Richard J. Norby Environmental Sciences Division Oak Ridge National Laboratory

1. Forests in a CO2-Rich World: Old Questions, New Challenges Richard J. Norby Environmental Sciences Division Oak Ridge National Laboratory International Botanical Congress Vienna, Austria 19 July 2005

2. Context is Important Forests are always changing…… large fluctuations in daily and seasonal weather periodic stresses of drought, flooding, wind pests and disease large-scale disturbance: fire The influence of rising atmospheric CO2 concentration is superimposed on all of these influences

3. The trees in this younger forest experience an increasing [CO2] each year – from 315 ppm in 1958 to nearly 380 ppm today

4. For over a millennium these old trees grew in an atmosphere with nearly unchanging [CO2] (~280 ppm)

5. Young seedlings growing today will experience a substantially different atmosphere as they mature over the next few decades

6. Elevated CO2 stimulates photosynthesis Trees grow faster in elevated CO2 and are bigger at the end of the experiment N concentrations are reduced No large changes in structure Stomatal conductance often is lower We know how trees respond to elevated CO2 There is a wealth of data from many CO2 enrichment studies demonstrating physiological responses of seedlings and young trees

7. Global Carbon Cycle The primary responses of trees to elevated CO2 have the potential to alter the net exchange of C between the atmosphere and biosphere. “CO2 fertilization” can create a negative feedback on the anthropogenic increase in [CO2] This negative feedback is represented in global models that couple the C cycle to climate models The description of the biospheric response to CO2 is poorly constrained by data The global carbon cycle provides the context for most of our research on forests in a CO2-rich world

8. Scale: the big challenge Are data from short-term experiments with young trees relevant to questions about the global C cycle? The large size and long life of trees preclude direct assessment of CO2 fertilization of intact, mature forests

9. - Paul Kramer, 1981 We cannot make reliable predictions concerning the global effects of increasing CO2 concentration until we have information based on long-term measurements of plant growth from experiments in which high CO2 concentration is combined with water and nitrogen stress on a wide range of species. Old question: how to relate what we know about physiology to forest response New Challenge: describing responses to inform models

10. Experimental Results with Young Trees Extrapolation of experimental results from young trees and seedlings can lead to a false view of forest response Wide variation in response is difficult to explain or summarize Belowground productivity is too often ignored Current free-air CO2 enrichment (FACE) studies help to resolve some of the uncertainty

11. Forest FACE Synthesis Project Objective: Quantify CO2 effect on NPP in a manner that will inform ecosystem and global models Explain differences in response between experiments Four experiments in which forest stands exposed to ~550 ppm CO2 for 3-8 years NPP from all plots and years after canopy development was complete

12. Response of NPP to elevated CO2 is consistent across a wide range of NPP Regression is significantly different from 1:1 line Regression defines a median response of 23% enhancement of NPP in ~550 ppm CO2 Response translates to a β-factor of 0.60

13. Functional vs. structural components This framework is used in interpretation of experiments, model implementation, and remote sensing NPP = ε * APAR ε = light-use efficiency APAR = absorbed light No significant effect of CO2 on LAI across all sites APAR increased in elevated CO2 at low LAI, but not at higher LAI

14. The importance of structural changes decreases as LAI increases Calculate the fraction of normalized gain in NPP attributable to gain in APAR In stands with low LAI, 60-90% of NPP gain was associated with increased APAR At higher LAI, NPP gain wholly attributable to increased LUE

15. Model – Data Comparison This synthesis of experimental evidence provides a standard to evaluate models Six dynamic global vegetation models predicted NPP response to elevated CO2 Response varied from 15 to 32% increase Average response of 22% very close to experimental evidence Another model prescribes a β-factor of 0.65, close to the experimental result Cramer et al. Global Change Biology (2001) 7, 357-373 Congruence of model and data on NPP response adds confidence to subsequent model results that depend on the biosphere-atmosphere feedback

16. This strong evidence describing NPP response does not resolve all issues about forests in a CO2-rich world • The median response masks spatial and temporal variability • Interactions with other global change factors may be significant • N feedbacks might limit response over the long term • The analysis did not include tropical or boreal forests • Will responses persist in more mature forests? • C partitioning patterns may determine the ultimate fate of the additional C

17. - Boyd Strain & Fakhri Bazzaz, 1983 The initial effect of elevated CO2 will be to increase NPP in most plant communities... A critical question is the extent to which the increase in NPP will lead to a substantial increase in plant biomass. Alternatively, increased NPP could simply increase the rate of turnover of leaves or roots without changing plant biomass.

18. Oak Ridge Experiment on CO2 Enrichment of Sweetgum • Liquidambar styraciflua monoculture plantation started in 1988 • the closed canopy constrains growth responses • full occupancy of the soil by the root system constrains the nutrient cycle • 2 elevated, 3 control plots • Each plot is 25 m diameter with ~90 trees • Full year of pre-treatment measurement in 1997 • CO2 exposure (550 ppm) started spring, 1998

19. Oak Ridge Experiment on CO2 Enrichment of Sweetgum Calculation of NPP Stem Allometry : DM = f(BA, H, taper, density) Coarse root Allometry: DM = f(BA) Leaf Litter traps Fine root Minirhizotrons and in-growth cores Understory Harvest

20. Oak Ridge Experiment on CO2 Enrichment of Sweetgum Net primary productivity • CO2 has consistently stimulated NPP • Average increase is 23% (16-38%) • LAI (~6) has not been increasing with time or CO2

21. NPP can be separated into structural and functional components 1999-2002 • Leaf area and APAR were not altered by elevated CO2 • Increase in NPP is attributable to increased light-use efficiency

22. Patterns of C partitioning have implications for C turnover and sequestration Inherent differences in C partitioning might explain differences between ecosystems in CO2 response The FACE synthesis showed widely divergent patterns in partitioning In the ORNL sweetgum FACE, the additional C is being partitioned primarily to fine roots Carbon partitioning is a key issue What is the fate of the additional C absorbed from the CO2-enriched atmosphere?

23. Oak Ridge Experiment on CO2 Enrichment of Sweetgum Aboveground woody increment • No difference in growth prior to treatment (1997) • CO2 significantly increased growth in 1st year of treatment (33%), but not in subsequent years (5-15%) • NPP increase not recovered in wood wood CO2 on

24. Oak Ridge Experiment on CO2 Enrichment of Sweetgum Fine root production • The increase in NPP is recovered primarily in fine root production • Annual fine root production has more than doubled since the 3rd year of treatment wood CO2 on fine root

25. Oak Ridge Experiment on CO2 Enrichment of Sweetgum Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct 1998 1999 2000 2001 2002 2003 Productivity was 2.2 times higher in elevated CO2 from 2000-2003 Annually, mortality matched production in both ambient and elevated CO2 Highly significant effect on peak standing crop. No effect on turnover (production/peak crop = 1.7 y-1) Norby et al. Proc. Nat. Acad. Sci. (2004) 101: 9689-9693

26. Oak Ridge Experiment on CO2 Enrichment of Sweetgum 1998 2003 Depth (cm) Root length (m m-2) Fine root distribution in soil In 1998 79% of root length was in top 30 cm In 2003, 63% was in top 30 cm in ambient CO2, but root length at 30-60 cm was significantly increased by elevated CO2 This response, although highly variable, could have important implications for C, N, and water cycling Norby et al. Proc. Nat. Acad. Sci. (2004) 101: 9689-9693

27. C partitioned to short-lived tissue is not sequestered in biomass What is fate of C in dead fine roots? a large fraction rapidly returns to the atmosphere but CO2 efflux from soil increased only occasionally in elevated CO2 more C is accumulating in soil time lags in response confound analysis Implications of fine root response to carbon sequestration

28. Soil carbon sequestration • Soil C increased in both ambient and elevated CO2 despite higher decomposition rate of old organic matter in elevated CO2 plots (W. M. Post) • C accrual in the top 5 cm due to CO2 enrichment was 44 g m-2 yr-1 (J. D. Jastrow) • The microaggregate fraction increased; this facilitates movement of C into long-lived pools

29. Context is Important The responses of forests to an increasing atmospheric CO2 concentration are largely positive increased net primary productivity potential for a negative feedback on increasing atmospheric CO2 But numerous other co-occurring changes will moderate those responses climatic warming, ozone, nitrogen limitations… We must not lose sight of how CO2 responses fit in to the more general picture of global change “CO2 fertilization” does not allow us to ignore the serious threats of atmospheric and climatic change to forest ecosystems and the goods and services they provide to humankind