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
liho
od
of
sup
pre
ssio
n
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: