O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Richard J. Norby Environmental Sciences Division Oak Ridge National Laboratory Forests in a CO 2 -Rich World: Old Questions, New Challenges International Botanical Congress Vienna, Austria 19 July 2005

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 CO 2 concentration is superimposed on all of these influences

The trees in this younger forest experience an increasing [CO 2 ] each year – from 315 ppm in 1958 to nearly 380 ppm today

For over a millennium these old trees grew in an atmosphere with nearly unchanging [CO 2 ] (~280 ppm)

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

Elevated CO 2 stimulates photosynthesis Trees grow faster in elevated CO 2 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 CO 2 There is a wealth of data from many CO 2 enrichment studies demonstrating physiological responses of seedlings and young trees

The primary responses of trees to elevated CO 2 have the potential to alter the net exchange of C between the atmosphere and biosphere. “CO 2 fertilization” can create a negative feedback on the anthropogenic increase in [CO 2 ] This negative feedback is represented in global models that couple the C cycle to climate models The description of the biospheric response to CO 2 is poorly constrained by data The global carbon cycle provides the context for most of our research on forests in a CO 2 -rich world Global Carbon Cycle

Scale: the big challenge The large size and long life of trees preclude direct assessment of CO 2 fertilization of intact, mature forests Are data from short-term experiments with young trees relevant to questions about the global C cycle?

We cannot make reliable predictions concerning the global effects of increasing CO 2 concentration until we have information based on long-term measurements of plant growth from experiments in which high CO 2 concentration is combined with water and nitrogen stress on a wide range of species. - Paul Kramer, 1981 Old question: how to relate what we know about physiology to forest response New Challenge: describing responses to inform models

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 CO 2 enrichment (FACE) studies help to resolve some of the uncertainty Experimental Results with Young Trees

Forest FACE Synthesis Project Objective: Quantify CO 2 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 CO 2 for 3-8 years NPP from all plots and years after canopy development was complete

Response of NPP to elevated CO 2 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 CO 2 Response translates to a β-factor of 0.60

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 CO 2 on LAI across all sites APAR increased in elevated CO 2 at low LAI, but not at higher LAI Functional vs. structural components

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 The importance of structural changes decreases as LAI increases

This synthesis of experimental evidence provides a standard to evaluate models Six dynamic global vegetation models predicted NPP response to elevated CO 2 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, Congruence of model and data on NPP response adds confidence to subsequent model results that depend on the biosphere- atmosphere feedback Model – Data Comparison

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 This strong evidence describing NPP response does not resolve all issues about forests in a CO 2 -rich world

The initial effect of elevated CO 2 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. - Boyd Strain & Fakhri Bazzaz, 1983

Oak Ridge Experiment on CO 2 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 CO 2 exposure (550 ppm) started spring, 1998

Calculation of NPP Oak Ridge Experiment on CO 2 Enrichment of Sweetgum Stem Allometry : DM = f(BA, H, taper, density) Fine root Minirhizotrons and in-growth cores Coarse root Allometry: DM = f(BA) Understory Harvest Leaf Litter traps

Oak Ridge Experiment on CO 2 Enrichment of Sweetgum CO 2 has consistently stimulated NPP Average increase is 23% (16-38%) LAI (~6) has not been increasing with time or CO 2 Net primary productivity

NPP can be separated into structural and functional components LAIAPARLUENPP (m 2 m -2 )(MJ m -2 y -1 ) (g MJ -1 )(g m -2 y -1 ) ambient elevated E/A Leaf area and APAR were not altered by elevated CO 2 Increase in NPP is attributable to increased light-use efficiency

Carbon partitioning is a key issue Patterns of C partitioning have implications for C turnover and sequestration Inherent differences in C partitioning might explain differences between ecosystems in CO 2 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 What is the fate of the additional C absorbed from the CO 2 -enriched atmosphere?

Oak Ridge Experiment on CO 2 Enrichment of Sweetgum No difference in growth prior to treatment (1997) CO 2 significantly increased growth in 1st year of treatment (33%), but not in subsequent years (5- 15%) NPP increase not recovered in wood Aboveground woody increment CO 2 on wood

Oak Ridge Experiment on CO 2 Enrichment of Sweetgum Fine root production CO 2 on wood fine root The increase in NPP is recovered primarily in fine root production Annual fine root production has more than doubled since the 3 rd year of treatment

Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Oak Ridge Experiment on CO 2 Enrichment of Sweetgum Productivity was 2.2 times higher in elevated CO 2 from Annually, mortality matched production in both ambient and elevated CO 2 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:

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

Implications of fine root response to carbon sequestration 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 CO 2 efflux from soil increased only occasionally in elevated CO 2 –more C is accumulating in soil –time lags in response confound analysis

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

Context is Important The responses of forests to an increasing atmospheric CO 2 concentration are largely positive increased net primary productivity potential for a negative feedback on increasing atmospheric CO 2 But numerous other co-occurring changes will moderate those responses climatic warming, ozone, nitrogen limitations… We must not lose sight of how CO 2 responses fit in to the more general picture of global change “CO 2 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