Post-fire recovery in black spruce forests

Analysis Summary

October 2007

Jill Johnstone

Intensive Sites

• 32 forest sites + 5 treeline sites

• Range of moisture * severity levels

• Seed and seedling applications in 2005

• Initial responses surveyed in 2006

• Site moisture– drainage class– mid-season soil moisture– soil texture

• Fire severity– visual severity rank– pre-fire organic layer from

adventitious roots• Post-fire seedbeds

– residual organic layer depth– cover of seedbed types– % cover of organics>3 cm– composite burn index (CBI)

• Post-fire recruitment– seedling densities – transplant growth

Field measurements

surfacefire

severity

organicseedbeds

pre-fireorganicdepth

site moisture

landscapeposition

Fire and Site Effects on Seedbeds:Hypotheses

• Assess using Structural Equation Modeling

Univariate correlations0 40 80

040

80

moss.cov

0 1 2 3 4 0 20 60 1 2 3 4 5 0.5 1.5 2.5 1 2 3 4 5 6

040

80

sqrt.org

01

23

4

org.cov

020

60

soil.sev.rank

12

34

5

CBI.substrate

0.5

1.5

2.5

1 2 3 4 5 6

12

34

56

drainage

Fire and Site Effects on Seedbeds:

Model Specification

severity index(1=low, 5=high consumption)

pre-fire organic depth (adv. root height in cm)

drainage index(1=subhygric, 6=xeric)

elevation (m)

OrganicSeedbeds

Composite BurnIndex

post-fire organicdepth (cm)

% cover of bare organics

Fire and Site Effects on Seedbeds:

Model Results

severity index

pre-fire organicdepth (adv. roots)

drainage index

elevation

OrganicSeedbeds

Composite BurnIndex

post-fire organicdepth

organic seedbedcover

-0.73

0.42

-0.42

Model χ2=13.5, p=0.2 (no significant lack of fit)

0.70

0.51

0.90

R2=0.96

dashed lines are ns

Summary: Fire Effects on Seedbeds• Organic soils dominate seedbeds where:

– Pre-fire organic layers were thick– High elevation restricts surface fire severity

• Variations in fire severity:– ~50% of the variation in organic seedbeds

• Expect impacts of organic seedbeds to be strongly controlled by fire severity

post-fireorganic

seedbed

coniferrecruitment

site moisture

landscapefactors

Site and Seedbed Effects on Tree Recruitment: Hypotheses

• Note: Seed availability is fixed for seeded plots and is not included in the model

deciduousrecruitment

Seedling counts0 1 2 3 4

01

23

4BS_sqrt

0.0 1.0 2.0 3.0 0 1 2 3 4 5 0 1 2 3 4 0 1 2 3 4

01

23

4

WS_sqrt

0.0

1.0

2.0

3.0

LP_sqrt

01

23

45

TA_sqrt

01

23

4

0 1 2 3 4

01

23

4

PB_sqrt

drainage

elev.

resid.org

BS WS LP

conifer

TA PB

deciduous

Used residual organic layer depth (sqrt transformed) to

represent seedbed effects (and fire severity)

Site and Seedbed Effects: Model specification

Seedling counts of black spruce, white spruce, lodgepole pine (highly intercorrelated, r>0.8)

Seedling counts of trembling aspen and paper birch (moderately correlated, r=0.7)

drainage

elev.

resid.org

BS WS LP

conifer

TA PB

deciduous

Model χ2=19.2, df=15, p=0.2 (no significant lack of fit)

0.310.36

0.50

-0.42

-0.48

-0.35R2=0.57

R2=0.48

0.98 0.91 0.87

0.86 0.85

Site and Seedbed Effects: ResultsPresenting standardized regression coefficients and sum of squared correlations for predicted seedling counts

Drivers of forest regeneration

• Differential sensitivity of functional groups– Deciduous highly sensitive to

post-fire seedbeds– Conifers more responsive to

site moisture

• Important role of fire severity in potentially tipping the balance between deciduous and conifer dominance

post-fireorganic

seedbed

seedlinggrowth

landscapefactors

site moisture

Seedling Growth: Hypotheses

• Note: Possibility for reciprocal effects between tissue N and seedling growth

tissuenitrogen

post-fireorganic

seedbed

black spruceheight growth

elevation

site moistureindex

Seedling Growth: Results

• Note: Possibility for reciprocal effects between tissue N and seedling growth

tissue% nitrogen

+-

+

-

R2 = 0.73

R2 = 0.43

+

Seedling Growth

Low severity

• Negative response to increasing organic layer depths

• Appears to mediated by N availability– need to clarify causal effects on

soil N as opposed to tissue N

• Generally consitent responses among species

• Greatest growth at driest sites

High severity

Resilience of Black Spruce Forests

Low severity

• Black spruce self-replacement– More poorly drained sites– Sites with thick organic layers

(cool & moist)

• Potential switch to deciduous dominance– Where high burn severity

exposes mineral soils– More frequent with climate

warming?

High severity

Thank you everyone!