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The Fate of the Land Carbon The Fate of the Land Carbon SinkSink
Stephen W. PacalaStephen W. PacalaDirector, Princeton Environmental InstituteDirector, Princeton Environmental Institute
Petrie Professor of EcologyPetrie Professor of Ecology
Failure of US climate legislation has condemned all of us to 10 years of additional delay.
450 ppmv is now not feasible.500 ppmv is closest feasible target.550 ppmv is more likely even if we succeed at the next likely opportunity.
- 444Pg+218Pg
Catastrophe from a global failure of CO2 fertilization at double pre-industrial CO2. Shevliakova et al. PNAS (2011)
Fertilization Persists Fertilization Fails
Total CO2 emissions
Atmosphere
Data: NOAA, CDIAC; Le Quéré et al. 2009, Nature Geoscience
CO2 P
artit
ioni
ng (P
gC y
-1)
1960 20101970 1990 20001980
10
8
6
4
2
Key Diagnostic of the Carbon CycleEvolution of the fraction of total emissions that remain in the atmosphere
Fate of Anthropogenic CO2 Emissions (2000-2008)
Le Quéré et al. 2009, Nature Geoscience; Canadell et al. 2007, PNAS, updated
1.4 PgC y-1
+7.7 PgC y-1
3.0 PgC y-1
29%
4.1 PgC y-1
45%
26%2.3 PgC y-1
Pan et al. 2011 Science 333. Synthesis of global forest inventory data.
Roughly half the missing sink is due to CO2
fertilization.
The other half is due to land use.
The land use sink will diminish through time. What about the CO2 sink?
Physiology of CO2 Fertilization
Le Chatelier's principle:
6CO2 + 6H2O C6H12O6 + 6O2
Increased water use efficiency:
Less stomatal opening needed for the same flux of CO2 in = less water loss per carbon gained.
The sink caused by CO2 fertilization should be:
1.Impeded by N-limitation (Liebig’s Law of the
Minimum).
2. Favored by water limitation.
Nitr
ogen W
ater
Can
opy
and
cano
py a
irS
oil/
snow
Atm
osph
ere
Pho
tosy
nthe
sis
Pla
nt a
nd s
oil r
espi
rati
on
En
ergy
an
d m
oist
ure
bal
ance
Car
bon
up
tak
e an
d r
elea
set~ 30 min
fine
ro
ots
Energy, water and carbon exchange
leav
essa
pwoo
dla
bile
woo
d
Car
bon
allo
cati
on a
nd g
row
th, t
~ 1
day
Phe
nolo
gy, t
~ 1
mon
th
Mor
tali
ty, n
atur
al a
nd f
ire
t ~ 1
yea
r
Bio
geog
raph
y, t
~ 1
year
Lan
d-us
e m
anag
emen
t, t ~
1 y
ear
Climate statistics
Carbon gain
Plant typeLAI, height,roots
Vegetation dynamics
Predictions of Global Biosphere Models
LM3V: Shevliakova, Milly, Pacala, Malyshev, Hurtt, Stoffer and many others.
Will the sink fail?
Current models of the global biosphere uniformly predict a large and persistent CO2 fertilization sink.
All models predict the water use efficiency benefit.
Many models lack an N cycle so they could not predict that N-limitation will stop the sink.
Those with an N cycle predict a sustained sink in the tropics because of N-fixing trees, and a weak or absent sink, because of Liebig’s Law, where N-fixers are absent.
Effects of N cycle on residual sink (C-only minus C-N)
CO2 fertilization is predicted to be N-limited in the high latitudes because of the absence of symbiotic N-fixing trees. Gerber et al. GBC 2009.
• “The danger in creating fully detailed models of complex systems is ending up with two things you don’t understand – the model and the system.”
Phillip England. Nature (2011) 469:38.
• “Give me four free parameters and I’ll make you and elephant. Give me a fifth and I make it wiggle its trunk.”
Attributed to J. von Neumann by Freeman Dyson. Nature (2004) 427.
But modern models of the global biosphere are extraordinarily complicated…
• 100’s of operational decisions = 100’s of free parameters in global biosphere models.
• If not for the crisis, I wouldn’t be ready to build such a model for many decades or a century or more.
• To design LM3, I had to loosen the scientific standards I use elsewhere.
Never have so many been asked to predict so much while knowing so little…
Friedlingstein et al. 2006. J. Climate 19: 3337–3353.
tuning
Then why do global biosphere models seem to get the right answer?
Cornucopia whenindependent models leave the tuning data
Duke FACE plus > 2 dozen others.
What do experiments tell us?
Norby et al. 2011 Ann Rev Eco. Syst. 42.:
Some FACE experiments show a persistent sink from enhanced wood growth (i.e. Duke) while others show a weak sink because trees invest primarily in short–lived tissues, especially fine roots (ORNL).
NPP is enhanced despite N-limitation.
Penulas et al. 2011 Global Ecology and Biogeography:
Tree wood growth has not been enhanced because of water saved.
Will the sink fail?
Global models correctly predict the CO2 fertilization of net photosynthesis seen in the Face experiments.
However, because they simply apply leaf-level relationships to the globe, they predict neither the observed persistence of the sink under N limitation nor the absence of the sink under water limitation. Instead they predict the opposite.
The problem must be in the scaling: the extrapolation from leaf to grid cell.
Mountain Pine Beetle Infestation of >160,000 km2 mixed conifer
forest in British Columbia
Dendroctonus ponderosae
Expected to reach ~375,000 km2 and release ~270 MtC (Kurz et al. 2008. Nature 452:887-890).
Correct Scaling in Forest Stand Simulators
Strategies to sustain the forest economy.
Salvage logging for < 15 years.
Regenerated pine after 35-50 years.
Economic collapse and depopulation from 15-35+ years.
In many stands, saplings of interior spruce and subalpine fir survive as advance regeneration.
Could these produce new spruce/fir stands that would fill the gap?
SORTIE and subsequently other forest stand simulators said yes. Empirical studies of
natural successional sequences confirmed.
Law passed in June 2008 prohibiting salvage logging of stands above a model-calculated threshold of spruce/fir advance regeneration.
Analytically Tractable Stand Simulator (Strigul et al. 2008. Ecol. Mono. 78 (4): 523-545)
Let canopy height z* be defined by:
1.0 N i(z, t) i(z*,z) dz
z*
i1
Number Species
where Ni (z,t) is the density of trees of species-i and height z at time t and αi(z*,z) is the tree’s crown area at height z*. Then:
N i(z,t)t
Gi(z,z
*, t)N i(z, t)
z i(z,z
*, t)N i(z, t)
N i(z0, t) Fi(z,z*, t)N i(z, t) dz
z0
/Gi(z0,z*,t)
N i(z, t)t
Gi(z,z
*, t)N i(z, t)
z i(z,z
*, t)N i(z, t)
N i(z0, t) Fi(z,z*, t)N i(z, t) dz
z0
/Gi(z0,z*, t)
3
2*
3
2*
*
*
2ln;
2lnˆ
,)(
,)(
**
L
L
D
D
L
L
D
D
DDG
DG
L
DG
D
GFGD
GFGHz
DDifeG
FDN
DDifeG
FDN
L
L
D
D
D
D
Analytics:
Equilibrium:
Stability criterion:
always met for reasonable parameters.
2*
D
D
GD
i
iL
iLii
jiD
jiDiij
GF
L
LGHz
3,
2,
,
,* 2ln
)(
)(ˆ
Analytical Condition for the Success of a Rare invading Species in an Equilibrium
Monoculture of a Resident Species.Adams et al. (2007), Strigul et al. (2008)
Species-i can invade species-j if : ** ˆˆ iiij zz
Species-j can invade species-i if : ** ˆˆ jjji zz
10
15
20
25
30
35
40
0 20 40 60 80 100
predicted
observed
stand age
Stand Successional
Chronosequence
Individual Growth
Mortality
Fit Predict
basa
l are
a
Forests in the Minnesota, Wisconsin and Michigan. Purves et al. (2008A,B)
FIA Forest Inventory Data
change in basal area from 15 to 100 years
-10
-5
0
5
10
15
20
Pin
u.b
an
k
Ac
er.
rub
r
Be
tu.p
ap
y
Po
pu
.tre
m
Pru
n.s
ero
Qu
er.
elli
Qu
er.
rub
r
Qu
er.
ve
lu
predictedobserved
8 most common species
chan
ge in
bas
al a
rea
Purves, Lichstein, Strigul, & Pacala. 2008. PNAS
xeromesic mesic hydromesic
species
-10
0
10
20
Ab
ie.b
als
Ac
er.
rub
r
Ac
er.
sa
cc
Be
tu.p
ap
y
Po
pu
.gra
n
Po
pu
.tre
m
Qu
er.
rub
r
Tili
.am
er
-10
-5
0
5
10
15
20
Pin
u.b
an
k
Ac
er.
rub
r
Be
tu.p
ap
y
Po
pu
.tre
m
Pru
n.s
ero
Qu
er.
elli
Qu
er.
rub
r
Qu
er.
ve
lu
predictedobserved
-10
0
10
20
Ab
ie.b
als
La
ri.la
ri
Pic
e.m
ari
Th
uj.o
cc
i
Ac
er.
rub
r
Be
tu.p
ap
y
Fra
x.n
igr
Po
pu
.tre
m
change in basal area from 15 to 100 years
Purves, Lichstein, Strigul, & Pacala. 2008. PNAS
Red Maple Paradox
Abrams (1998)
Too Little Cedar, Too
Much Black Ash and Red Maple
If the model is correct:• On mesic soils, red maple should be gaining on
sugar maple.• On wet soils, red maple and black ash should be
gaining on cedar.
Lichstein, Purves, & Pacala (in preparation)
ecosystem:• biomass• NPP
individual:• growth• mortality
physiology:• photosynthesis• respiration
current PPA
next-generation global model
lightwaternitrogenCO2
temp.
ESS Analysis:VMAX
Specific Leaf Area
Leaf Longevity
Leaf Nitrogen
Wood density
Height Allometry
Crown Allometry
LAI
Fine root area
Fecundity
Carbohydrate Storage
Seed size
Litter Chemistry
Soil Water Hydrology Submodel
Photosynthesis & Transpiration
Submodels
Light
Root Uptake Submodels
N-Cycling Submodel
Stem Mass = αsDν+1
Hei
ght
= α
zDν-
1
Crown Area = αcDν
N Deposition & Fixation
Rain E&T
Lit
ter
Plant Strategy
Fitness
Optimal StrategiesMeritocracy
Competitive strategies
Inva
sion
Pot
entia
l
Invader successful
Invader unsuccessfulResident
Tournament With No Fair Play.
Inva
sion
Pot
entia
l
Invader successful
Invader unsuccessfulResident
Tournament With No Fair
Play.
Competitive strategies
Inva
sion
Pot
entia
l
Invader successful
Invader unsuccessfulResident
Ecologically or evolutionarily stable strategy
Tournament With No Fair
Play.
Competitive strategies
Empirical Fingerprints of Competitive EquilibriumGersani et al. 2001 – Soybean plants
Poi
nts
= F
LUX
NE
T d
ata
Line
s =
Mod
el p
redi
ctio
ns
1. As N-availability changes:
No tradeoff between leaves and roots, but a dramatic tradeoff between wood and roots.
Dybzinski et al. (2010)
ESS Predictions
• Leaves increase with N
• Roots increase with water addition
• Water and N have a significant interaction
* ** *
2. Complex results from simple experiments.(Farrior, Tilman and Pacala, in prep.)
ESS Predictions
4. N-fixing canopy trees are common in the tropics but absent at higher latitudes, even though temperate and boreal ecosystems are thought to be the most N-limited.
3. As N-mineralization increases, leaf N increases within each species, but leaf N of the dominant species decreases (Dybzinski et al. in review).
ESS Predictions
So what does the model predict about the effects of nitrogen and water limitation on a CO2 fertilization sink?
1. Add fines root to rare invader that competes better for N. 2. Build more leaf with extra N (most shaded leaf).3. If carbon gain from new leaf > cost of new root + new leaf
then:
More N
Light
Nitrogen
+ C-gain
1. Replace resident strategy with invader .2. Repeat until the cost of a change in strategy is always less
than benefits.
Nitrogen Limited ESS = fine roots that exactly cancel the net carbon gain of the most shaded leaf.
Prediction for CO2 Fertilization with Limited N (Dybzinski et al. in prep.)
Elevated CO2 increases net photosynthesis, increasing the value of the most shaded leaf.
Competitive optimal strategy is to add fine roots that exactly cancel this value.
Extra investment in new fine roots is small if the understory is dark because the most shaded leaf has little value. Most extra C goes to wood, so big sink.
Reverse if understory is light. Then most extra C goes to short-lived fine roots, so small sink.
Prediction for CO2 Fertilization with limited N – a Weakening of Liebig’s Law
Elevated CO2 creates a large long-lived sink if LAI is relatively large (understory is dark), but not if LAI is relatively small.
LAI increases with N-mineralization.
N-limitation
Severe Present
Extra C goes mostly to fine
roots.
Extra C goes mostly to
wood.
1. Add root to rare invader that competes better for water. 2. Photosynthesis is proportional to transpiration.3. If carbon gain from extra photosynthesis > cost of new root
then:
More Water
Light
Water
Extra C-gain Prop. to
Extra Water
1. Replace resident strategy with invader .2. Repeat until the cost of a change in strategy is always less
than benefits.
Water Limited ESS = roots that exactly cancel the net carbon gain of the ENTIRE CANOPY during water-limited periods.
Prediction for CO2 Fertilization with Limited Water (Farrior et al. in prep.)
Elevated CO2 increases net photosynthesis, increasing the value of EVERY leaf.
Competitive optimal strategy is to add fine roots, whose cost exactly cancels this increase in value.
Thus all of the extra carbon from CO2 fertilization during periods of water limitation goes to short lived fine roots and does not create a large sink.
Conventional Wisdom: Sustained carbon sinks caused by CO2 fertilization are more likely if water is limiting and less likely if nitrogen is limiting.
Our Model Predictions: Exactly the opposite.
Norby et al. 2011 Ann Rev Eco. Syst. 42.:
NPP is enhanced in FACE experiments despite N-limitation.
Some FACE experiments exhibit a strong persistent sink because of increased wood growth and limited fine root proliferation despite N-limitation. Others show a weak sink and have large fie root proliferation.
Penulas et al. 2011 Global Ecology and Biogeography:
Tree wood growth has not been enhanced because of water saved.
What does this mean for the globe?
The answer depends upon the mix of water limitation, severe N-limitation and relatively weak N-limitation.
My guess is that sites that N-limited sites with relatively high LAI are responsible for most global NPP.
If so, we predict a long-lived global sink.
CONCLUSIONS
1.The future of humanity literally depends upon the future of the carbon sink.
2.Existing global models do not predict the observed failure of the sink under water limitation and the persistence of the sink under nitrogen limitation.
3.The observed responses are predicted as the most competitive strategies (Nash Equilibria).
4.These strategies happily imply a large and long-lived global benefit from CO2 fertilization.