View
213
Download
0
Category
Preview:
Citation preview
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!
Recommended