how do forest ecosystems respond to environmental change?
Post on 31-Dec-2015
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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
1.5
2.0
2.5
3.0
3.5
0 1 2 3 4 5 6 7 8 9 10
organic layer depth depth (cm)
rela
tiv
e s
ee
dlin
g e
sta
blis
hm
en
t
aspen
black spruce
white spruce
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0.1
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10.0
100.0
0 5 10 15 20 25 30
organic layer depth (cm)
de
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ity
ra
tio
(d
ec
id:s
pru
ce
)
0
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10000
0 5 10 15 20 25 30
organic layer depth (cm)
bio
ma
ss
ra
tio
(d
ec
id:s
pru
ce
)
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)
0.26
0.34
0.55 (8) 0.20 (12)
0.00 (6)
Regression Treepseudo-R2=0.65
Deciduous vs. Spruce Recruitment
Natural seedling densities 2 yrs post-fire
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0 5 10 15 20 25 30 35
organic layer depth (cm)
pro
po
rtio
n o
f d
ecid
uo
us
seed
lin
gs
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?
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