Mechanisms of Current Terrestrial Carbon Sinks and Future Persistency Josep Canadell GCP and GCTE International Office Canberra, Australia [

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 CO 2 fertilization Nitrogen fertilization Reforestation Surprises through changes in biodiversity

IPCC 2001 Schimel 2001 Achard et al Malhi& Grace 2000 Terrestrial Carbon Sources and Sinks [1990’s] Pg C/yr

Terrestrial Carbon Sinks [1990’s] Pg C/yr Net Sink Gross Sink

Terrestrial Biosphere C Sink Cramer et al Why do we need to know the mechanisms? IPCC 2001 Future atmospheric CO 2 concentrations and stabilization scenarios

Late 1990’s: Cropland establishment and abandonment, CO 2 and Climate (IPCC 2001). Mid 1990’s: There was more than CO 2. N deposition with unrealistic uptake rates of up to 80%. Early 2000’s: All due to past land use practices (US-lead), (using forest demography and age structure). CO 2 no effect. Early 1990’s: All due to CO 2 fertilization (biogeochemistry models/Physiological) Sink Mechanisms – The 90’s understanding

CO 2 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)` Candidate Mechanisms of Current Terrestrial Sinks Direct human induced

Global Sink Attribution by IPCC 2001 [ ] S1 = CO 2 S2 = CO 2 + Climate* S3 = CO 2 + Climate + Cropland Establishment and Abandonment** *Climate effect is inferred by S2 - S1 ** Land-use effect is inferred by S3 - S2 In the 1980s North extra-tropics: CO 2 : -0.2 to -1.6; Climate: +0.4 to –0.2; Land use: 0.0 to -0.4 Tropics: CO 2 : -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. McGuire et al. (2001)

Forest trees0.15 Other forest 0.15 Cropland soils0.04 Woody encroach Wood products0.07 Reservoirs0.04 Exports - Imports0.09 US-Fixed expt.rivers0.04 Sinks in the Coterminous U.S. [ ] PgC yr PgC yr -1 apparent U.S. Pacala et al. 2001

Forest trees0.15 Other forest 0.15 Cropland soils0.04 Woody encroach Wood products0.07 Reservoirs0.04 Exports - Imports0.09 US-Fixed expt.rivers % of the total Sink due to trees Sinks in the Coterminous U.S. [ ] PgC yr -1

35% of the sink is susceptible CO 2 and N deposition fertilization Forest trees0.15 Other forest0.15 Cropland soils0.04 Woody encroach Wood products0.07 Reservoirs0.04 Exports - Imports0.09 US-Fixed expt.rivers0.04 Sinks in the Coterminous U.S. [ ] PgC yr -1

Forest trees0.15 Other forest 0.15 Cropland soils0.04 Woody encroach.0.13 Wood products0.07 Reservoirs0.04 Exports - Imports0.09 US-Fixed Ex.Rivers % of total Sink due to other less commonly accounted mechanisms Sinks in the Coterminous U.S. [ ] PgC yr -1

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

Forest Regrowth in Abandoned Croplands 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 CO 2 Nitrogen Deposition Climate Change Caspersen et al ’s-1990’s Forest Inventory 1 Forest Inventory 2 Growth Rate 2% 98% time

Sink Strength due to Forest Regrowth Jiquan Chen, Univ of Toledo Net Ecosystem Productivity (Mg.ha-1) t 3 t 4 t 1 t 2 Years

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

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

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

Disturbances in Canada’s forests [1920 – 1995] Kurz & Apps 1999

Variable TempConstant Temp Tg C / yr Source Sink Decrease after 1970 Net ecosystem C fluxes in Canada [1920 – 1995] Kurz & Apps 1999

Photo: Martin 1975, Arizona 1903 & 1941 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. Maximum Potential C sequestration in the absence of fire = 2 Pg C yr -1 (upper value) Scholes and Hal 1996 Estimated CO 2 sink: USA : 0.17 PgC/yr for the 1980s ( Houghton et al., 1999) NE Australia : 0.03 PgC/yr (Burrows, 1998) Woody Encroachment

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

Biomass Stimulation (%) GCTE Synthesis. Mooney et al Biomass Responses to Elevated CO 2

Growth Enhancement CO 2 concentration (ppm) Photo: R. Jackson [Texas, USA] Increasing aCO 2 Effects on Plant Growth Canadell et al. (in preparation)

g S (mol m -2 s -1 ) Jackson et al Stomatal acclimation - Solanum (C 3 forb ) Intercellular [CO 2 ] 600 ppm CO 2 H2OH2O Saturation of CO 2 Increased Water Use Efficiency

Fossil-fuel N Deposition on Land (kg/km 2 ) 1990 Townsend et al Net primary production (g C m -2 y –1 ) , Nitrogen input (g N m -2 y -1 ) NPP Responses to N fertilization Schlesinger 1997 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)

Houghton 2002 Reforestation: Annual Flux of Carbon in China [ ]

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

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

Surprises

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

Ambient CO ppm Native annuals Bromus Relative ratio (elevated/ambient CO 2 ) Density Seed rain Biomass Invasive Bromus takes over at elevated CO 2 FACE - Nevada Desert Smith et al. 2000

1. Major terrestrial biospheric sinks are in mid-latitudes (net sink) and in the tropics (gross sink). Conclusions (i) 2. Legacy of past land use practices is a major driver of the current Northern hemisphere C sink, and CO 2 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 CO 2 fertilization may drive part of the sink. Why CO 2 should increase NEP in the tropics and not in temperate forests?

5. CO 2 fertilization is likely to have a larger effect in the coming decades but not beyond 600 ppm. Conclusions (ii) 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.

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. Conclusions (iii)

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