1. The Fate of the Land Carbon Sink Stephen W. Pacala Director, Princeton Environmental Institute Petrie Professor of Ecology

2. 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.

3. - 444Pg+218Pg Catastrophe from a global failure of CO 2 fertilization at double pre-industrial CO 2. Shevliakova et al. PNAS (2011) Fertilization Persists Fertilization Fails

4. Total CO 2 emissions Atmosphere Data: NOAA, CDIAC; Le Quéré et al. 2009, Nature Geoscience CO 2 Partitioning (PgC y -1 ) 1960 20101970 1990 2000 1980 10 8 6 4 2 Key Diagnostic of the Carbon Cycle Evolution of the fraction of total emissions that remain in the atmosphere

5. Fate of Anthropogenic CO 2 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

6. Pan et al. 2011 Science 333. Synthesis of global forest inventory data. Roughly half the missing sink is due to CO 2 fertilization. The other half is due to land use. The land use sink will diminish through time. What about the CO 2 sink?

7. Physiology of CO 2 Fertilization Le Chatelier's principle: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Increased water use efficiency: Less stomatal opening needed for the same flux of CO 2 in = less water loss per carbon gained.

8. The sink caused by CO 2 fertilization should be: 1.Impeded by N-limitation (Liebig’s Law of the Minimum). 2. Favored by water limitation. Nitrogen Water

9. Canopy and canopy air Soil/snow Atmosphere Photosynthesis Plant and soil respiration Energy and moisture balance Carbon uptake and release t~ 30 min fine roots Energy, water and carbon exchange leaves sapwood labile wood Carbon allocation and growth, t ~ 1 day Phenology, t~ 1 month Mortality, natural and fire t ~ 1 year Biogeography, t ~ 1 year Land-use management, t ~ 1 year Climate statistics Carbon gain Plant type LAI, height, roots Vegetation dynamics Predictions of Global Biosphere Models LM3V: Shevliakova, Milly, Pacala, Malyshev, Hurtt, Stoffer and many others.

10. Will the sink fail? Current models of the global biosphere uniformly predict a large and persistent CO 2 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.

11. Effects of N cycle on residual sink (C-only minus C-N) CO 2 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.

12. “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…

13. 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…

14. Friedlingstein et al. 2006. J. Climate 19: 3337–3353. tuning Then why do global biosphere models seem to get the right answer? Cornucopia when independent models leave the tuning data

15. Duke FACE plus > 2 dozen others. What do experiments tell us?

16. 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.

17. Will the sink fail? Global models correctly predict the CO 2 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.

18. Mountain Pine Beetle Infestation of >160,000 km 2 mixed conifer forest in British Columbia Dendroctonus ponderosae Expected to reach ~375,000 km 2 and release ~270 MtC (Kurz et al. 2008. Nature 452:887-890). Correct Scaling in Forest Stand Simulators

19. 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?

20. 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.

21. Analytically Tractable Stand Simulator (Strigul et al. 2008. Ecol. Mono. 78 (4): 523-545) Let canopy height z * be defined by: where N i (z,t) is the density of trees of species-i and height z at time t and α i (z *,z) is the tree’s crown area at height z *. Then:

23. Analytics: Equilibrium: Stability criterion: always met for reasonable parameters.

24. 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 : Species-j can invade species-i if :

25. stand age Stand Successional Chronosequence Individual Growth Mortality FitPredict basal area Forests in the Minnesota, Wisconsin and Michigan. Purves et al. (2008A,B) FIA Forest Inventory Data

26. change in basal area from 15 to 100 years 8 most common species change in basal area Purves, Lichstein, Strigul, & Pacala. 2008. PNAS

27. xeromesic mesic hydromesic species 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

28. 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)

29. ecosystem: biomass NPP individual: growth mortality physiology: photosynthesis respiration current PPA next-generation global model light water nitrogen CO 2 temp.

30. ESS Analysis: V MAX 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 = α s D ν+1 Height = α z D ν-1 Crown Area = α c D ν N Deposition & Fixation RainE&T Litter

31. Plant Strategy Fitness Optimal Strategies Meritocracy

32. Competitive strategies Invasion Potential Invader successful Invader unsuccessful Resident Tournament With No Fair Play.

33. Invasion Potential Invader successful Invader unsuccessful Resident Tournament With No Fair Play. Competitive strategies

34. Invasion Potential Invader successful Invader unsuccessful Resident Ecologically or evolutionarily stable strategy Tournament With No Fair Play. Competitive strategies

35. Empirical Fingerprints of Competitive Equilibrium Gersani et al. 2001 – Soybean plants

36. Points = FLUXNET data Lines = Model predictions 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

37. 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

38. 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

39. So what does the model predict about the effects of nitrogen and water limitation on a CO 2 fertilization sink?

40. 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

41. 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.

42. Prediction for CO 2 Fertilization with Limited N (Dybzinski et al. in prep.) Elevated CO 2 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.

43. Prediction for CO 2 Fertilization with limited N – a Weakening of Liebig’s Law Elevated CO 2 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 SeverePresent Extra C goes mostly to fine roots. Extra C goes mostly to wood.

44. 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

45. 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.

46. Prediction for CO 2 Fertilization with Limited Water (Farrior et al. in prep.) Elevated CO 2 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 CO 2 fertilization during periods of water limitation goes to short lived fine roots and does not create a large sink.

47. Conventional Wisdom: Sustained carbon sinks caused by CO 2 fertilization are more likely if water is limiting and less likely if nitrogen is limiting. Our Model Predictions: Exactly the opposite.

48. 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.

49. 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.

50. 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 CO 2 fertilization.