aiacc biodiversity course ecosystem function modelling bob scholes csir environmentek, pretoria...

AIACC Biodiversity course

Ecosystem function modelling

Bob Scholes

CSIR Environmentek, Pretoria

[email protected]


Aspects and levels of biodiversity

Composition: what it contains

Structure:what it looks like Function: how it works

Gene

Population

Ecosystem

Biome envelope modelsSpecies niche models

Ecosystem functionmodels


Dynamic Vegetation Models

• All the major Global Circulation Models have ‘coupled’ models of the land surface– Simulate carbon uptake/loss, albedo, bulk

stomatal conductivity, surface roughness– Have a crude representation of biomes or

‘functional types’– Some of the better ones (LPJ, Sheffield)

have fire and mammals in them


DGVMs continued…

• Problems:– Very hard to use unless you have a

supercomputer– Results are not freely available, unlike the

GCM outputs– Mostly not optimised for Africa– Scale is inappropriate for protected areas– Equations are complex and untransparent


A ‘reduced form’ ecosystem model for savannas under climate change

• ‘Functional types’ are restricted to those occuring in savannas (but are expanded beyond the generic global types)

• Includes effects of temperature, rainfall, seasonality, CO2, soil texture, fire and megaherbivores

• ‘Quasi-mechanistic’ equations– Simple, reduced forms based on emergent

properties at ecosystem scale• Timestep of one year (‘implicit seasonality’)

and ‘patch’ spatial scale


Temperature

CO2

Rainfall

Sand %

Tree ht & BA

Sour grass

Tree prodnMixed

Browsers

Coarse graz

Fine grazers

Carnivore

Fire intens

Sweet grass

Elephants

Fire freq

Basic savanna system model


Water balance modelling

G = Rain/E0 * daysmonth if Rain<E0, else R/E= 1months

E0 = open water evaporation ~ 0.8 x {700(T+0.006A)/(100-L)+15(0.0023A+0.37T*0.53(TxTn) +0.35Tann-10.9))/(80-Tx)}

T = mean air temperature ( C)Tx= max air tempTn=min air tempA=altitude (m)L=latitude (deg) [Linacre 1984]


Controls on grass growthat the annual timescale

• Rainfall in the current growing season– Actually, it is the duration of growth opportunity

that matters– This is affected by evaporation as well as rain,

and is mediated by soil texture

• The fertility of the soil• The amount of tree cover• Daytime temperature• [CO2]


Linear relation betweengrass production and rainfall

0

100

200

300

400

500

0 200 400 600 800 1000

Annual rainfall

Gra

ss A

G N

PP

(g

/m2/

y)

clay soil

sand soil


Slope: Rain Use Efficiency(g/m2/mm)

0

2

4

6

8

10

12

50 60 70 80 90 100

Sand %

Rain

use e

ffic

ien

cy (

kg

/ha/m

m)


Intercept dependent on soil water holding capacity

covaries with the rain use efficiency

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

0 5 10 15

Rain use efficiency (kg/ha/mm)

Inte

rcep

t o

f A

GG

NP

P v

s R

ain

(kg

/ha/y

)


Effect of trees on grass

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.000 5.000 10.000 15.000 20.000

Tree basal area (m2/ha)

Fra

ctio

n o

f tr

eele

ss g

rass

NP

P

P = P0 * e-0.1BA


Maximum tree basal area


What controls the growth rate of trees?

• Size of the tree• Competition with other trees

y = 0.9998x

R2 = 0.6487

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.00 2.00 4.00 6.00 8.00 10.00

Observed Relative Annual Increment (%)

Pre

dic

ted

Rel

ativ

e A

nn

ual

In

crem

ent

(%)

Ptree = 1+ 19 e -0.2d * e -3(A/Amax) where A = basal area, d = diameter (cm)

Shackleton data


Effect of CO2 on NEP

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800

CO2 in atmosphere

Ret

ativ

e p

erfo

rman

ce

grass

trees

F (CO2) = 1+ ln([CO2]/[CO2ref]

~ 0.4 for trees, 0.2 for grass[CO2ref] = 360 ppm


Effects of temperature on NEP

0.000

0.200

0.400

0.600

0.800

1.000

1.200

0 10 20 30 40 50

Mean daytime temperature (C)

Rel

ativ

e p

erfo

rman

ce

trees

grass

ƒ[T] = ec*(1-{[(b-T)/(b-a)]^d }/d *(b-T)/(b-a)c

a = position of optimum ~ 28°C for trees, ~33°C for grassesb =temperature below which no growth occurs ~5C trees, 10C grassc = steepness of curve below optimum ~3d = steepness of curve above optimum ~7


What controls tree mortality?

• Fire• Elephants

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 50 100 150 200 250

Gdays

Fra

ctio

n o

f la

nd

scap

e b

urn

ed


Mammal dynamics

dN/dt = rN - offtake

r = rmax * fN/cPrey biomass

Ln(rmax) = 3.269-0.00081 Body mass

f = food requirement (kg/head/d)

C = fraction of prey biomass than can be consumed in a year


Keeping it together!

• Complete competitors cannot coexist– Give each herbivore a partly unique

resource

• The faster-growing prey must be more prefered by predators– Preference = N2/N2

• Predators must grow slower than prey


Test 1: trees, grass and fire

0

10

20

30

40

50

60

2000 2020 2040 2060 2080 2100

Year

Pla

nts

(m

2/h

a),

(g/m

2),

(m)

Basal Area

Grass prod

Tree ht


Test 2: +herbivores, carnivores

0

500

1000

1500

2000

2500

3000

2000 2020 2040 2060 2080 2100

Year

Herb

ivore

s (kg

/km2)

Elephants

Coarse

Fine

Browser

Mixed

0

5

10

15

20

25

2000 2020 2040 2060 2080 2100

Year

Ca

rniv

ore

s (

kg

/km

2)


Test 3: + elephants

0

1000

2000

3000

4000

5000

2000 2020 2040 2060 2080 2100

Year

He

rbiv

ore

s (

kg

/km

2)

Elephants

Coarse

Fine

Browser

Mixed

0

10

20

30

40

50

60

2000 2020 2040 2060 2080 2100

Year

Pla

nts

(m

2/h

a),

(g/m

2),

(m)

Basal Area

Grass prod

Tree ht


The experiment design

B2 Scenario

Hadley model

550 ppm

A2 Scenario

Hadley Model

Upper estimate

+5C, -6% rain

B2 Scenario

CCModel

Lower estimate

+2.2C, -1.2%

A2 Scenario

CCModel

700 ppm


Change in production drivers

0.0

0.5

1.0

1.5

2.0

2000 2020 2040 2060 2080 2100

Year

Pro

du

cti

on

dri

ve

rs

Water balance

f(CO2)g

f(CO2)t

f(T)g

f(T)t

0.0

0.5

1.0

1.5

2.0

2000 2020 2040 2060 2080 2100

YearP

rod

uc

tio

n d

riv

ers

Water balance

f(CO2)g

f(CO2)t

f(T)g

f(T)t


Change in vegetation structure

0

10

20

30

40

50

60

2000 2020 2040 2060 2080 2100

Year

Plan

ts

(m2/

ha),(

g/m

2),(m

)

Basal Area

Grass prod

Tree ht

0

10

20

30

40

50

2000 2020 2040 2060 2080 2100

Year

Plan

ts (m

2/ha

),(g/

m2)

,(m)

Basal Area

Grass prod

Tree ht


Change in herbivores

0

500

1000

1500

2000

2500

3000

2000 2020 2040 2060 2080 2100

Year

Her

biv

ore

s (k

g/k

m2)

Elephants

Coarse

Fine

Browser

Mixed

0

500

1000

1500

2000

2500

2000 2020 2040 2060 2080 2100

Year

Her

bivo

res

(kg/

km2)

Elephants

Coarse

Fine

Browser

Mixed


Preliminary conclusions

• Water and temperature effects can overwhelm the CO2 effect

• Substantial changes in herbivore stocking rate are possible in the future

• Elephants at high density put the tree cover into a stable coppice state

• The outcome of climate-change induced habitat change depends on how you manage fires and elephants

aiacc biodiversity course ecosystem function modelling bob scholes csir environmentek, pretoria...

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