aiacc biodiversity course ecosystem function modelling bob scholes csir environmentek, pretoria...
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AIACC Biodiversity course
Ecosystem function modelling
Bob Scholes
CSIR Environmentek, Pretoria
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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]
AIACC Biodiversity course
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]
AIACC Biodiversity course
Linear relation betweengrass production and rainfall
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100
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300
400
500
0 200 400 600 800 1000
Annual rainfall
Gra
ss A
G N
PP
(g
/m2/
y)
clay soil
sand soil
AIACC Biodiversity course
Slope: Rain Use Efficiency(g/m2/mm)
0
2
4
6
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10
12
50 60 70 80 90 100
Sand %
Rain
use e
ffic
ien
cy (
kg
/ha/m
m)
AIACC Biodiversity course
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
)
AIACC Biodiversity course
Effect of trees on grass
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0.20
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0.40
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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
AIACC Biodiversity course
Maximum tree basal area
AIACC Biodiversity course
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
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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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
What controls tree mortality?
• Fire• Elephants
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0.050
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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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
Test 1: trees, grass and fire
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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
AIACC Biodiversity course
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)
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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
AIACC Biodiversity course
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