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

“Ghost Island”

Former tree island that has been deforested

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

Full representations of kernel function under different levels of anisotropy

0

0.25

0.5

• 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