The Fate of the Land Carbon Sink

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

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~ 1

mon

th

Mor

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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

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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

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L

LGHz

3,

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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

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predictedobserved

8 most common species

chan

ge in

bas

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rea

Purves, Lichstein, Strigul, & Pacala. 2008. PNAS

xeromesic mesic hydromesic

species

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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.