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Modeling Nutrient Limitation: Ecosystem Consequences of
Resource Optimization
Nature should weed out sub-optimal strategies of acquiring resources from
the environment
E.B. RastetterThe Ecosystems CenterMarine Biological Laboratory Woods Hole, MA 02543
Captiva Island Meeting March 2011
U1 = g B f(R1) f(T)Uncoupled:
U1 = g B min{f(R1), f(R2), f(R3)...} f(T)U2 = q2 U1
U3 = q3 U1...
Liebig Limitation:
U1 = g B {f(R1) f(R2) f(R3)...} f(T)U2 = q2 U1
U3 = q3 U1...
Concurrent Limitation:
Strategies for modeling resource acquisition:
0
100
200
0
100
200
0
100
200
Len
gth
of
pri
mar
y la
tera
l ro
ots
(%
co
ntr
ol)
PO4 NO3 K NH4
Nutrient limiting in all layers
Nutrient supplied to
top and bottom layers
Nutrient supplied to
middle layer
0 - 4 cm
4 - 8 cm
8 - 12 cm
Data from Drew 1975
0
0.25
0.5
0.75
1
0.2 0.4 0.6 0.8 1
N P S
0
0.25
0.5
0.75
1
0.2 0.4 0.6 0.8 1
N P S K Mg Mn
Data from Wikström and Ericsson 1995
1
0.25
0.5
0.75
00.2 0.4 0.6 0.8 1
Concentration of nutrient in the plant(fraction of optimum)
Ro
ot:
sho
ot
rati
o
Response of birch seedlings to element limitation
0
10
20
30
40
Wet sedge Toolik inlet
Wet sedge Toolik outlet
Tussock
Root
NPP
as %
of t
otal
NPP
Control
fertilized
Nadelhoffer et al. 2002
0
5
10
15
20
25
30
170 340 510 680
CO2 Concentration (ppm)
Ph
oto
syn
thes
is (
m
ol m
-2 s
-1)
680 ppm
340 ppm
Response of an arctic cotton grass to elevated CO2
data from Tissue & Oechel 1987
U1 = g1 B V1 f(R1) f(T)Optimized:
U2 = g2 B V2 f(R2) f(T)U3 = g3 B V3 f(R3) f(T)
...
V1 + V2 + V2 ... = 1
U2 = q2 U1
U3 = q3 U1...
Maximize U1 under the constraints that
Ui = gi B Vi f(Ri) f(T)
Adaptive (Optimizing):
dVi/dt = a ln{Φ qi U1/Ui } Vi
Select Φ so that ∑dVi/dt = 0 (i.e., ∑Vi = 1):
q1 ≡ 1
Φ = π(Ui/(qi U1))Vi
Uncoupled: U1U = g B f(R1) f(T)
Liebig: UiL = qi g B min{f(R1), f(R2), f(R3)} f(T)
Concurrent: UiC = qi g B {f(R1) f(R2) f(R3)} f(T)
Optimized: UiO = gi B Vi f(Ri) f(T)
If f(R1) doubles: U1U 2×UiL ≥ 0×, ≤ 2×UiC 2×UiO > 0×, < 2×
Comparison of responses for 3-resource models:
U1U 2×UiL 2×UiC 8×UiO 2×
If all three f(Ri) double:
Uncoupled: U1U = g B f(R1) f(T)
Liebig: UiL = qi g B min{f(R1), f(R2), f(R3)} f(T)
Concurrent: UiC = qi g B {f(R1) f(R2) f(R3)} f(T)
Optimized: UiO = gi B Vi f(Ri) f(T)
Comparison of responses for 3-resource models:
PlantsBC: 43550
BN: 74BP: 11
InorganicEN: 2.6EP:0.26
Microbes and soil organic
matterDC: 19960
DN: 420DP: 42
UN: 6.5UP: 1.2
INF: 0.28IND: 0.2IP: 0.05
LC: 770LN: 6.5LP: 1.2
QN: 0.014QP: 0.025
QOC: 255QON: 0.47QOP: 0.025
UmN: 19.98UmP: 1.387
MN: 26MP: 2.6
Rm: 515
Pn: 770UNfix: 0
Based on Sollins et al. 1980HJ Andrews Forest
Rastetter 2011
1010T
aC
aCmaxAC Q
Ck
CVPBU
1010T
iN
iNNmaxAN Q
Nk
NVUBU
1010T
iP
iPPmaxAN Q
Pk
PVUBU
ii
ii VU
Ra
td
Vd
ln
PNC V
P
P
V
N
N
V
C
C
R
U
R
U
R
U
Adaptive:
ii
i
R
U
At steady state:
1.18
1.20
1.22
1.24
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)
2 x CO2
600
700
800
900
1000
1100
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)
2 x CO2
600
700
800
900
1000
1100
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)
2 x CO2
600
700
800
900
1000
1100
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)
2 x CO2
600
700
800
900
1000
1100
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
600
700
800
900
1000
1100
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
5.8
5.9
6.0
6.1
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
1.18
1.20
1.22
1.24
0 20 40 60 80 100
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)0
500
1000
1500
2000
0 20 40 60 80 100
2 x CO2 + 4oC
0
1
2
3
0 20 40 60 80 100
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)
2 x CO2 + 4oC
0
500
1000
1500
2000
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
66
02
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)0
500
1000
1500
2000
0 20 40 60 80 100
2 x CO2 + 4oC
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
Uncoupled Liebig
Concurrent Acclimating
Years
NP
P(g
C m
-2 y
r-1 )
Net
N
min
eral
izat
ion
(g
N m
-2 y
r-1 )
Net
P
min
eral
izat
ion
(g P
m-2
yr-1
)0
500
1000
1500
2000
0 20 40 60 80 100
2 x CO2 + 4oC
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
0
1
2
3
0 20 40 60 80 100
0
5
10
15
0 20 40 60 80 100
0
500
1000
1500
2000
0 20 40 60 80 100
66
02
Conclusions:
1. Acclimation toward optimal resource use adds additional dynamics that propagate through and interact with ecosystem resource cycles.
2. These dynamics reflect adjustments within the biotic components of the ecosystem to maintain a metabolic balance and meet stoichiometric constraints
3. These dynamics are not represented in either “Liebig’s Law of the minimum” or “Concurrent” models of resource acquisition.
4. Because of these additional dynamics, initial responses are not likely to reflect long-term responses, making long-term projections from short-term experiments or observations difficult.
5. The optimization of resource use will tend to synchronize ecosystem resource cycles in the long term unless disturbance resets the system.
6. These “acclimation” responses act on many time scales and include activation/deactivation of enzyme systems, allocation of resources to individual tissues, replacement of suboptimal species with other species with more “optimal” uptake characteristics, and even natural selection of more “optimal” uptake characteristics.
Conclusions:
6. Resource optimization in my model is simulated through the reallocation of an abstract quantity I call “effort” (Vi)
7. Because the allocation of “effort” represents many processes within the vegetation, it is difficult to quantify except in terms of the observed long-terms dynamics of the ecosystem; this is the model’s biggest weakness.
8. Currently the allocation of “effort” is tied to a single rate constant. Because of the many processes involved in acclimation, a formulation with several rate constants might be more realistic
9. Model description in Rastetter EB. 2011. Modeling coupled biogeochemical cycles. Frontiers in Ecology and the Environment 9:68-73.
10. Executable code, sample data files, and instructions are available at http://dryas.mbl.edu/Research/Models/frontiers/welcome.html