Stand structure and fire behaviour. Dave Schroeder Wildfire Management Branch Alberta Agriculture and Forestry

Transcription

1 Stand structure and fire behaviour Dave Schroeder Wildfire Management Branch Alberta Agriculture and Forestry

2 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.

3 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

4 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

5 What are forest fuel management objectives at a stand scale? Except for complete removal, fuel management will not STOP a wildfire Limit ember production Increase opportunities for direct suppression

6 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

7 Fuel management: How it is done Methods Removal Isolation Conversion Displacement Tactics Manual Mechanical Chemical

8 Proven methods for dry belt conifer 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)

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

10 Partial removal thinning Jack Pine HFI: 10,000 15,000 kw/m FPInnovations report: Schroeder, 2010

11 Partial removal under burnning Jack Pine Prescribed fire to remove surface fuels. FPInnovations report: Baxter, 2013

12 Partial removal - Black Spruce 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

13 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 Partial Removal Better for ground crews site lines, access Blowdown a challenge for mature conifer stands

14 Complete Removal Grass: Effective for one fire season.

15 Conversion: Species Management 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

16 Displacement Crown fuel to surface fuel, mostly by mulching.

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

18 kw/m Ground suppression with proper resources kw/m Ground suppression still possible with air support

19 Complete Mulch Direct suppression possible, during conditions that would support crown fire (FWI = 25). Short range (<100 m) ember transport. Needed pump and hose for suppression. Instant response to wind gusts.

20 Likelihood of suppression Thinking about thresholds certain Why don t we just choose this? possible Treatment intensity, e.g. amount removed unlikely None 4,000 Intermittent Crown fire 10,000 Continuous Crown fire? limit of test burns HFI (kw/m) for Natural Conifer Stands? 100,000 + Slave Lake And others

21 Summary 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?

22 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) Research Promoting less flammable species

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