# Presentation: 553562

## Forest Sensitivity to Elevated Atmospheric CO2 and its Relevance to Carbon Management

- Richard J. Norby
- Oak Ridge National Laboratory
- Aspen Global Change Institute
- October 19, 2001

## Trees that are planted today will be growing in a higher CO2 concentration tomorrow

- Trees that are planted today will be growing in a higher CO2 concentration tomorrow

## Forests play an important role in the global C cycle

- Increased rate of CO2 uptake as atmospheric [CO2] rises could slow the rate of [CO2] increase

## Forestry Issues with Rising Atmospheric CO2

- Forestry Issues with Rising Atmospheric CO2

**Global Carbon Cycle**
**flux   storage**

- Net primary
-  productivity

**Vegetation Response**
**“****CO****2 ****fertilization**”

- Forest composition
- and biomass

- Deforestation,
- reforestation

**Climatic Change**
**“****greenhouse effect**”

## 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**

## 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**

## Alternative Research Questions

- What trees should be planted to take advantage of rising [CO2]?
- Will trees prevent global warming?
- Can high CO2 be used to sequester more C in trees?

***Alternative Research Questions***

## Mature Forests Not Necessarily CO2 Sinks

**Madison, Wisconsin** – Some scientists and policy-makers claim forests can absorb enough carbon dioxide to cut the risk of further global warming.  But at least some forests may not be up to the job.
- 	--ScienceNow, 10 August 2001

**Role of Trees in Curbing Greenhouse Gases Is Challenged**
- Two new studies challenge the idea that planting forests could be an effective way to absorb emissions of carbon dioxide, heat-trapping gas that many scientists believe is causing global warming.
- 	--New York Times, 24 May 2001

**Trees Aren’t a ‘Magic Answer’ on Warming**
- Studies suggest forests won’t soak up excess CO2.
- 	--New York Times, 2 June 1992

## We know how trees respond to elevated CO2

**We know how trees respond to elevated CO****2**

**Elevated CO****2**** stimulates photosynthesis and photosynthetic enhancement is sustained in the field**
**Trees in elevated CO****2**** are bigger at the end of the experiment**
**N concentrations are reduced**
**No large changes in structure**
**Stomatal responses are inconsistent**

## Response of tree productivity is likely to be less for mature trees in a closed forest

**How can data from relatively short-term experiments be used to address the long-term responses of a forest?**

## Hypotheses for new experiments

**Maximum stand leaf area will increase with increasing CO****2**
**Growth per unit leaf area will increase after canopy closure**
**Fine root standing crop will not change but turnover will increase**
**Downregulation of growth will occur through N cycle**
**Stand water use will be decoupled from effects on stomata**

## Oak Ridge Experiment on CO2 Enrichment of Sweetgum

**Oak Ridge Experiment on**** CO****2**** Enrichment of Sweetgum**

**To understand how the eastern deciduous forest will be affected by CO****2**** enrichment of the atmosphere, and what are the feedbacks from the forest to the atmosphere. **
**This goal will be approached by measuring the integrated response of an intact forest ecosystem, with a focus on stand-level mechanisms.**

**Goal**

## 
Each plot is 25 m diameter (20 m diameter inside buffer) 
Full year of pre-treatment measurement in 1997
CO2 exposure (560 ppm) started spring, 1998
Exposure is 24 hours per day, April through October
Brookhaven exposure system

**Each plot is 25 m diameter (20 m diameter inside buffer) **
**Full year of pre-treatment measurement in 1997**
**CO****2**** exposure (560 ppm) started spring, 1998**
**Exposure is 24 hours per day, April through October**
**Brookhaven exposure system**

**CO****2**** Enrichment of a Deciduous Forest: The Oak Ridge Sweetgum Experiment**

***Liquidambar styraciflua***** plantation started in 1988**
**2 elevated, 3 control plots (2 with blowers)**

## Calculation of NPP

- <date/time>

**Calculation of NPP**

- Oak Ridge Experiment on CO2 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**

## Net Primary Productivity

- <date/time>

**Net Primary Productivity**

**CO****2**** effect on NPP was:                27% in 1998,   17% in 1999, 19% in 2000.**
**GPP was 27% higher in elevated CO****2****.**
**Allocation of the extra C changed from stem to fine root.**

- Oak Ridge Experiment on CO2 Enrichment of Sweetgum

## Aboveground Woody Increment

- <date/time>

**Aboveground Woody Increment**

**No difference in growth prior to treatment (1997).**
**CO****2**** significantly increased growth in 1st year of treatment (35%), but not in 2nd (15%) or 3****rd**** (7%).**
**The CO****2**** effect on stem growth also has disappeared in the Duke prototype FACE ring.**

- Oak Ridge Experiment on CO2 Enrichment of Sweetgum

## Fine Root Productivity

- <date/time>

**Fine Root Productivity**

**Delay in fine root response may be related to accumulation of CHO in coarse roots.**
**Fine roots C turns over faster than wood C through soil processes.**
**Analyze soil heterotrophic respiration:**
**	****	****- ****integrate CO****2**** efflux**
**	****	****- subtract root respiration**
**	****	****- add litter decomposition**

- Oak Ridge Experiment on CO2 Enrichment of Sweetgum

## Net Ecosystem Productivity

- <date/time>

**Net Ecosystem Productivity**

**NEP was positive each year.**
**CO****2**** effect on heterotrophic respiration was less than effect on NPP. **

- Oak Ridge Experiment on CO2 Enrichment of Sweetgum

## Dynamics of Carbon Storage

**Short-term C sequestration (NEP) was enhanced by CO****2**** enrichment**
**With an increasing fraction of the additional C storage allocated to short-term pools rather than to wood, the gains are likely to be short-lived**

**Dynamics of Carbon Storage**

## Additional carbon entering the soil through root systems has the potential to add to long-lived soil organic matter pools, but most analyses suggest this is unlikely.

## Regardless of the fate of the extra C taken up in the Oak Ridge and Duke FACE experiments, both of these rapidly-growing tree plantations are sequestering substantial amounts of C under current conditions.

## Interactions between CO2 and Water

**CO****2**** effects on leaf-level transpiration are mitigated as the scale increases to a whole canopy, stand, and region**

**Interactions between CO****2**** and Water**

**Elevated CO****2**** often reduces stomatal conductance and leaf-level transpiration and increases water-use efficiency**

## Interactions between CO2 and Warming

**Effects are non-linear, time-dependent, and highly dependent on initial conditions**
**Warming can stimulate productivity through increased photosynthesis and longer growing season**
**Warming can decrease productivity through increased stress**
**Elevated CO****2**** is likely to ameliorate negative effects of warming**

**Interactions between CO****2**** and Warming**

## Prediction of NPP with Three Biogeochemical Models

**Prediction of NPP with Three Biogeochemical Models**

- Predictions are for 2025-2034 (425 ppmv CO2)
- All three models predict increased NPP with climate change and increased CO2 for both climate simulations
- Increases are smaller (or become decreases) when CO2 is not included
- Increases are less with the Canadian climate simulation

## CO2 x Temperature Interactions

- Elevated CO2 increased growth of maples trees by 73%

## CO2 x Temperature Interactions

- Elevated temperature reduced growth by 35% because of increased stress

## CO2 x Temperature Interactions

- Positive effects of CO2 and negative effects of temperature were additive

## Should the certainty of rising atmospheric CO2 influence forest management decisions?

- A factor in evaluation of sequestration projects?

**Should the certainty of rising atmospheric CO****2**** influence forest management decisions?**

## Wide range in response to high CO2?

- Is variation due to species or experiment?
- Dry matter increase is confounded by increasing leaf area and exponential growth
- These results do not predict the response of a forest

## Species may be fundamentally similar

- Expressing annual growth per unit leaf area adjusts for exponential growth pattern
- Response is uniform across species and conditions: 29% increase at ~650 ppm CO2
- Growth per unit leaf area separates the functional response from the structural response

## CO2 Effect on Sequestration is Relatively Small

- NEP gain

- wood gain

- NPP gain

- CO2 effect on NEP must be discounted somewhat because of lack of permanence and because increase in [CO2] will be gradual
- Any remaining effect is small compared to the substantial measurement errors

**CO****2**** Effect on Sequestration is Relatively Small**

*CO**2** fertilization need not be considered part of the evaluation of a C sequestration project*

## FACE experiments are a useful testbed for sequestration projects

- Full C budgets
- New instrumentation
- Increasing focus on belowground C accounting

**FACE experiments are a useful testbed for sequestration projects**

## Acknowledgements

- Research funded by U.S. Department of Energy
- Artwork provided by Vincent van Gogh
- (www.vangoghgallery.com)