OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

Richard J. Norby

Environmental Sciences Division

Oak Ridge National Laboratory

Forests in a CO2-Rich World:Old Questions, New Challenges

International Botanical CongressVienna, 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 CO2 concentration is superimposed on all of these influences

The trees in this younger forest experience an increasing [CO2] 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 [CO2] (~280 ppm)

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

• 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

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There is a wealth of data from many CO2 enrichment studies demonstrating physiological responses of seedlings and young trees

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

Global Carbon Cycle

Scale: the big challenge

The large size and long life of trees preclude direct assessment of CO2 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 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.

- 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 CO2 enrichment (FACE) studies help to resolve some of the uncertainty

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Experimental Results with Young Trees

Forest FACE Synthesis Project

Objective: Quantify CO2 effect on NPP in a manner that will inform ecosystem and global modelsExplain 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

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

This framework is used in interpretation of experiments, model implementation, and remote sensing

NPP = ε * APARε = light-use efficiencyAPAR = 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

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 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

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 CO2-rich world

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.

- Boyd Strain & Fakhri Bazzaz, 1983

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

Calculation of NPP

Oak Ridge Experiment on CO2 Enrichment of Sweetgum

Stem

Allometry : DM = f(BA, H, taper, density)

Fine rootMinirhizotrons and in-growth cores

Coarse rootAllometry: DM = f(BA)

UnderstoryHarvest

LeafLitter traps

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Oak Ridge Experiment on CO2 Enrichment of Sweetgum

• CO2 has consistently stimulated NPP

• Average increase is 23% (16-38%)

• LAI (~6) has not been increasing with time or CO2

Net primary productivity

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NPP can be separated into structural and functional components

LAI APAR LUE NPP

  (m2 m-2) (MJ m-2 y-1) (g MJ-1) (g m-2 y-1)

ambient 5.66 1225 1.71 2070

elevated 5.93 1226 2.11 2561

E/A 1.05 1.00 1.24 1.24

1999-2002

• Leaf area and APAR were not altered by elevated CO2

• 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 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

What is the fate of the additional C absorbed from the CO2-enriched atmosphere?

Oak Ridge Experiment on CO2 Enrichment of Sweetgum

• 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

Aboveground woody increment

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Oak Ridge Experiment on CO2 Enrichment of Sweetgum

Fine root production

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• 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

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Oak Ridge Experiment on CO2 Enrichment of Sweetgum

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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum

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Root length (m m-2)

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

Fine root distribution in soil

Norby et al. Proc. Nat. Acad. Sci. (2004) 101: 9689-9693

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 CO2 efflux from soil increased only

occasionally in elevated CO2

– more C is accumulating in soil– time lags in response confound analysis

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

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