herbaceous Material

Very patchy – Clean

Most of this is intuitive

To do:

Link to previous summer’s rainfall

Use as training samples for remotely sensed data

How “spatial” is fire intensity?

Fire intensity has deterministic drivers (fuel moisture, available oxygen, fuel amount)

Do these vary enough around trees to drive potential effects on p(top kill)?

Ground level

3 cm

5 cm

30 cm

Sand

iButton

Classic point clouds.

Similar to earlier studies (Moustakas et al. 2013), at wetter end, grass biomass is decoupled from canopy coverage

Variation in fuel moisture content ~ Canopy Coverage.

Better assessment of intensity from temperature curves

Canopy opening patch size effect on fire intensity, controlling for RH%

When measured or modeled, intensity doesn’t appear to predict woody cover.

…so does it matter as much as we think?

Maybe we need to shift away from intensity as a metric..

And what, exactly, does fire severity look like in savannas?

LAL (lighting activity level)

Dispersion (atmospheric stability)

Mixing Height (“ceiling”)

Haines Index (atmospheric driven large fire growth)

ERC is the potential energy released from a headfire

Calculated based on dynamic fuel models

Fuel moisture calculated based on magnitude/intensity of rain, RH%, live and dead fuel moisture

Extensive monitoring of live and dead fuel moisture

Oak savanna research: 10 hr live fuel moisture content key to top kill

Haines Index: Likelihood of “firestorm”