Tree Islands: Landforms and Underlying Biotic Feedbacks
Paolo D’Odorico (University of Virginia) Joel Carr (University of Virginia) Vic Engel (Everglades National Park)
TREE ISLANDS • More elevated - woody vegetation
Wet Season
Dry Season
Okavango Delta Florida Everglades
Similar Dynamics - more complex landscape: Everglades freshwater system
5
6
1 Okefenokee Swamp, GA 2 Marshall Loxahatchee NWR, FL 3 Everglades National Park, FL 4 Quiet & Calabash Marshes, Belize 5 The Pantanal, Brazil 6 Okavango Delta, Botswana
1 2,3
4
1
10
100
1000Fr
eq
ue
ncy
Area (m2)
1940 n = 1053 mean = 84777 sd = 161500
1
10
100
1000
Fre
qu
en
cy
Area (m2)
1995 n = 578 mean = 59397 sd = 126683
Large change in areal extent in the 55 year period
DEM-derived (survey verified) Island Height (1995 Islands only, and Ghost Island areas)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
10 20 30 40 50 60 70 80 90 100 110 120 130 More
Frac
tio
nal
fre
qu
en
cy
Island height (DEM adj) (cm)
1995 islands
Ghost islands
n = 422 mean = 69.5 sd = 12.3
n = 499 mean = 74.8 sd = 11.4
Two-phase Landscape • Tree islands: - vegetation: trees
- more elevatedless flooded - phosphorus “rich”
• Marshes/Wet Prairies: - “Herbaceous vegetation”
- less elevatedmore flooded - phosphorus “poor”
H
S
I
Hammock Hardwood
Species
Intermediate Species
Swamp Species
Tree islands are nutrient rich “Fertility Islands” A “Savanna” (Wetzel et al, 2005): mosaic of tree and “herbaceous” patches coexisting in the same landscape
Tree Islands
Wet Prairies
(sawgrasses, graminoids)
(D’Odorico, et al., 2011)
TREE ISLAND DYNAMCS
• How do we explain this coexistence?
• Stability & resilience of tree islands
• Impact of changes in tree cover or water level
Tree Islands
Wet Prairies
Stable coexistence of two states bistability what causes this bistability?
Plant Community
Physical Environment
Feedback between the
system and its environment
Ecogeomorphic Processes
Effects of positive feedbacks on ecosystem dynamics
State of the system
Pote
nti
al f
un
ctio
n
State of the system
Pote
nti
al f
un
ctio
n
Bistable System “Unistable System”
Tree Island Marsh
Positive feedbacks: trees create their own “habitat”
Available Phosphorous: trees enhance P availability
(Wetzel et al., 2005; Lawrence, D’Odorico
et al., 2007; DeLonge, D’Odorico et al, 2008)
Additional P-
inputs to treed areas
Canopy
Growth (P-limited)
Increased
Canopy “trapping”
Soil formation/accumulation : trees favor soil formation
Tree
Growth
Soil Accretion
island formation
Accumulation of soil
organic matter &
precipitation of carbonates
(McCarthy et al., 1994; Graf et al., 2008 )
Atmospheric Deposition of P
Canopy trapping
Enhanced P Deposition
Other mechanisms of P accumulation in Tree Islands
P P P P
“Evapotranspirational Pumping “ (Ross et al., 2006)
Guano Deposition (Frederick and Powell, 1994)
Other feedbacks: interactions of peat accretion with soil P cycle
Feedbacks Bistability Process-based zero-dimensional Model
• Tree growth limited by prolonged waterlogging and P availability
• In the absence of these limitations trees would have competitive advantage over herbaceous vegetation
TTTadt
dTcc
T tree biomass
2 cc
dGa G G T G
dt G herbaceous biomass
ccT Carrying capacity for trees
ccG Carrying capacity for grasses
Dynamics of Tree Biomass: TTTadt
dTcc
a
2
Based on band dendrometer data (D’Odorico, Engel, Oberbauer, et al.)
Feedbacks Bistability Process-based zero-dimensional Model
• Tree growth limited by prolonged waterlogging and P
TTTadt
dTcc
T tree biomass
Carrying capacity for trees (depends on P & hydroperiod)
)(0 PfTTcc
∆h
To
0 0.8 m -0.4 m
flooded
Not flooded
1
h hw
∆h=h-hw
Ground Surface
Tree Soil Accretion
• Depends on tree biomass and hydroperiod
LossSoilonAccumulatiSoildt
hd
)(
1 2
( )( )
d hT h k
dt
Soil dynamics much slower than vegetation dynamics
Soil Respiration & Fires
h hw
∆h=h-hw
• Tobon et al. [2004]: P in throughfall, stemflow, & precipitation in 4 adjacent Amazonian forest sites
Trees Soil P Balance
Pin= T +
= strength of canopy
dependent P-input
outin PPdt
dP
P Availability & Elevation Tree dynamics
)(0 PfTTcc )(Pf
1
P
h hw
∆h=h-hw
∆h
To
0 0.8 m -0.4 m
flooded
Not flooded
1
Dependence of T on P q
bPPf
1
11)(
Dependence of T on ∆h
Equilibrium States In the short term ∆h≈const Trees establish in elevated & P-rich areas
Pin= T +
Background rate of P
deposition
=2.0 kg P ha-1
P deposition
(m)
=0.2 kg P ha-1
(m)
Stable Unstable Alternative stable States
Ghost Island
In the long term: dependence on the soil feedback
1 2
( )( )
d hT h k
dt
h hw
∆h=h-hw
'kTh eqeq
Ground Surface
At Equilibrium:
0.5 1 1.5 2 2.5
0
0.2
0.4
0.6
0.8
1
Teq B
0.5 1 1.5 2 2.5
0
0.2
0.4
0.6
0.8
1
Teq A
losssoil
accretionsoil
2
1
Marsh
Tree Island
State of the system
Pote
nti
al f
un
ctio
n Bistable System
Tree Island Marsh
0 1 2 3 4 5
0.4
0.45
0.5
0.55
Bistable Landscape
Tree Islands & Marshes Prairies
Stable
Marshes & Prairies
Bistable Landscape
(Tree Islands & Marshes)
Stable Marshes
losssoil
accretionsoil
2
1
Background levels of P Deposition
How do tree islands get established? • Trees colonize marshes/prairies during prolonged droughts
• Rock outcrops provide microsites for tree establishment
Feedbacks Bistability Process-based dimensional Model
• Extend the zero point model to incorporate spatial dynamics
• Pattern formation?
T tree biomass ccT Carrying capacity for trees
(depends on hydroperiod only)
1 2
( )( )
d hT h k
dt
( , )
cc
dT x taT T T
dt
2 2' '
' '
1 2 3
1 2
1 1
exp exp ( , )cc
x x x xr r
r rdTaT T T b b b T x t dx
dt d d
khTdt
hd
21
)(
2 2' '
' '
1 2 3
1 2
1 1
exp exp ( , )
x x x xr r
r rb b b T x t dx
d d
P P P P
“Evapotranspirational Pumping “ (Ross et al., 2006)
Guano Deposition (Frederick and Powell, 1994)
Magnitude of facilitation inhibition
Determine the distance at which maximum inhibition occurs
Limited by atmospheric phosphorus deposition
2 2' '
' '
1 2 3
1 2
1 1
exp exp ( , )
x x x xr r
r rb b b T x t dx
d d
Advection
0
0.25
0.5
flow
There is also a general hydraulic gradient across
the landscape
Atmospheric Deposition of P
Canopy trappin
g Enhanced P Deposition
• Small time step and long runs to account for soil feedbacks.
• Patterns often depend on initial distribution of elevation and vegetation.
• Changes in soil elevation effectively limit the spreading of trees.
• These mounds are stable in location.
• Under low anisotropic conditions, elliptical islands form
• These islands migrate slowly in the down flow direction.
• Under medium anisotropic conditions, elongated islands form
• These islands migrate down flow direction.
• Under high anisotropic conditions, the islands migrate and extend until the form into full bands, in general here the rate of island migration exceeds the rate of elevation loss.
• Under high anisotropic conditions, with moderately low atmospheric phosphorus input the islands migrate and extend but are unable to from full bands.
• Under high anisotropic conditions and low atmospheric P deposition the islands migration “pressure” exceeds the ability of vegetation to stabilize and raised elongated treeless islands form.
• These islands migrate only so long as they have trees, and treeless islands are eventually lost.
Conclusions
Acknowledgement:
Steve Oberbauer, Michael Ross & Jay Sah (FIU)
Supported by NPS-Everglades National Park (CESI) Grant # #H5284080004
• Feedbacks between Trees and P Deposition & Soil Accretion may lead to bistable landscapes
• In the long run Tree Islands and Marshes are alternative equilibria
• The state of elevated island with no trees is a transient (short term) feature
• Tree island are a metastable state coexisting with the marsh state only limited resilience
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