tropical plant trait evolution and the consequences for savanna-forest transitions william a....
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![Page 1: Tropical plant trait evolution and the consequences for savanna-forest transitions William A. Hoffmann North Carolina State University Collaborators Augusto](https://reader035.vdocuments.net/reader035/viewer/2022070415/5697bf791a28abf838c828f5/html5/thumbnails/1.jpg)
Tropical plant trait evolution and the consequences for savanna-forest transitions
William A. HoffmannNorth Carolina State University
CollaboratorsAugusto FrancoM. HaridasanErika Geiger
Sybil GotschLucas SilvaDavi Rossatto
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The Cerrado
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What determines tree cover in savannas?
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What determines the distribution of
forest and savanna?
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•Dense tree cover•no grass
•Hot, dry, windy microclimate•Frequent fire
•Cool, moist microclimate•Infrequent, mild fire
•Sparse tree cover•Dense grass
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Miconia cuspidata
Miconia pohliana
(Forest species)
(Savanna species)
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Symplocos mosenii(Forest species)
Symplocosrhamnifolia
(Savanna species)
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Large areas of the tropics have climates in which either savanna or forest vegetation in possible
Staver et al (2011)
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Cerling et al 1997 Nature 389:153-158
C4 grasses became abundant only in the past 8 million years
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Simon et al 2009
Savanna trees and shrubs began to arise from forest ancestors approximately 10 million years ago.
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There have been at least 115 independent origins of savanna trees or shrubs
Black = forest taxaRed = savanna taxa
Hoffmann (unpublished)
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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?
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The multiple, independent origins of savanna lineages is ideal for comparative studies
Black = forest taxaRed = savanna taxa
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Savanna Forest
Hei
ght a
t mat
urity
(m
)
0
2
4
6
8
10
Savanna Forest
Leaf
Are
a In
dex
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Note that this is a comparison of forest and savanna species.
Savanna ForestS
ee
dlin
gro
ot:
sho
ot
ratio
0
1
2
3
4
Savanna and forest species differ substantially in traits that have large implications for ecosystem structure and dynamics
Savanna Forest
Bar
k th
ickn
ess:
stem
rad
ius
0.0
0.1
0.2
0.3
Savanna ForestDia
met
er g
row
th r
ate
(mm
yr-
1)0
1
2
3
4
5
6
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53% of the evolutionary transitions from forest to savanna were associated with a shift to a smaller growth form.
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Question 2:
What are the consequences of these adaptations for ecosystem properties and vegetation dynamics?
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Savanna species Forest species
Pos
t-fir
e su
rviv
al (
%)
0
20
40
60
80
100
Savanna and forest species survive fire equally well
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The fire trap (Bell 1984)
Long time without fire
Fire
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The fire trap (Bell 1984)
Short time without fire
Long time without fire
Fire
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The fire trap (Bell 1984)
Short time without fire
Long time without fire
FireFire
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The fire trap (Bell 1984)
Short time without fire
Long time without fire
Fire
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The fire trap (Bell 1984)
Short time without fire
Long time without fire
FireFire
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An analogous threshold exists for ecosystems
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An analogous threshold exists for ecosystems
Short timewithout fire
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An analogous threshold exists for ecosystems
Short timewithout fire
Fire
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An analogous threshold exists for ecosystems
Short timewithout fire
Long timewithout fire
Fire
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An analogous threshold exists for ecosystems
Short timewithout fire
Long timewithout fire
Fire Fire
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CONTROL
WATER
NUTRIENTS
WATER + NUTRIENTS
10m x 70m plots
Resource manipulation experiment
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Water availability
Not irrigated Irrigated
Gro
wth
rat
e (m
m y
r-1)
0
1
2
3
4No added nutrientsAdded nutrients
Tree growth is more strongly limited by nutrients than by water
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High-resource sites permit more rapid canopy closure
Time since fire (y)
Cano
py c
over
High resource site
Low resource site
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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?
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Hoffmann et al (2012) Ecology Letters
Bark thickness (mm)0.1 1 10 100
Ste
m s
urvi
val (
%)
0
20
40
60
80
100Low fire intensity
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Hoffmann et al (2012) Ecology Letters
Bark thickness (mm)0.1 1 10 100
Ste
m s
urvi
val (
%)
0
20
40
60
80
100Low fire intensity
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A growing stem becomes fire resistant when its bark thickness exceeds 6 mm
Hoffmann et al (2012) Ecology Letters
Bark thickness (mm)0.1 1 10 100
Ste
m s
urvi
val (
%)
0
20
40
60
80
100Low fire intensity
5.9 mm
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A growing stem becomes fire resistant when its bark thickness exceeds 6 mm
Hoffmann et al (2012) Ecology Letters
Bark thickness (mm)0.1 1 10 100
Ste
m s
urvi
val (
%)
0
20
40
60
80
100Low fire intensityHigh fire intensity
5.9 mm 9.1 mm
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-30 -10 10 30Distance (m)
Microclimate •Wind speed•Relative humidity•Temperature
Fuels •Mass•Moisture•Bulk density
BehavePlus 5(fire behavior model)
Hoffmann et al (2012) Austral Ecology
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Win
d sp
eed
(km
hr-1
)
0.0
0.5
1.0
1.5
2.0
2.5
Min
. rel
ativ
e hu
mid
ity (
%)
35
40
45
50
55
Distance (m)-30 -20 -10 0 10 20 30
Max
air
tem
p.(C
)
202122232425262728
Distance (m)-30 -20 -10 0 10 20 30
Fin
e fu
el m
oist
ure
(%)
6
8
10
12
14
16
18
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Fla
me
leng
th
(m)
0.00.20.40.60.81.01.21.4
Fire
line
inte
nsity
(k
W m
-2)
0
100
200
300
400
Distance (m)-30 -20 -10 0 10 20 30
Rat
e of
spr
ead
(Km
hr-
1)
0.00
0.05
0.10
0.15
0.20
Fire simulations with BehavePlus
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Distance along transect (m)-30 -20 -10 0 10 20 30
Fue
l bul
k de
nsi
ty (
kg m
-3)
0
10
20
30
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Flammability of savanna is determined primarily by the presence of grass
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The canopy density at which grasses are excluded is a critical transition between savanna and forest.
Tree LAI
0 1 2 3 4 5
Gra
ss L
AI
0
1
2
3
4
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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.
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Stem diameter (cm)
1 10 100
Bar
k th
ickn
ess
(cm
)
0.01
0.1
1
10Savanna speciesForest species
As savanna trees grow, they accumulate bark thickness more quickly than forest species
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Stem diameter (cm)
1 10 100
Bar
k th
ickn
ess
(cm
)
0.01
0.1
1
10Savanna speciesForest species
As savanna trees grow, they accumulate bark thickness more quickly than forest species
Threshold bark thickness
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Stem diameter (cm)
1 10 100
Bar
k th
ickn
ess
(cm
)
0.01
0.1
1
10Savanna speciesForest species
As savanna trees grow, they accumulate bark thickness more quickly than forest species
Threshold bark thickness
4.7 cm 10.2 cm
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Recall that forest species grow more quickly than savanna species when growing in the same environment
Savanna ForestDia
met
er g
row
th r
ate
(mm
yr-
1)
0
1
2
3
4
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Savanna species Forest speciesFire interval required to ensure <50% topkill
8 years 14 years
5-year mean return interval
25 years 108 years
2-year mean return interval
510 years 32800 years
Expected total time under stochastic fire regime
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Under a typical fire regime a forest tree has little chance of reaching maturity in savanna
But, there is safety in numbers
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•Dense tree cover•no grass
•Hot, dry microclimate•Frequent fire
•Cool, moist microclimate•Infrequent, mild fire
•Sparse tree cover•Dense grass
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Forest species have greater leaf area than savanna species when growing in the same environment
Diameter (cm)0.1 1 10
Le
af a
rea
(m
2 )
0.0001
0.001
0.01
0.1
1
10
100
Savanna speciesForest species
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Forest species permit more rapid canopy closure
0
1
2
3
4
5
6
Time (years)
Leaf
are
a in
dex
Site occupied by forest species
Site occupied by savanna species
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Forest species permit more rapid canopy closure
0
1
2
3
4
5
6
Time (years)
Leaf
are
a in
dex
Site occupied by forest species
Site occupied by savanna species
Threshold canopy density
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Forest species permit more rapid canopy closure
0
1
2
3
4
5
6
Time (years)
Leaf
are
a in
dex
Site occupied by forest species
Site occupied by savanna species
Threshold canopy density
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Distance along transect (m)-30 -20 -10 0 10 20 30
Dia
met
er g
row
th r
ate
(mm
yr-1
)
0
2
4
6
8
10
12
14
16
Forest species
Savanna species
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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
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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.