How do forest ecosystems respond to environmental

change?

Resilience & Ecosystem Feedbacks

Dominant species

RecruitmentInteractions

Competition

Functional traits

Disturbance

Resilience Feedbacks

Black spruceLow

nutrient cycling

Cold soils

Conservative growth

Mosses accumulate

Aerial seed bank

Recruitment on organic soils

Typical successional trajectories

• Self-replacement of black spruce

• Asexual regeneration of understory

• Little compositional change after disturbance

Deep burning fires can shift successional trajectories

Is there a threshold of organic soil consumption that has predictable effects

on regeneration pathways?

Recruitment declines with organic depth

2-yr recruitment from seeding experiments (Alaska/Yukon)n=4 to 16 (total plots = 60)

0.0

0.5

1.0

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0 1 2 3 4 5 6 7 8 9 10

organic layer depth depth (cm)

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black spruce

white spruce

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organic layer depth (cm)

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(d

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0 5 10 15 20 25 30

organic layer depth (cm)

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Species differences

lead to strong effects on post-fire

dominance

Seedling Density

Aboveground Biomass

Burned spruce forest • Alaska 40,000 ha burn

• 8 yrs post-fire• n=19 stands

Can we detect thresholds across real landscapes?

• Alaska 2004 fires • 3 fire complexes• 90 black spruce sites

|org.dep < 7.95

elev < 661elev < 415

moist.rank<2.5

elev < 525

org.dep<15

0.57 (59)prop.decid (n)

0.820.77

0.830.90 (11) 0.73 (8)

0.60 (6)0.95 (8)

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0.55 (8) 0.20 (12)

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Regression Treepseudo-R2=0.65

Deciduous vs. Spruce Recruitment

Natural seedling densities 2 yrs post-fire

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organic layer depth (cm)

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Fire & regeneration thresholds

• Residual organic layer determines seedbed quality

• Differences in species sensitivity lead to strong composition effects

• Increased fire severity => crossing threshold of residual organics => shift in successional trajectory

Disturbance & climate interact to alter black spruce resilience

tundra black spruce deciduous

dynamic equilibrium

directional change

Resilience Feedbacks

Species dominant

Recruitment

Interactions

Organic seedbeds

Conservative growth

Low nutrient cycling

Mosses accumulate

Critical Research

• Moving deeper in time – What are the longer term consequences of

variations in fire severity?

• Understanding space– Which parts of the landscape are

vulnerable to shifts in trajectories, and which aren’t?

• Can we test anticipated changes?

Climate response dynamics

• Key feedbacks support resilience

• Do threshold responses measure the depth of resilience?

• At the landscape scale, is this the primary pathway for climate change response?