Fire and Forest

Dynamics

in Northern Boreal

Forests

Jill JohnstoneBiology, University of Saskatchewan

Northern

boreal forest

• Conifer

dominated

• Cool soils,

slow growth &

decomposition

• Resistant to

change?

Fire and Global Change

Stocks et al. 1998

Chapin et al. 2005

Can we expect changes in

forest composition?

What are those likely to be?

Resilience and Response Dynamics

dynamic

equilibrium

directional

change

Resilience & Ecosystem Feedbacks

Dominant

species

RecruitmentInteractions

Competition, herbivory

Functional

traits

Disturbance

Black spruce

dominant

Local seed &

ResproutingSlow

growth

FIRE

Poor quality

seedbeds

(organic soil)

Slow nutrient turnover

Low competition

High moisture

High moss

Cool soils

Broadleaf

dominant

Resprouting &

Seed dispersalRapid

growthHigh quality

seedbeds

(mineral soil)

Rapid nutrient turnover

High competition

Low moisture

Low moss

Warm soils

FIRE

A. Black spruce domain B. Broadleaf forest domain

Alternate successional cycles

Johnstone et al. 2010, Can. J. Forest Research

Black spruce

dominant

Local seed &

ResproutingSlow

growth

FIRE

Poor quality

seedbeds

(organic soil)

Slow nutrient turnover

Low competition

High moisture

High moss

Cool soils

Broadleaf

dominant

Resprouting &

Seed dispersalRapid

growthHigh quality

seedbeds

(mineral soil)

Rapid nutrient turnover

High competition

Low moisture

Low moss

Warm soils

FIRE

A. Black spruce domain B. Broadleaf forest domain

Johnstone et al. 2010, Can. J. Forest Research

Alternate successional cycles

How do fire characteristics shape

patterns of forest resilience?

• Why study fire?

– Ubiquitous in western boreal region

– Sensitive to climate

– Post-fire recovery determines future forest

composition

Fire and successional

trajectories in black spruce

forests

Fire severity affects

seedbed quality

Burning of organic soils influences

patterns of post-fire recruitment

Patch effects of fire severityLow severity (organic)

– Poor seedbeds

– Recruitment requires high

seed inputs

– Favors serotinous conifers

High severity (mineral)

– Higher quality seedbeds

– Creates opportunities for

deciduous establishment

How does this influence forest

dynamics across

heterogeneous landscapes?

Fire severity and post-fire recovery

• Alaska 2004 fires

• 90 black spruce sites

• Initial stand recovery

• Environmental conditions

– Potential site moisture

– Elevation

– Potential insolation

• Pre-fire stand structure

– Stem density

– Stem basal area

• Fire severity

– Composite Burn Index (CBI)

– Residual organic layer depth

• Post-fire recruitment

– Tree seedling density

– 4 years post-fire

Field Data

Spruce seedling density

Boosted regression tree, prediction error=0.54

Johnstone et al. 2010, Global Change Biology

Deciduous seedling density

Boosted regression tree, prediction error=0.44

Johnstone et al. 2010, Global Change Biology

Relative spruce dominance:

Recovery of spruce trajectory

Boosted regression tree, prediction error=0.42

Johnstone et al. 2010, Global Change Biology

Controls on spruce forest resilience

• Severe fires reduce the

competitive advantage of spruce

and favor deciduous species

• Severe fires alter soil microclimate

• Site moisture

– Warm, dry soils favor aspen

– Severe fires are also more likely

• Young stands vulnerable to

change

Studies of fire frequency using

overlapping fires

historic fire

recent fire

overlap zones:

rapid disturbance return

image courtesy of David Milne, Yukon Gov.

0

5000

10000

15000

20000

25000

30000

35000

40000

total Picea Pinus Populus

ste

m d

en

sity (

#/h

a)

Burned at >80 yr.

Burned at <30 yr.

***

***

***

ns

Johnstone & Chapin 2006, Ecosystems

Repeat fires alter tree regeneration

Seed rain

Brown & Johnstone, unpublished

Seedling establishment

Brown & Johnstone, unpublished

How old does a stand need to be

before there is sufficient cone

production to support regeneration?

Cone Production

0

0.5

1

1.5

2

2.5

0 20 40 60 80

Tree Age

Nu

mb

er

of

Co

ne

s P

res

en

t o

n T

ree

(Lo

g1

0 s

ca

le)

n=170, p<0.001, r=0.360

n=14, p<0.001, r=0.723

Cones/tre

e (

log s

cale

)

Viglas & Johnstone, unpublished

Fire interval effects

• Repeat fires interrupt

conifer regeneration

cycles

– Reduced cone production

• Confers a regeneration

advantage to wind-

dispersed seeds

• Net effect is to shift

trajectories to deciduous

dominance

Black spruce

dominant

Organic

seedbeds

Slow growth

Low competition

High moisture

Cool soils

Slow turnover

Deciduous

dominant

Resprouting &

seed dispersal

Mineral soil

seedbeds

Rapid growth

High competition

Low moisture

Warm soils

Rapid turnover

Shifts in resilience cycles

Fire

Black spruce

dominant

Organic

seedbeds

Slow growth

Low competition

High moisture

Cool soils

Slow turnover

Deciduous

dominant

Resprouting &

seed dispersal

Mineral soil

seedbeds

Rapid growth

High competition

Low moisture

Warm soils

Rapid turnover

Shifts in resilience cycles

severe or short-interval

fire

long fire interval

Fire

Why is this important?

• Changes in forest cover affect:

– Carbon storage

– Energy and water transfer

– Wildlife and subsistence resources

– Feedbacks to future fire behavior

Fire severity and succession:

Impacts on future fire behavior

• High fire severity transforms black

spruce to deciduous forest

• Deciduous forest has lower flammability

Can fire-initiated changes create a

negative feedback to climate-driven

increases in fire activity?

ALFRESCO simulation experiment

• Spatial simulation model for boreal landscapes

• Succession influenced by fire severity

• 3 Severity Scenarios:

– Low (LSS): All fires burn with low severity (spruce trajectory)

– High (HSS): Maximum extent of high severity (decid. trajectory)

– Mix: Intermediate scenario

• High and moderate scenarios of climate warming

Area = ~ 2500 Area = ~1000

KEY:

Green & Yellow = Low Sev.

Red = High Sev. in HSS

Black = High Sev. in Mix + HSS

Cumulative area burned

High warming

Low warming

Johnstone, Rupp, et al., in review

Disturbance & climate interact

to alter black spruce resilience

tundra black spruce deciduous

dynamic

equilibrium

directional

change

Future Research

• Mechanistic understanding of plant-soil-

microbial feedbacks

• Quantifying thresholds and tipping

points

• Landscape prediction of vulnerability to

change

Conclusions

• Fire is both catalyst and driver of

change

• Critical post-fire reorganization phase

• Both frequency and severity shape future

succession

• Landscape context => vulnerability to

change

• Understanding the drivers of resilience is

key to predicting future change

Acknowledgements

Co-authors:

Carissa Brown

Terry Chapin

Teresa Hollingsworth

Michelle Mack

Mark Olsen

Scott Rupp

Ted Schuur

David Verbyla

Jayme Viglas