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Tropical plant trait evolution and the
consequences for savanna-forest transitions
William A. Hoffmann
North Carolina State University
Collaborators
Augusto Franco Sybil Gotsch
M. Haridasan
Lucas Silva
Erika Geiger
Davi Rossatto
The Cerrado
What determines tree cover in savannas?
What determines
the distribution
of forest and
savanna?
•Sparse tree cover
•Dense grass
•Hot, dry, windy microclimate
•Frequent fire
•Dense tree
cover
•no grass
•Cool, moist microclimate
•Infrequent, mild fire
Miconia pohliana
(Savanna species)
Miconia cuspidata
(Forest species)
Symplocos
rhamnifolia
(Savanna species)
Symplocos mosenii
(Forest species)
Large areas of the tropics have
climates in which either savanna or
forest vegetation in possible
Staver et al (2011)
C4 grasses became abundant only in the
past 8 million years
Cerling et al 1997 Nature
389:153-158
Savanna trees and shrubs began to arise from forest
ancestors approximately 10 million years ago.
Simon et al 2009
There have been at least
115 independent origins of
savanna trees or shrubs
Black = forest taxa
Red = savanna taxa
Hoffmann (unpublished)
Main questions
• What selective pressures have shaped
the evolution of tree species in savanna?
• What are the consequences of savanna
tree adaptations for ecosystem
properties and vegetation dynamics?
The multiple, independent origins of
savanna lineages is ideal for
comparative studies
Black = forest taxa
Red = savanna taxa
0.3
0.2
0.1
0.0
6
5
4
3
2
1
0
Savanna Forest
3.5
8
6
4
2
0
3.0
2.5
2.0
1.5
1.0
0.5
3
2
1
0
0.0
Savanna Forest
4
Seedling
root:shoot ratio
10
Leaf Area Index
Height at maturity (m)
Savanna Forest
Diameter growth rate (mm yr-1)
Bark thickness:stem radius
Savanna and forest species differ substantially in traits that
have large implications for ecosystem structure and dynamics
Savanna Forest
Savanna Forest
Note that this is a comparison of forest and savanna species.
53% of the evolutionary transitions from forest to savanna were
associated with a shift to a smaller growth form.
Question 2:
What are the consequences of these
adaptations for ecosystem properties
and vegetation dynamics?
Savanna and forest species survive fire
equally well
Post-fire survival (%)
100
80
60
40
20
0
Savanna species
Forest species
The fire trap (Bell 1984)
Long time
without fire
Fire
The fire trap (Bell 1984)
Long time
without fire
Short time
without fire
Fire
The fire trap (Bell 1984)
Long time
without fire
Short time
without fire
Fire
Fire
The fire trap (Bell 1984)
Long time
without fire
Short time
without fire
Fire
The fire trap (Bell 1984)
Long time
without fire
Short time
without fire
Fire
Fire
An analogous threshold exists for ecosystems
An analogous threshold exists for ecosystems
Short time
without fire
An analogous threshold exists for ecosystems
Short time
without fire
Fire
An analogous threshold exists for ecosystems
Long time
without fire
Short time
without fire
Fire
An analogous threshold exists for ecosystems
Long time
without fire
Short time
without fire
Fire
Fire
Resource
manipulation
experiment
10m x 70m plots
CONTROL
NUTRIENTS
WATER
WATER + NUTRIENTS
Tree growth is more strongly limited
by nutrients than by water
Growth rate (mm yr-1)
4
No added nutrients
Added nutrients
3
2
1
0
Not irrigated
Irrigated
Water availability
High-resource sites permit more rapid canopy closure
Canopy cover
High resource site
Low resource site
Time since fire (y)
At what point is each thresholds
reached?
• How big must a tree be to avoid topkill?
• How dense must the canopy be to substantially
reduce flammability?
Stem survival (%)
100
Low fire intensity
80
60
40
20
0
0.1
1
10
100
Bark thickness (mm)
Hoffmann et al (2012) Ecology Letters
Stem survival (%)
100
Low fire intensity
80
60
40
20
0
0.1
1
10
100
Bark thickness (mm)
Hoffmann et al (2012) Ecology Letters
A growing stem becomes fire resistant when its bark
thickness exceeds 6 mm
Stem survival (%)
100
Low fire intensity
80
60
40
5.9 mm
20
0
0.1
1
10
100
Bark thickness (mm)
Hoffmann et al (2012) Ecology Letters
A growing stem becomes fire resistant when its bark
thickness exceeds 6 mm
Stem survival (%)
100
Low fire intensity
High fire intensity
80
60
40
9.1 mm
5.9 mm
20
0
0.1
1
10
100
Bark thickness (mm)
Hoffmann et al (2012) Ecology Letters
-30
30
10
-10
Distance (m)
Microclimate
•Wind speed
•Relative humidity
•Temperature
Fuels
•Mass
•Moisture
•Bulk density
BehavePlus 5
(fire behavior model)
Hoffmann et al (2012) Austral Ecology
55
Min. relative
humidity (%)
-1
(km hr )
Wind speed
2.5
2.0
1.5
1.0
0.5
45
40
35
0.0
28
27
26
25
24
23
22
21
20
Fine fuel
moisture (%)
Max air temp.
(C)
50
-30 -20 -10
0
10 20 30
Distance (m)
18
16
14
12
10
8
6
-30
-20
-10
0
10
Distance (m)
20
30
0.20
0.15
0.10
0.05
0.00
-30 -20 -10
0
10 20 30
Distance (m)
400
(kW m-2)
Fireline intensity
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(Km hr-1)
Rate of spread
Flame length
(m)
Fire simulations with BehavePlus
300
200
100
0
-3
Fuel bulk density (kg m )
30
20
10
0
-30
-20
-10
0
10
20
Distance along transect (m)
30
Flammability of savanna is determined primarily
by the presence of grass
Grass LAI
4
3
The canopy density at which
grasses are excluded is a critical
transition between savanna and
forest.
2
1
0
0
1
2
3
Tree LAI
4
5
Thus we have two critical thresholds
• A tree reaches a fire-resistance threshold
when it accumulates a bark thickness of about
6 mm.
• The ecosystem reaches a fire-suppression
threshold when it attains a leaf area index of
about 3.
Bark thickness (cm)
10
As savanna trees grow, they accumulate bark
thickness more quickly than forest species
Savanna species
Forest species
1
0.1
0.01
1
10
Stem diameter (cm)
100
Bark thickness (cm)
10
As savanna trees grow, they accumulate bark
thickness more quickly than forest species
Savanna species
Forest species
1
Threshold bark
thickness
0.1
0.01
1
10
Stem diameter (cm)
100
Bark thickness (cm)
10
As savanna trees grow, they accumulate bark
thickness more quickly than forest species
Savanna species
Forest species
1
Threshold bark
thickness
0.1
4.7 cm
0.01
1
10.2 cm
10
Stem diameter (cm)
100
Diameter growth rate (mm yr-1)
Recall that forest species grow more quickly than
savanna species when growing in the same environment
4
3
2
1
0
Savanna Forest
Fire interval required to
ensure <50% topkill
Savanna species
8 years
Forest species
14 years
Expected total time under stochastic fire regime
5-year mean return
interval
25 years
108 years
2-year mean return
interval
510 years
32800 years
Under a typical fire regime a forest
tree has little chance of reaching
maturity in savanna
But, there is safety in numbers
•Sparse tree cover
•Dense grass
•Hot, dry microclimate
•Frequent fire
•Dense tree cover
•no grass
•Cool, moist microclimate
•Infrequent, mild fire
Forest species have greater leaf area than savanna
species when growing in the same environment
100
2
Leaf area (m )
10
1
0.1
0.01
Savanna species
Forest species
0.001
0.0001
0.1
1
Diameter (cm)
10
Forest species permit more rapid canopy
closure
6
Site occupied
by forest species
Leaf area index
5
4
Site occupied by
savanna species
3
2
1
0
0
5
10
15
20
25
30
Time (years)
35
40
45
50
Forest species permit more rapid canopy
closure
6
Site occupied
by forest species
Leaf area index
5
4
3
Threshold
canopy density
Site occupied by
savanna species
2
1
0
0
5
10
15
20
25
30
Time (years)
35
40
45
50
Forest species permit more rapid canopy
closure
6
Site occupied
by forest species
Leaf area index
5
4
3
Threshold
canopy density
Site occupied by
savanna species
2
1
0
0
5
10
15
20
25
30
Time (years)
35
40
45
50
-1
Diameter growth rate (mm yr )
16
14
Forest species
12
10
8
6
Savanna species
4
2
0
-30
-20
-10
0
10
20
Distance along transect (m)
30
Conclusions
•Many indepedent origins of savanna trees
•Fire has exerted strong selection on traits
•Nutrients, not water, limits tree growth in these
mesic savannas.
•Two critical thresholds govern savanna-forest
dynamics:
(1) bark thickness at which a stem becomes fire
resistant.
(2) canopy density at which grasses are excluded
• Reaching a forest state requires forest species,
but these are particularly constrained by the high
frequency of fire
Vegetation models should
•Represent fire and its feedbacks with vegetation
•Represent topkill and reprouting
•Represent of savanna and forest tree functional
types OR tradeoffs involving bark, carbohydrate
storage, shade tolerance, canopy density.
•Not assume that tree cover in mesic savannas
are water limited.
•Robustly simulate savannas over large areas in
response to water deficits, multiple nutrient
deficiencies, seasonal flooding, and physical soil
constraints.