Stand structure and fire behaviour

Dave Schroeder Wildfire Management Branch

Alberta Agriculture and Forestry

Specifically…

Can stand structure be managed to reduce likelihood of wildfire damage to values for an

acceptable level of risk?

Most damage from wildfire occurs from: • Generation of flying embers = #1 cause of structure

ignitions.

• Exposure to convective and radiative energy.

What is a damaging fire?

One that supports high rate of spread and intensity

Crown fire: In conifer dominated stands.

Conifers primary source of embers.

Grass fire: In cured (dead) grass

Logging debris: influenced by harvest method

Brown and Davis (1973)

How are forest fuels structured? Crown fire is dependent on:

1. Surface fire intensity

2. Conifer canopy density

3. Ladder fuels

Except for complete removal, fuel management will not STOP a wildfire

• Limit ember production

• Increase opportunities for direct suppression

What are forest fuel management objectives at a stand scale?

Setting target thresholds Head Fire Intensity (HFI) = output of Canadian Forest Fire Behaviour Prediction System

HFI 2,000 - 4,000 kW/m: Intermittent crown fire occurrence becomes likely considered a limit for ground based attack Flying embers at least 100m ahead of fire

HFI up to 10,000 kW/m: direct suppression becomes difficult or impossible, indirect attack may be used

HFI > 10,000 kw/m: embers fly 1000m + , e.g., Athabasca River not a barrier Suppression at rear and flanks only

• Removal

• Isolation

• Conversion

• Displacement

Methods Tactics

• Manual

• Mechanical

• Chemical

Fuel management: How it is done

Maintain fire resistant species with low intensity fire

• Remove surface fuel

• Large gap between surface and crown fuel

(Agee and Skinner, 2005)

Will these methods work in boreal conifer stands?

• Thin barked, weak fire resistance

• Surface fuel often influenced by deep organic layer

• Mature conifers subject to blowdown if thinned

• May propagate other flammable fuels (e.g. grass)

Proven methods for dry belt conifer

Remove ladder fuels (cut understory, prune lower branches). Remove some crown fuel (thinning), pile and burn debris.

Partial Removal

Partial removal – thinning Jack Pine

HFI: 10,000 – 15,000 kW/m FPInnovations report: Schroeder, 2010

Partial removal –

under burnning Jack

Pine

Prescribed fire to remove surface fuels.

FPInnovations report: Baxter, 2013

• Light thinning, surface removal and pruning has a low threshold for effectiveness. – ICFME and Alaska (modeled)

• Saskatchewan and NWT case studies: more intense thinning + removal allowed suppression FPInnovations report: Mooney 2013

• FACTOR: Black spruce surface influenced by organic /feathermoss versus pine

Partial removal - Black Spruce

• Effective in pine, jury is out for spruce.

• Current treatment methods costly relative to other treatments

• Open stands, dry out more quickly relative to natural stands.

• More fire potential at moderate danger rating – when suppression still effective.

• Equalize at extreme end

• Greater tanker drop effect

• Better for ground crews – site lines, access

• Blowdown a challenge for mature conifer stands

Partial Removal

Complete Removal

Grass: Effective for one fire season.

Promote less flammable species

• Conifer to Aspen

• Spring hazard but no crown fire

• Conifer to grass?

• Need to burn annually

• Spruce to tamarack?

• Currently initiating research

• Feathermoss to sphagnum?

• Formal research project started at Pelican Mountain

Conversion: Species Management

Displacement

Crown fuel to surface fuel, mostly by mulching.

Strip Mulch

Narrow strips (~4m), no treatment in residual. HFI: 14,000 – 28,000 kW/m, FPInnovations report: Hvenegaard et al, 2016

500- 2000 kW/m

Ground suppression with proper resources

2000 – 4000 kW/m

Ground suppression still possible with air support

Complete Mulch Direct suppression possible, during conditions that would support crown fire (FWI = 25). Short range (

Thinking about thresholds

4,000 Intermittent

Crown fire

10,000 Continuous

Crown fire

100,000 + Slave Lake

And others

Like

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sup

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certain

possible

unlikely

None

HFI (kW/m) for Natural Conifer Stands

? – limit of test burns

Treatment intensity, e.g. amount removed

Why don’t we just choose this ?

?

• Managing vegetation, including stand structure WILL affect fire behaviour and can enhance suppression.

• 2,000 – 2,500 ha grass burned annually

• Alberta ~ 10,000 ha standing timber treated with various methods/tactics

• We do not know the upper limits for effectiveness (e.g. HFI 50,000 + kW/m).

• We do not know what the acceptable risk is. • E.g. do the public expect almost ZERO risk for even the most

extreme conditions?

• Will they accept almost ZERO risk landscapes?

Summary

• Test burns

• Ft Providence, NWT; Pelican Mountain, AB; Horse Creek, AB

• Removal

• Black spruce manual treatments

• Debris management

• Displacement

• Fire behaviour

• Cluster retention

• Underburn boreal pine

• Physical models (Firetec)

• Promoting less flammable species

Research

Thanks to: Colleagues in Wildfire Management Branch And research partners: