Mechanisms of Current Terrestrial Carbon Sinks and Future Persistency

1. Mechanisms of Current Terrestrial Carbon Sinks and Future Persistency  Josep Canadell GCP and GCTE International Office Canberra, Australia [Email: pep.canadell@csiro.au]

2. Outline • Distribution and strength of terrestrial sinks • Candidate sink mechanisms • Where IPCC-2001 left the issue • US terrestrial sink case study • Mechanisms: present and future stability • Land use change legacy • Fire suppression • Woody encroachment • Climate change • CO2 fertilization • Nitrogen fertilization • Reforestation • Surprises through changes in biodiversity

3. Terrestrial Carbon Sources and Sinks [1990’s] - 1.7 - 0.8 + 0.7 + 0.3 + 0.7 Pg C/yr IPCC 2001 Schimel 2001 Achard et al. 2002 Malhi& Grace 2000

4. Terrestrial Carbon Sinks [1990’s] Net Sink Pg C/yr - 1.7 - 0.8 - 0.7 - 0.3 - 0.7 Gross Sink

5. Terrestrial Biosphere C Sink Cramer et al. 2000 Why do we need to know the mechanisms? Future atmospheric CO2 concentrations and stabilization scenarios IPCC 2001

6. Sink Mechanisms – The 90’s understanding Early 1990’s: All due to CO2 fertilization(biogeochemistry models/Physiological) Mid 1990’s: There was more than CO2. N deposition with unrealistic uptake rates of up to 80%. Late 1990’s: Cropland establishment and abandonment, CO2 and Climate (IPCC 2001). Early 2000’s: All due to past land use practices (US-lead), (using forest demography and age structure). CO2 no effect.

7. Direct human induced Candidate Mechanisms of Current Terrestrial Sinks • CO2 fertilization • Nitrogen fertilization • Climate change • Regrowth in abandoned croplands • Regrowth in previously disturbed forests • Logging, fire, wind, insects • Fire suppression (woody encroach., forest thickening) • Decreased deforestation • Improved agriculture • Sediment burial • Future: Carbon Management (e.g., reforestation)`

8. McGuire et al. (2001) • In the 1980s • North extra-tropics: CO2: -0.2 to -1.6; Climate: +0.4 to –0.2; Land use: 0.0 to -0.4 • Tropics: CO2: -0.6 to -1.4; Climate:+ 0.7 to -0.1; Land use: +0.5 to +1.2 • The analyses included only 3 out of 10 sink mechanisms thought to be important. Global Sink Attribution by IPCC 2001 [1920-1992] S1 = CO2 S2 = CO2 + Climate* S3 = CO2 + Climate + Cropland Establishment and Abandonment** *Climate effect is inferred by S2 - S1 ** Land-use effect is inferred by S3 - S2

9. Sinks in the Coterminous U.S. [1980-90] PgC yr-1 Forest trees 0.15 Other forest 0.15 Cropland soils 0.04 Woody encroach. 0.13 Wood products 0.07 Reservoirs 0.04 Exports - Imports 0.09 US-Fixed expt.rivers 0.04 0.71 PgC yr-1 apparent U.S. Pacala et al. 2001

10. Sinks in the Coterminous U.S. [1980-90] PgC yr-1 Forest trees 0.15 Other forest 0.15 Cropland soils 0.04 Woody encroach. 0.13 Wood products 0.07 Reservoirs 0.04 Exports - Imports 0.09 US-Fixed expt.rivers 0.04 21 % of the total Sink due to trees

11. Sinks in the Coterminous U.S. [1980-90] PgC yr-1 Forest trees 0.15 Other forest 0.15 Cropland soils 0.04 Woody encroach. 0.13 Wood products 0.07 Reservoirs 0.04 Exports - Imports 0.09 US-Fixed expt.rivers 0.04 35% of the sink is susceptible CO2 and N deposition fertilization

12. Sinks in the Coterminous U.S. [1980-90] PgC yr-1 Forest trees 0.15 Other forest 0.15 Cropland soils 0.04 Woody encroach. 0.13 Wood products 0.07 Reservoirs 0.04 Exports - Imports 0.09 US-Fixed Ex.Rivers 0.04 32% of total Sink due to other less commonly accounted mechanisms

13. 1. Are the sink mechanisms permanent features? 2. Will they increase in strength? Sink Strength Sink Strength time time time time 3. Will they saturate? 4. Will they disappear? Sink Strength Sink Strength Future Dynamics of Carbon Sink Mechanisms

14. 2% Growth Rate 98% time Forest Inventory 1 Forest Inventory 2 Forest Regrowth in Abandoned Croplands 1980’s-1990’s • Eastern United States (5 states) • 98% of the C sink attributed to land use change: • Forest regrowth after crop abandonment • Reduced harvesting • Fire suppression • 2% remaining attributed to: • Increasing CO2 • Nitrogen Deposition • Climate Change Caspersen et al. 2000

15. Sink Strength due to Forest Regrowth 4 t 2 2 t Net Ecosystem Productivity (Mg.ha-1) 3 0 t 4 t 1 -2 0 20 40 60 80 100 Years Jiquan Chen, Univ of Toledo

16. Climate as a Driver of C Sinks in the U.S. 1950-1993/Biome-BGC 2/3 of forest growth rate explained by increased precipitation and extension of growing season due to warming 8% increase in precipt. [1.39 mm yr-1] No continental T change [increased in west and decreased on East] Decrease annual vapor deficit Nemani et al. 2002

17. Annual Flux of C (TgC yr-1) Houghton et al. 2000 Carbon Sink: Fire suppression Fire exclusion has increased C storage in forests [last 100 yrs] Total Area Burned (US) Eliminating fire completely, US forest could accumulated 2.6 Pg C by 2140 Photos: M. Flannigan [Canada]

18. Sinks, for how long and at which cost? Time Bomb Swetnam et al.

19. Increase after 1970 10 8 6 Area (million ha) 4 2 0 1920 1940 1960 1980 2000 ClearCut Fire Insects Total Disturbances in Canada’s forests [1920 – 1995] Kurz & Apps 1999

20. Decrease after 1970 400 Sink 300 200 Tg C / yr 100 0 -100 Source -200 1920 1940 1960 1980 2000 Variable Temp Constant Temp Net ecosystem C fluxes in Canada [1920 – 1995] Kurz & Apps 1999

21. Maximum Potential C sequestration in the absence of fire = 2 Pg C yr-1 (upper value) Scholes and Hal 1996 Woody Encroachment Woody plant encroachment has promoted C sequestration in grassland and savanna ecosystems of N and S America, Australia, Africa, and Southeast Asia over the past century. Estimated CO2 sink: USA: 0.17 PgC/yr for the 1980s (Houghton et al., 1999) NE Australia: 0.03 PgC/yr (Burrows, 1998) Photo: Martin 1975, Arizona 1903 & 1941

22. Carbon accumulation due to woody encroachment • There is a • Maximum limit. • We may be over- • Estimating C gain • in wet regions. Jackson et al. 2002 Goodale and Davidson 2002

23. Biomass Responses to Elevated CO2 Biomass Stimulation (%) GCTE Synthesis. Mooney et al. 1999

24. Growth Enhancement 200 400 600 800 1000 200 400 600 800 1000 CO2 concentration (ppm) Increasing aCO2 Effects on Plant Growth Canadell et al. (in preparation) Photo: R. Jackson [Texas, USA]

25. 600 ppm CO2 H2O Saturation of CO2 Increased Water Use Efficiency Stomatal acclimation - Solanum (C3 forb) gS (mol m-2 s-1) Intercellular [CO2] Jackson et al. 2002

26. NPP Responses to N fertilization Fossil-fuel N Deposition on Land (kg/km2) 10,000 1000 Net primary production(g C m-2 y–1) 100 10 1990 Townsend et al. 1996 Schlesinger 1997 1 10-2 10-1 100 101 102 103 Nitrogen input (g N m-2 y-1) Nitrogen Deposition • N deposition explains 100% of current sink 80%20%15% • (Holland et al. 1995, 97, Nadelhoffer et al. 1999, McGuire (in preparation)). • The fertilization effect reaches a saturation. • N deposition will not stimulate C uptake in the tropics (Hall & Matson 1999)

27. Reforestation: Annual Flux of Carbon in China [1850-2000] Houghton 2002

28. More realistic scenario: Half of the cropland returns to native 20 ppm 700 ppm (by 2100) down to 680 ppm Maximum potential of C sink with reforestation Historically, 450 Pg of C emitted (ff+lucc) (200 Pg from deforestation) 90 ppm (40 ppm from deforestation) Nothing-to-eat Scenario: 700 ppm (by 2100) down to 660 ppm Ramakutty & Foley 1999 Prentice et al. 2001

29. 1. Are the sink mechanisms permanent features? 2. Will they increase in strength? Sink Strength Sink Strength time time time time 3. Will they saturate? 4. Will they disappear? Sink Strength Sink Strength Future Dynamics of C Sink Mechanisms None Increased Precipitation (depending on timing of warming) Forest Regrowth CO2 fertilization Woody Encroachment N deposition Cropland Soils

30. Surprises

31. Increasing Dominance of Lianas in Amazonian Forest Lianas have increased 1.7-4.6% yr-1 relative to trees (over last two decades). Lianas increase mortality and decrease tree growth. Photo: R. Hays Cummins Tropical sink may decrease sooner than predicted. Phillips et al. 2002

32. 4 3 Native annuals Relative ratio (elevated/ambient CO2) Bromus 2 1 0 Density Biomass Seed rain Invasive Bromus takes over at elevated CO2 Ambient CO2 FACE - Nevada Desert 550 ppm Smith et al. 2000

33. Conclusions (i) 1. Major terrestrial biospheric sinks are in mid-latitudes (net sink) and in the tropics (gross sink). 2. Legacy of past land use practices is a major driver of the current Northern hemisphere C sink, and CO2 and N fertilization may play a much smaller role than previously thought. 3. Management practices and disturbances that affect the age structure and demography of ecosystems are critical for understanding current and future C sinks. Both need to be coupled to biogeochemical and ecophysiological models. 4. The causes of the tropical gross sink are less clear but CO2 fertilization may drive part of the sink. Why CO2 should increase NEP in the tropics and not in temperate forests?

34. Conclusions (ii) 5. CO2 fertilization is likely to have a larger effect in the coming decades but not beyond 600 ppm. 6. Globally, N deposition is responsible for less than 15% of the current sink, much less than previously thought. 7. Timing of precipitation and temperature will determine the net effect of climate change on C sinks. 8. Surprises in sink strength may arise in the future via changes in biodiversity.

35. Conclusions (iii) 9. There are no permanent sink mechanisms that will ensure indefinite terrestrial sinks. Many of the current sinks are likely to decrease or disappear over the next half a century.

36. End